Evaluation of prominent PIO susceptibility criteria - garteur

Version: 3- i - Date: October 15 th , 1999GARTEUR/TP-120-01List of AuthorsHafid Smaïli NLR Chapter 1, 5, contribution to chapter 2, 3 and editorStefano Scala CIRA Contribution to chapter 2 and 3F. Amato UNAP Contribution to chapter 3Rogier van der Weerd DUT Contribution to chapter 3Gunnar Hovmark FFA Contribution to chapter 4Hans-Joachim Mehl DLR Contribution to chapter 4Hans-Christoph Oelker DASA DASA industrial viewLars Rundqwist SAAB SAAB industrial view

Version: 3- ii - Date: October 15 th , 1999GARTEUR/TP-120-01SummaryThis report ‘Evaluation of Prominent PIO Susceptibility Criteria’ presents a literature survey oncurrent criteria for the prediction of pilot-in-the-loop oscillations. The document is a deliverableof Workpackage 1 (task 1.1) ‘Analysis Challenge’ of GARTEUR action group FM(AG12)‘Pilot-in-the-Loop Oscillations; Analysis and Test Techniques for their Prevention’.The document provides a description of criteria for the prediction of category I and II PIOs. Thetheoretical backgrounds of the criteria are described including their effectiveness to predictPIOs in the aforementioned categories. The goal of the report is to provide the analysis teams inthe project a consolidated view on the applicability of the most prominent set of PIOsusceptibility criteria. The selected criteria in this report will be used by the design teams todevelop analysis and evaluation techniques, which prove that a given highly augmented aircraftis sufficiently free from PIO proneness. In this process, priority will be given to thedevelopment of prediction procedures for Category II PIOs. Analysis methods for PIOsusceptibility assessment will be addressed separately by the design teams.The results in this report indicate that the Category I and II prominent PIO criteria have a highdegree of success in predicting PIOs for the cases evaluated. However, the Category II criteria,although promising in predicting PIO, should be developed further.This report was established with contributions from CIRA, UNAP, INTA, FFA, DLR and NLR.Part of the material in this report has been provided by AGARD/RTO Working Group 23 onFlight Control. Integration and final editing of the supported material was performed by NLR.

Version: 3- iii - Date: October 15 th , 1999GARTEUR/TP-120-01ContentsList of AuthorsSummaryList of FiguresList of TablesList of Symbols and AbbreviationsDistribution Listiiivviiviiixi1. Introduction 12. Aircraft-Pilot Coupling and Pilot-in-the-Loop Oscillations 32.1 General 32.2 Pilot-in-the-Loop Oscillations (PIO); Background and Classifications 32.3 Prominent PIO prediction criteria 52.3.1 Category I PIO Criteria 72.3.2 Category II PIO Criteria 72.3.3 Criterion Evaluation 83. Category I PIO Criteria Evaluation 113.1 General 113.2 Neal-Smith Criterion 113.2.1 Criterion Background 113.2.2 Criterion Evaluation 123.3 Bandwidth / Phase Delay Criterion 133.3.1 Criterion Background 133.3.2 Criterion Evaluation 143.4 Smith-Geddes Criterion 153.4.1 Criterion Background 153.4.2 Criterion Evaluation 163.5 Phase Rate Criterion and Gain Phase Template (Average Phase Rate) 173.5.1 Criterion Background 173.5.2 Criterion Evaluation 183.6 Gibson Time Domain Dropback 223.6.1 Criterion Background 223.6.2 Criterion Evaluation 253.7 Discussion of Category I PIO Criteria 273.7.1 Criterion Effectiveness 273.7.2 Criterion Gaps and Extensions 29

Version: 3- iv - Date: October 15 th , 1999GARTEUR/TP-120-014. Category II PIO Criteria Evaluation 314.1 General 314.2 Time Domain Neal-Smith Criterion 314.2.1 Criterion Background 314.2.2 Criterion Evaluation 384.3 Open Loop Onset Point Criterion (OLOP) 424.3.1 Criterion Background 424.3.2 Criterion Evaluation 445. Conclusions 47Industrial View 49References 53

Version: 3- v - Date: October 15 th , 1999GARTEUR/TP-120-01List of FiguresFigure 1: Application of the Neal-Smith criterion to the landing databases LAHOS, HAVE PIO,HAVE CONTROL. ............................................................................................................ 12Figure 2: Application of the bandwidth/phase delay criterion to the landing databases LAHOS,HAVE PIO, HAVE CONTROL. ........................................................................................ 14Figure 3: Application of the Smith-Geddes criterion to the landing databases LAHOS, HAVEPIO, HAVE CONTROL. .................................................................................................... 16Figure 4: Application of the phase rate criterion to the landing databases LAHOS, HAVE PIO,HAVE CONTROL. ............................................................................................................ 18Figure 5: The gain phase template part of the Average Phase Rate criterion, with the evaluationof configuration LAHOS 5_1. ............................................................................................ 19Figure 6: Evaluation of the LAHOS configuration 5_1 by the Gibson gain-phase template. .... 21Figure 7: Pitch axis step response characteristics for application of time domain dropbackcriterion. .............................................................................................................................. 23Figure 8: Original dropback criterion boundaries. ...................................................................... 23Figure 9: Frequency-domain definition of dropback using the frequency response of pitch rateto control inceptor force q/F es (s). ........................................................................................ 24Figure 10: Generic step responses showing the relationship between pitch attitude and flightpath angle. ........................................................................................................................... 25Figure 11: Application of Gibson Dropback criterion to the landing databases LAHOS, HAVEPIO, HAVE CONTROL ..................................................................................................... 25Figure 12: Application of the modified dropback criterion to the landing databases LAHOS,HAVE PIO, HAVE CONTROL ......................................................................................... 26Figure 13: The closed-loop tracking task. ................................................................................... 31Figure 14: A typical Simulink system for use of the criterion. ................................................... 33Figure 15: A typical time history, for the Neal-Smith configuration 1c. .................................... 34Figure 16: Level 1, 2 and 3 HQ limits from ref. [45]. ................................................................ 34Figure 17: Constraints for rmsθeat the smallest feasible D vs. D, θc= 5°. .............................. 36∂rmsθ Figure 18: Constraints for minimume vs. D, θc= 5°. .................................................. 36∂D2∂ rmsθ Figure 19: Constraints for maximum e2 vs. D, θc= 5°................................................. 37∂DFigure 20: Constraints for rmsθeat the smallest feasible D vs. D, θc= 2.5°. ........................... 37Figure 21: Constraints for rmsθeat the smallest feasible D vs. D, θc= 10°. ............................ 38Figure 22: 565 deg/s rate limit. ................................................................................................... 39Figure 23: 35 deg/s rate limit. ..................................................................................................... 40Figure 24: 15 deg/s rate limit. ..................................................................................................... 41

Version: 3- vi - Date: October 15 th , 1999GARTEUR/TP-120-01Figure 25: Jump phenomenon after rate limiting onset. ............................................................. 43Figure 26: Physical significance of the OLOP parameter. .......................................................... 44Figure 27: Validation of the OLOP criterion using the FOSIM data. ......................................... 46

Version: 3- vii - Date: October 15 th , 1999GARTEUR/TP-120-01List of TablesTable 1: Evaluation of PIO prediction. ......................................................................................... 8Table 2: Landing databases for assessment of PIO criteria. ......................................................... 9Table 3: PIO prediction with the Neal-Smith criterion. .............................................................. 13Table 4: PIO prediction with the Bandwidth-Phase Delay criterion. ......................................... 15Table 5: PIO prediction with the Smith-Geddes criterion. ......................................................... 16Table 6: PIO prediction with Average Phase Rate plus gain-phase template criterion. ............. 20Table 7: PIO prediction with Gibson gain-phase template. ........................................................ 21Table 8: PIO prediction with Gibson dropback criterion. ........................................................... 26Table 9: PIO prediction with modified dropback criterion. ........................................................ 27Table 10: Performance Indices of Category I PIO prediction criteria. ....................................... 28Table 11: Lateral databases for PIO research. ............................................................................ 45

Version: 3- viii - Date: October 15 th , 1999GARTEUR/TP-120-01List of Symbols and AbbreviationsADMIRE Aero-Data Model in Research EnvironmentEREA Association of European Research Establishments in AeronauticsAG Action GroupAPC Aircraft-Pilot CouplingAPR Average Phase RateARE Algebraic Riccati EquationCEV Centre d’Essais en VolCIRA Centro Italiano Ricerche AerospazialiDAv Dassault AviationDASA DaimlerChrysler AerospaceDE GermanyDERA Defence Evaluation and Research AgencyDLR Deutsches Zentrum fűr Luft-und Raumfahrt e.V.DUT Delft University of TechnologyEG Exploratory GroupES SpainFCS Flight Control SystemFFA Flygtekniska Főrsőksanstalten (The Aeronautical Research Institute of Sweden)FM/GoR Flight Mechanics Group of ResponsablesFOSIM ForskningssimulatorFR FranceGARTEUR Group for Aeronautical Research and Technology in EURopeINTA Instituto Nacional de Técnica AeroespacialIT ItalyLAAS Laboratoire d’Analyse et d’Architecture des SystèmesLMI Linear Matrix InequalitiesNL The NetherlandsNLR Nationaal Luchtvaart- en RuimtevaartlaboratoriumNSF National Simulation FacilityOLOP Open Loop Onset PointONERA Office National d’Études et de Recherches AérospatialesPIO Pilot-Involved (or Pilot-Induced / Pilot-in-the-Loop) OscillationsPVS Pilot-Vehicle SystemSAAB Saab ABSE SwedenSUE Super-Etendard Simulator

Version: 3- ix - Date: October 15 th , 1999GARTEUR/TP-120-01TBCTBDUNAPWPXCTo Be ConfirmedTo Be DefinedUniversità degli Studi di Napoli Federico IIWork PackageGARTEUR Executive Committee

Version: 3- x - Date: October 15 th , 1999GARTEUR/TP-120-01

Version: 3- xi - Date: October 15 th , 1999GARTEUR/TP-120-01Distribution List(Distribution is via e-mail and Wide Area Network if not otherwise specified)GARTEUR Executive Committee (hardcopy)F. Abbink (NL) NLRP. Garcia Samitier (ES) INTAA. Gustafsson (SE) FFAD.E. Mowbray – Chairman XC (UK) DERAD. Nouailhas (FR) ONERAH.J. Schepers (DE) DLRGARTEUR Secretary (hardcopy)O.K. Sodha (UK) DERAGARTEUR Flight Mechanics Group of Responsables (hardcopy)B. Brännström (SE) FMVP. Caap (SE) FFAJ. Hall (UK) DERAH.T. Huynh – Monitoring Responsable FM(AG12) (FR) ONERAP.G.A.M. Jorna (NL) NLRA. Kröger (DE) DASAF. Muñoz – Chairman FM-GoR (ES) INTAR. Rodloff (DE) DLRGARTEUR Flight Mechanics Industrial Points of Contact (hardcopy)J. Choplin (FR) DAvE. Kullberg (SE) SAABGARTEUR FM(AG12) ParticipantsF. Amato (IT) UNAPS. Bennani (NL) DUTJ.M. Biannic (FR) ONERAJ. Choplin (FR) DAvM. Crouzet (FR) CEVB. Dang-Vu – Chairman FM(AG12) (FR) ONERAG. Duus – Vice-Chairman FM(AG12) (DE) DLRB. Escande (FR) ONERA

Version: 3- xii - Date: October 15 th , 1999GARTEUR/TP-120-01M. Garcia (FR) LAASG. Hovmark (SE) FFAR. Iervolino (IT) UNAPA. Knoll (DE) DASAH.J. Mehl (DE) DLRT. Norén (SE) FFAH.-Chr. Oelker (DE) DASAC. Pittet (FR) LAASI. Queinnec (FR) LAASL. Rundqwist (SE) SAABS. Scala (IT) CIRAM.H. Smaïli (NL) NLRS. Tarbouriech (FR) LAASP. Vicente (ES) INTAR. van der Weerd (NL) DUTT. Wilmes (DE) DLROthersM. Bauschat (DE) DLRL. Forssell (SE) FFAF. Johansson (SE) FFAF. Karlsson (SE) SAABR. Luckner (DE) DASAJ.F. Magni (FR) ONERAM. Selier (NL) NLRR. van der Sluis (NL) DUTA. Varga (DE) DLRC. Vidal (IT) CIRA

Version: 3- 1 - Date: October 15 th , 1999GARTEUR/TP-120-011. IntroductionThe development of fly-by-wire flight control systems for modern aircraft initiated an increasein problems encountered in the aircraft man-machine interface. These problems expressthemselves as adverse interactions between the human pilot and the aircraft dynamics and areindicated as Pilot-in-the-Loop Oscillations or PIO. Currently, PIO is considered as a subclass ofAircraft-Pilot Coupling or APC as the more general definition for these interactions.PIO can be considered as a closed-loop destabilisation of the aircraft-pilot loop, triggered by arich variety of diverse phenomena in terms of effective aircraft dynamics and pilot behaviour. Inmost cases, a PIO event is triggered by a sudden change of the vehicle dynamics during a highdemanding flying task in which the pilot is unable to adapt himself.Aircraft handling qualities research throughout the years has established a subset ofrequirements that can be used in aircraft design and analysis for the prevention of PIO.Although most established PIO criteria were determined to be suitable for the prediction oflinear PIOs, a set of criteria to evaluate non-linear PIO phenomena is still under investigationand are not yet adequate enough. In addition, current industry standards lack a specificguideline providing unified PIO test techniques and analysis tools for aircraft design.The GARTEUR action group FM(AG12) ‘Pilot-in-the-Loop Oscillations; Analysis and TestTechniques for their Prevention’ addresses the need for the development of a guideline on PIOtesting and analysis. The goal of the project is the development of analysis and test procedureswhich prove that a given highly augmented aircraft is sufficiently free from PIO proneness. Inthis process, an emphasis is made on the development of Category II PIO prediction procedures.As part of the project’s objectives, design teams will work on the application of existing andnew mathematical analysis tools, thereby comparing their capability to predict whether or not agiven pilot-vehicle system is PIO prone. This report provides the analysis teams a consolidatedview on the applicability of the most prominent set of Category I and II PIO susceptibilitycriteria. After a brief description on the background and characteristics of PIO and APC inchapter 2, the most prominent criteria for Category I and II PIO susceptibility assessment aredescribed (chapter 3 and 4). Following a description on its theoretical background, eachcriterion will be evaluated on its effectiveness to predict the relevant PIO category. Theapplicability of the prominent criteria will be summarised in chapter 5.

Version: 3- 2 - Date: October 15 th , 1999GARTEUR/TP-120-01Part of the material in this report has been provided by AGARD/RTO Working Group 23 onFlight Control, which edited a report on best practices in flight control design. A selection ofreference material is provided in the literature list at the end of this report.

Version: 3- 3 - Date: October 15 th , 1999GARTEUR/TP-120-012. Aircraft-Pilot Coupling and Pilot-in-the-Loop Oscillations2.1 GeneralThis part presents an introduction to Aircraft-Pilot Coupling (APC) or Pilot-in-the-LoopOscillations (PIO). The description will start with the background on the PIO phenomenon andthe classifications that categorize PIO (2.2). Following this, a selection of criteria will be madethat are the most prominent for assessing Category I and II PIOs (2.3).2.2 Pilot-in-the-Loop Oscillations (PIO); Background and ClassificationsPIO is a phenomenon in the field of Handling Qualities of aircraft which has been encounteredand studied well before the advent of active control technology and fly-by-wire flight controlsystems. Its origin is a misadaptation between the pilot and the aircraft during some task inwhich tight closed-loop control of the aircraft is required from the pilot, with the aircraft notresponding to pilot commands as expected by the pilot himself. This situation can trigger a pilotaction capable of driving the aircraft out of pilot control, which in some cases can only berecovered by the pilot releasing the column and exiting from the control loop.For the active role played by the pilot during the PIO, the original significance given to theacronym was Pilot Induced Oscillations. A more rigorous analysis of the causes of PIOhighlighted the fact that PIO are indeed caused by a deficient FCS design more than by piloterrors, and the term Aircraft-Pilot Coupling [5] has been recommended for use. More recently,AGARD/RTO Working Group 23 has suggested the expression Pilot-in-the-Loop Oscillationsto satisfy both of the two needs: first to use the PIO abbreviation which is so well-known to theaeronautical community, and second to be more generous to the pilot. At the same time the newproposed expression highlights the fundamental closed-loop behaviour of PIO.The introduction of fly-by-wire FCS has in a sense exacerbated the problem of PIO, since themultitude of FCS modes, which can be easily designed and included into the new Digital FCS,can very easily disorientate the pilot in the interpretation of the aircraft response to his actions.Indeed, the three elements which are considered in PIO analysis are:1. The pilot2. The aircraft dynamics3. The trigger

Version: 3- 4 - Date: October 15 th , 1999GARTEUR/TP-120-01The trigger is defined as an event which can introduce the misadaptation [2]. Examples of thetrigger are a mode change or an unexpected non-linear behaviour in the FCS, or a variation inthe pilot control behaviour, such as an increase of the pilot gain.This situation has forced the U.S. military authorities to write down since 1982 explicit FlyingQualities Requirements for PIO in their Military Standard Specification Documents [6].An even greater emphasis is given to PIO detection criteria in the new issues of this Standard[27]. The formal definition of PIO given is:There shall be no tendency for pilot-induced oscillations, that is, sustained or uncontrollableoscillations resulting from the efforts of the pilot to control the airplaneBecause of the highly destructive potential of the PIO phenomenon a great effort has been spentin the last years in many research programs both in USA and Europe [28, 26, 5] to study PIO, inorder to derive methods which will be able to predict the tendency of aircraft to develop PIO.As defined in the MIL specs, PIO is an umbrella under which the same phenomenon (closedlooppilot vehicle oscillation) can show with very different behaviours, mainly depending on theunderlying cause of the PIO occurrence.A classification of PIO (from [5]) is given below. It takes into account some possible differentbehaviours of the closed-loop pilot vehicle system during the PIO.In the given classification three different behaviours are recognised, leading to three PIOcategories:PIO Category IThe closed-loop pilot vehicle system has a linear behaviour. The PIOs in this category resultfrom identifiable phenomena such as excessive time delay, excessive phase loss due to filters,improper control/response sensitivity, etc. As they are the simplest to model, they can be verywell understood to prevent PIO in this category.PIO Category IIThe closed-loop pilot vehicle system has a non-linear behaviour, mainly characterised by thesaturation of position or rate limited elements. These PIOs can in general be modeled as linearevents in which an identifiable nonlinear contribution may be treated separately.

Version: 3- 5 - Date: October 15 th , 1999GARTEUR/TP-120-01PIO Category IIIThe closed-loop pilot vehicle system has a highly non-linear behaviour, with no further peculiarcharacteristic. These PIOs rarely occur and are difficult to recognize. When they do occur, thePIOs in this category are the most severe.This classification allows to categorise PIO detection criteria according to their potentiality toreveal the PIO tendency for the various categories.PIO Criteria PrerequisitesThe National Research Council’s Committee on the Effects of Aircraft-Pilot Coupling on FlightSafety [29] defined three prerequisites for PIO prediction criteria. These prerequisites are:Validity“implies that a criterion embodies properties and characteristics that define the environment ofinterest and are associated with parameter spaces covering the vast majority of knowncases….The criterion must relate to closed-loop, high-gain, aggressive, urgent, and precisepilot-control behavior.”Selectivity“demands that the criterion differentiate sharply between ‘good’ and ‘bad’ systems…. The mostimportant selectivity feature is the capability of distinguishing configurations that may besusceptible to severe PIOs from those that are not.”Ready applicability“requires that the criterion be easily and conveniently applied. Expression of the criterion interms of readily available system parameters should be compact.”In the next section we present a survey of some PIO prediction criteria which have beenproposed by the research community.2.3 Prominent PIO prediction criteriaThe main objective for developing PIO criteria is to use them as design guidelines, in order toprevent PIO in newly designed aircraft. Another benefit of PIO criteria is from their applicationto flight test data, where analysis of data can highlight potential problems not emerged duringthe development phase. Finally, new PIO detection criteria have been proposed in the last year,which claim to be able to identify a PIO in flight, thus allowing a warning signal to be delivered

Version: 3- 6 - Date: October 15 th , 1999GARTEUR/TP-120-01to the pilot or a corrective action to be taken by an automatic flight control system; these criteriaare not addressed in this report, since they will be analysed in other reports of FM/AG12.The databases used for the generation and validation of PIO criteria have been established inseveral research programs which have involved both piloted simulations and flight tests withvariable stability aircraft (in-flight simulations). Most of the experiments were aimed at theanalysis of linear effects in the augmented aircraft dynamics, such as the Neal-Smith [8],LAHOS [9], HAVE PIO [10], HAVE CONTROL [11]. Incidents such as those of the YF22 andGripen have highlighted the strong impact that rate limits in the actuator or flight control systemdynamics can have on the generation of PIO. As a consequence in the last years someexperiments to study the influence of rate limiters on PIO were conducted, such as the HAVELIMITS [12], and a German/Swedish study on the FOSIM flight simulator [13].The review of criteria for the analysis and prediction of PIO will be presented according to thegiven classification of PIO, in Category I and II PIO. For Category III PIO no specific detectioncriterion has been proposed by researchers, therefore this Category will not be analysed in thisreport.PIO detection criteria, which have been proposed by the Handling Qualities and Flight Controlresearch community in the past years [29], are summarized as follows:Category I PIO• Bandwidth/Phase Delay• Gibson Gain/Phase Templates & Average Phase Rate• Pitch Attitude Dropback• Neal-Smith with Requirements Adjusted by Moscow Aviation Institute• Time Domain Neal-Smith• Smith-Geddes for Attitude Dominant PIOs• Hess Pilot Proprioceptive Feedback PSD Templates• Röger Criterion [56]Category II PIO• Bandwidth/Phase Delay with amplitude sensitivity• Open Loop Onset Point (OLOP)• Hess Pilot Proprioceptive Feedback PSD Templates• Time Domain Neal-Smith

Version: 3- 7 - Date: October 15 th , 1999GARTEUR/TP-120-012.3.1 Category I PIO CriteriaThe most prominent criteria [1, 5] presented in this report are:1. Neal-Smith Criterion [4].2. Bandwidth/Phase Delay, [16].3. Smith-Geddes, [17, 18, 19].4. Gibson Average Phase Rate/ω 180 + Gain/Phase Template, [20, 21, 22].5. Gibson Time Domain [4].Some of the above listed criteria are good indicators of the tendency of the aircraft to PIO, aswill be shown in the following.All these criteria, originally designed for ‘normal’ handling quality assessment, address stabilityaspects of closed-loop aircraft-pilot systems.The Neal-Smith and Smith-Geddes criteria define a pilot model and use it for the analysis of theclosed-loop system. The bandwidth/phase delay and phase rate criteria only use the open-loopaircraft with no direct model of the pilot. Implicit inclusion of the pilot is obtained by plottingsome parameters of the aircraft model into plots where boundaries of PIO proneness/safety havebeen derived from the analysis of the parameters of configurations whose PIO properties wereknown from piloted tests.Major research effort in the past has been on deriving criteria for the pitch axis, due to themaximum importance of stability and control in this axis for the safe operation of the aircraft.Therefore in the following we will present examples of application of the PIO criteria to thepitch axis. Applicability of the criteria to the roll axis is a matter of current research, as well astheir extension to transport aircraft.2.3.2 Category II PIO CriteriaThe most prominent criteria presented in this report are:1. Time Domain Neal-Smith Criterion (TDNS)2. Open Loop Onset Point Criterion (OLOP)The Category II PIO phenomena are characterised by quasilinear pilot-vehicle systemoscillations. In these events, the main nonlinear effect is actuator rate and/or position limiting.The rate limiting causes an additional time delay which can have a catastrophic influence on theaircraft handling qualities. The time delay due to rate limiting can easily be identified as the

Version: 3- 8 - Date: October 15 th , 1999GARTEUR/TP-120-01cause for the PIO, but its magnitude in a specific flight control system is difficult to predict. Theabove mentioned criteria may predict the tendency for Category II PIO. The first one is amethod based on the time domain, while the last one is based on frequency domain analysis.Like the Category I PIO criteria, also the above mentioned criteria address stability aspects ofthe closed-loop aircraft pilot system. For Category II PIO, the focus is now on the nonlinearbehaviour induced by rate limiters. The TDNS criterion may, however, also be applied to moregeneral nonlinearities. All Category II PIO criteria use a defined pilot model with differentcomplexity.2.3.3 Criterion Evaluation2.3.3.1 Category I PIOIn the following pages a quantitative evaluation of the different category I PIO predictioncriteria is presented, to complement the description of the methods. The evaluation is based onthe use of the following table, where the number of cases predicted to be PIO prone/free iscompared to the actual number of flight test PIO.Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by the NO PIO (B) (A)analysed criterionPIO (C) (D)Table 1: Evaluation of PIO prediction.From the numbers in the table it is possible to evaluate the effectiveness of the PIO criteria inpredicting PIO, according to different effectiveness measures. Two indices of effectivenessproposed in ref. [27] are the global success rate, i.e. the percentage of cases which are correctlypredicted to be PIO free or prone, and an index of conservatism, i.e. the percentage of casespredicted PIO prone which have actually undergone PIO in reality with respect to the totalnumber of predicted PIO prone cases:I1) Global success rate = (B+D)/(A+B+C+D)I2) Index of conservatism = D/(C+D)

Version: 3- 9 - Date: October 15 th , 1999GARTEUR/TP-120-01In ref. [3] it was proposed to add a further significant index of effectiveness, the percentage ofcases which are predicted by the criterion to be PIO prone, with respect to the total number offlight test PIO cases:I3) Safety index = D/(A+D)The aim is to maximise this measure, since failing to identify cases which can produce PIO canlead to very dangerous situations. It is interesting to note that index I2 highlights theconservatism of the method (the higher the index the less conservative the method is). Theindex indicates the probability that a configuration which has been identified as PIO prone bythis method will actually develop a PIO in flight. Index I3 highlights how safe the use of themethod is, indicating the probability that PIO prone configurations are identified by the method.The databases used for the assessment of the PIO criteria are the three landing databases:LAHOSLanding High Order System: Influence of highorder effects on landing and approach handlingqualities, 49 configurations, [9]HAVE PIO PIO investigations during landing, 18configurations, [10]HAVE CONTROL PIO investigations during landing, 12configurations, [11]Table 2: Landing databases for assessment of PIO criteria.2.3.3.2 Category II PIOEvaluation of the Category II PIO criteria, to complement the background of the method, willbe done by means of a qualitative description.

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Version: 3- 11 - Date: October 15 th , 1999GARTEUR/TP-120-013. Category I PIO Criteria Evaluation3.1 GeneralThis part provides a description of the evaluation of the most prominent Category I PIOsusceptibility criteria as defined in chapter 2. Each criterion will be highlighted by its theoreticalbackground followed by an evaluation of its effectiveness to predict PIO for three databases(paragraphs 3.2 to 3.6). The results of the criteria evaluation are finally discussed andsummarized (3.7).3.2 Neal-Smith Criterion3.2.1 Criterion BackgroundThe Neal-Smith criterion for closed-loop systems was originally developed for highlyaugmented fighter aircraft performing precision pitch attitude tracking tasks [14]. The criterionincludes a pilot model that contains a gain, lead/lag compensation and a time delay. Definitionof the pilot model is done using a certain performance standard or degree of aggressiveness ofthe pilot, which is characterised by the bandwidth frequency ω . The pilot model parametersmust be adjusted so that the closed-loop frequency response satisfies the followingrequirements:bw• the aircraft-pilot phase angle at the bandwidth frequency must be –90°.• the low frequency amplitude droop must be less than –3 dB.To apply the criterion, the following steps have to be followed:1. Specification of the bandwidth appropriate for the task:Category A flight phases:Category B and C flight phases:Category C landing phase:ωbw=3.5 rad/secωbw=1.5 rad/secω =2.5 rad/secbw2. Adjustment of the parameters of the pilot model to meet the performance standard definedby the bandwidth frequency ω using a fixed pilot model time delay of 0.3 seconds.bw

Version: 3- 12 - Date: October 15 th , 1999GARTEUR/TP-120-013. Determination of the pilot phase compensation and closed-loop resonance and comparisonto the proposed handling qualities boundaries (Figure 1).The criterion was extended to the approach and landing tasks using the databases in Table 2.This resulted into new handling quality boundaries that provide a good correlation between thepredictions of the criterion and the Cooper Harper ratings of about 90%.3.2.2 Criterion EvaluationFigure 1 presents the evaluation of the Neal-Smith criterion with the three databases presentedin Table 2 clarifying the correlation between the criterion parameters and PIO ratings obtainedwithin the experiments. The data indicates that the modified Neal-Smith boundaries are wellsuited to predict PIO during the landing task.Titel:Autor:ErstellDatum :dashed: original boundaries cat. Asolid: new boundaries for landingFigure 1: Application of the Neal-Smith criterion to the landing databases LAHOS, HAVE PIO,HAVE CONTROL.From the figure it can be seen that if the criterion parameters are located within the Level 1area, the PIO rating of a configuration is very likely to be less than 2.5.In Table 3 a summary of the results of the application of the Neal-Smith criterion to the threelanding databases is presented.

Version: 3- 13 - Date: October 15 th , 1999GARTEUR/TP-120-01Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by Neal- NO PIO 6 (B) 1 (A)SmithPIO 24 (C) 36 (D)Table 3: PIO prediction with the Neal-Smith criterion.From the numbers in Table 3 it is possible to evaluate the value of the three effectivenessindices introduced above:I1) Global success rate (BD/ABCD) = 42/67=81.6%I2) Index of conservatism (D/CD) = 36/60= 60%I3) Safety index (D/AD) = 36/37 = 97.3%3.3 Bandwidth / Phase Delay Criterion3.3.1 Criterion BackgroundThe bandwidth/phase delay criterion was developed using the Neal-Smith database for categoryA flight phases and the LAHOS database for category C flight phase [15, 27]. This criterion isbased on the analysis of the aircraft attitude transfer function. No pilot model is used. Twoparameters must be computed from the stick force to attitude transfer function:1. the bandwidth ωbw, defined as the frequency at which the phase margin is 45° or the gainmargin is 6 dB, whichever frequency is lower [15]; the physical interpretation of bandwidthis the highest frequency for which the aircraft will respond to pilot commands. A pilottrying to control the aircraft at a higher frequency would raise his gain so much thatinstability will occur.Φ(2ω180) − Φ(ω180) π2. the phase delay τ p , defined as τp=. This is a measure of2ω180180degthe slope of the phase angle at frequencies above the bandwidth. A higher phase delayimplies that a pilot trying to control the aircraft above the bandwidth will find a rapidlyreducing phase margin. Thus instability is again likely to occur. This parameter is the mostimportant with respect to PIO prediction. Cases whose τ p is above a prescribed value arepredicted PIO prone. The boundary value depends on the aircraft class and flight phase.The steps involved in the application of the criterion are:

Version: 3- 14 - Date: October 15 th , 1999GARTEUR/TP-120-011. Compute the two criterion parameters from the stick force to attitude transfer function:2. Plot the two parameters in the criterion plane and compare them to the boundary for PIO.A thorough analysis of the bandwidth/phase delay criterion is presented in ref. [27], where theinclusion of two secondary parameters, the flight path bandwidth and the dropback parameterwas suggested to cope with some cases not well predicted by the primary parameters. Inparticular an excessive dropback indicates that a configuration can be PIO prone even if itsphase delay satisfies the prescribed boundary.3.3.2 Criterion EvaluationFigure 2 presents the evaluation of the bandwidth/phase delay criterion with the three databasespresented in Table 2 clarifying the correlation between the criterion parameters and PIO ratingsobtained within the experiments.Figure 2: Application of the bandwidth/phase delay criterion to the landing databases LAHOS,HAVE PIO, HAVE CONTROL.In Table 4 a summary of the results of the application of the bandwidth criterion by Mitchelland Hoh to the three landing databases is presented.

Version: 3- 15 - Date: October 15 th , 1999GARTEUR/TP-120-01Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by NO PIO 28 (B) 10 (A)Bandwidth-Phase Delay PIO 4 (C) 34 (D)Table 4: PIO prediction with the Bandwidth-Phase Delay criterion.From the numbers in Table 4 it is possible to evaluate the value of the three effectivenessindices introduced above:I1) Global success rate (BD/ABCD) = 62/76 = 81.6%I2) Index of conservatism (D/CD) = 34/38 = 89.5%I3) Safety index (D/AD) = 34/44 = 77.3%These figures are reasonably good for a PIO prediction criterion.3.4 Smith-Geddes Criterion3.4.1 Criterion BackgroundThree types of PIO are considered by Ralph Smith [17]:Type IType IIType IIIInitiated by resonance of the closed-loop aircraft-pilot system during attitudetracking. PIO triggered by switching from attitude to normal acceleration control.Initiated by resonant open-loop dynamics, such as due to low damping.Initiated by resonance of the closed-loop aircraft-pilot system during attitudetracking, regardless of acceleration dynamics without any switching.A PIO criterion based on a simple procedure has been proposed by Ralph Smith for predictionof the attitude-dominant type III PIO. The criterion uses a very simple linear formula for theaircraft-pilot crossover frequency ω , based on the crossover frequency data of single axiscrtracking tasks [25]. The crossover criterion frequency ω cr depends on the average slope S ofthe aircraft amplitude response in the crossover region according to the formula:ωcr= 6 .0 + 0. 24SFor the application of the attitude dominant Smith-Geddes criterion to the pitch axis thefollowing steps have to be performed:

Version: 3- 16 - Date: October 15 th , 1999GARTEUR/TP-120-011. Determine the slope of pitch attitude to stick force amplitude response S over thefrequency range of 1 to 6 rad/sec;2. Calculate the crossover criterion frequency ω crand the criterion phase angle of pitchattitude to stick force frequency response Φcr;3. The aircraft is type III PIO sensitive if3.4.2 Criterion EvaluationΟΦcr< −160and PIO prone ifΦ < −180Οcr.Figure 3 presents the evaluation of the Smith-Geddes criterion with the three databasespresented in Table 2 clarifying the correlation between the criterion parameters and PIO ratingsobtained within the experiments.7HQ BoundariesSmith−Geddes65No PIOPIOData Bases:LAHOSHAVE PIOHAVE CONTROLmean PIOR435_1PIO Ratings:PIOR < 1.5PIOR 1.5 − 2.52PIOR 2.5 − 3.5PIOR 3.5 − 4.51PIOR 4.5 − 50−50−100−150phase angle [deg]−200−250Figure 3: Application of the Smith-Geddes criterion to the landing databases LAHOS, HAVEPIO, HAVE CONTROL.Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by NO PIO 24 (B) 7 (A)Smith-GeddesPIO 8 (C) 37 (D)Table 5: PIO prediction with the Smith-Geddes criterion.

Version: 3- 17 - Date: October 15 th , 1999GARTEUR/TP-120-01From the numbers in Table 5 it is possible to evaluate the value of the three effectivenessindices introduced above:I1) Global success rate (BD/ABCD) = 61/76 = 80.3%I2) Index of conservatism (D/CD) = 37/45 = 82.2%I3) Safety index (D/AD) = 37/44 = 84.1%From the figures above this criterion is reasonably good in predicting PIO. The Smith-Geddescriterion was validated in the pitch axis using the Neal-Smith database for up-and-away flight[18]. The criterion can be considered to be effective in detecting PIO prone configurations,since all configurations with PIOR higher than three are predicted to be PIO prone. But, a largescattering is found in the data, such as some very good configurations (PIOR 1.5) are predictedto be PIO prone. Thus it appears that the Smith-Geddes criterion parameter Φ cr alone is notsufficient as a PIO indicator. This is confirmed by the fact, that the important influence of thehigh frequency phase rolloff is not addressed by this criterion. Investigations based on theHAVE PIO database have shown that the crossover frequency ω cr is highly correlated with thefrequency of PIO cases that have occurred [5].3.5 Phase Rate Criterion and Gain Phase Template (Average Phase Rate)3.5.1 Criterion BackgroundThe phase rate criterion was introduced as a simple design criterion to predict PIO due to highorder effects in modern flight control systems [21]. The phase rate parameter is defined as thegradient of the phase angle with respect to the frequency in the neutral stability region, whichmeans 180° phase delay. Therefore, it is a direct measure of the high frequency phase roll-off.The phase rate parameter has a strong influence on the PIO generation process. A pilotperforming a critical task will generally raise his gain to increase the bandwidth available forthe task; this causes an increase of the phase delay proportional to the phase rate parameter.Now the pilot feels the aircraft is not responding fast enough to his commands and this inducesa further increase of the pilot gain. This situation will in the end drive the pilot into a PIO, sinceany increase in crossover frequency results in a severe loss of phase margin.Originally, the phase rate parameter was defined as the local slope of the phase angle around180° phase delay:PR180− ΔΦ(ω)=ΔωΦ(ω ) =−180 deg

Version: 3- 18 - Date: October 15 th , 1999GARTEUR/TP-120-01But more recently the average phase rate is used [23]. In that case the phase angle slope isdetermined within a wider frequency range: Δ ω = 2ω180−ω180, thus taking into account theeffect of a greater increase in bandwidth which is the situation we would look to when the PIOprocess generated by the pilot is initiated. It is obvious that the average phase rate parameter isdirectly proportional to the phase delay parameter τ of the bandwidth criterion (see above).Hence, in this context the local phase rate is considered in the following discussion. Minordifferences exist between the criterion boundaries for local and average phase rate. For theevaluation of the criterion, the phase rate parameter PR180in deg/Hz and the neutral stabilityfrequency f180in Hz have to be determined from the pitch attitude frequency response.p3.5.2 Criterion EvaluationFigure 4 presents the evaluation of the phase rate criterion with the three databases presented inTable 2, clarifying the correlation between the criterion parameters and PIO ratings obtainedwithin the experiments. The figure indicates that the PIO rating of a configuration is very likelyless than 2.5 if the criterion parameters are located within the Level 1 area.Figure 4: Application of the phase rate criterion to the landing databases LAHOS, HAVE PIO,HAVE CONTROL.The criterion has been modified in the sense that the boundaries of the regions between Levels 1to 3 have been changed (Figure 4).

Version: 3- 19 - Date: October 15 th , 1999GARTEUR/TP-120-01The first part of the Average Phase Rate (APR) criterion just described is very similar to theBandwidth/Phase Delay criterion. A second part of the criterion has been proposed in ref. [22,23], in order to include an evaluation of the effects of the actual gain of the aircraft dynamics.This part of the criterion plots the pitch attitude transfer function on a Nichols (gain-phase)diagram with a focus on the "PIO region", i.e. the area with phases ranging in [-200°, -180°]. Inthis area bounds are given both for the gain at -180° phase and for the slope of the transferfunction in the phase range [-200°, -180°]. Figure 5 presents the gain-phase template with theprescribed boundaries, including the evaluation of LAHOS configuration 5_1. It is evident that,contrary to the APR part of the criterion, the gain phase part of the criterion successfullypredicts this configuration to be PIO prone. It is worth noting again that the added value of thegain-phase part of the criterion is in the inclusion of a gain driven evaluation criterion.Figure 5: The gain phase template part of the Average Phase Rate criterion, with the evaluationof configuration LAHOS 5_1.In Table 6 a summary of the results of the application of the Average Phase Rate + gain-phasetemplate criterion by Gibson to the three landing databases is presented.

Version: 3- 20 - Date: October 15 th , 1999GARTEUR/TP-120-01Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by Phase NO PIO 12 (B) 1 (A)Rate + gain-phase PIO 20 (C) 43 (D)templateTable 6: PIO prediction with Average Phase Rate plus gain-phase template criterion.From the numbers in Table 6 it is possible to evaluate the value of the three effectivenessindices introduced above:I1) Global success rate (BD/ABCD) = 55/76 = 72.4%I2) Index of conservatism (D/CD) = 43/63 = 68.3%I3) Safety index (D/AD) = 43/44 = 97.7%It is evident that this criterion is highly effective with respect to the safety point of view (indexI3).A further criterion using a template in the gain phase plane has been proposed by Gibson in ref.[20], for handling qualities evaluation. The analysis is performed by plotting the pitch attitudetransfer function in the Nichols plane against boundaries derived from a database ofconfigurations with known handling qualities. A relative gain transfer function is plotted, wherethe attitude gain is scaled such that the 0 dB line is crossed at -120° phase. Areas of particularinterest for handling qualities behaviour are labelled in the plot. Other than a satisfactory area,also a PIO area and a "pitch bobble" one are indicated on the template. The satisfactory area iscentred on a K/s behaviour of the response, by assuming that this kind of response isparticularly well behaved.Since the criterion parameter is the transfer function itself and not some global parameters, thiscriterion is not suited to plot a whole set of configurations, because the spread of the graphs onthe plot could hide the peculiarities of the single one. On the other hand looking at the wholetransfer function can give more indications than just looking at some global parameters.In Table 7 a summary of the results of the application of the Gibson gain-phase templatecriterion to the three landing databases is presented.

Version: 3- 21 - Date: October 15 th , 1999GARTEUR/TP-120-01Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by NO PIO 13 (B) 1 (A)Gibson freq. domain PIO 19 (C) 43 (D)templateTable 7: PIO prediction with Gibson gain-phase template.From the numbers in Table 7 it is possible to evaluate the value of the three effectivenessindices introduced above:I1) Global success rate (BD/ABCD) = 56/76 = 73.7%I2) Index of conservatism (D/CD) = 43/62 = 69.4%I3) Safety index (D/AD) = 43/44 = 97.7%It is evident that this criterion is highly effective with respect to the safety point of view (indexI3).Figure 6 presents the evaluation of LAHOS configuration 5_1. The gain-phase templatecriterion correctly predicts the PIO proneness of this configuration. The transfer function exitsfrom the prescribed bounds both in the low frequencies region (above 0dB of relative gain),where attitude dropback is predicted, and in the higher frequency region (below 0dB of relativegain), where the PIO region is crossed.Figure 6: Evaluation of the LAHOS configuration 5_1 by the Gibson gain-phase template.

Version: 3- 22 - Date: October 15 th , 1999GARTEUR/TP-120-013.6 Gibson Time Domain Dropback3.6.1 Criterion BackgroundGibson obtained a vast experience in aircraft control and handling qualities on a wide variety ofaircraft, both conventional and fly-by-wire, over several decades working on many advancedprograms in the Great Britain. His work has lead to many design guidelines and criteria, oftenreferred to as the ‘Gibson Criteria’, some of which have been incorporated in specifications andrequirements of both European and American military and commercial regulations.Gibson’s design guidelines can be split into a time domain and a frequency domain approach.The background of the criteria in the pitch axis is the quality of the combined responsecharacteristics of both pitch attitude and flight path, and how well the combined behavior iswhen the pilot excites the aircraft through the stick. Many of Gibson’s ideas involve open-loopanalysis of the aircraft response. However, the connection with closed-loop control by the pilotis always present, in the end the requirements result in systems that behave like systems whosecharacteristics are known to be preferable for tight pilot control.Two important Gibson criteria that are defined in the time domain are addressed here: pitchattitude dropback and flight path angle time delay. Both criteria were developed over aconsiderable time span, and the latest definitions can be found in ref. [46].The dropback criterion in its original form was introduced in ref. [20] as one of a set of designguidelines for highly augmented fighter aircraft. A subsequent analysis for transport aircraft isreported in ref. [55]. Originally, it was defined in terms of limiting values of the pitch rateovershoot ratio and the ratio of attitude dropback to steady state pitch rate. The definition ofthese parameters is shown in Figure 7, which depicts typical responses of a rate command typeaircraft to a box-car stick input. The parameters involved in the criterion are the peak pitch rate,q m , and the attitude dropback, DB, normalized to the steady state pitch rate, q s , as defined inFigure 7. Dropback is computed as the difference between the pitch attitude at the time the stickis released and the steady state attitude after the stick is released. A positive value of thisdifference is referred to as dropback, while a negative value is named overshoot. Because of theuse of a steady state pitch rate, the criterion is applicable only to pitch rate command andconventional response types with constant velocity (short period approximations).Regions of typical pilot comments are defined in the criterion plane (DB/q s ,q m /q s ) (Figure 8).Here, criterion mappings are related to qualitative descriptions of the response such asabruptness, sluggishness and bobbling. Negative dropback is an indication of sluggishness,

Version: 3- 25 - Date: October 15 th , 1999GARTEUR/TP-120-01The second time domain Gibson criterion addressed here is the flight path time delay, t γ .Figure 10 shows how t γ is determined from the flight path response to a step input. This isachieved by taking the best fit tangent to the flight path angle response at around 4 seconds. Theflight path time delay is defined by the intersection of the tangent and the time axis. Basically, t γis a measure of the delay that is observed between stick input and a noticeable flight pathresponse. Ref. [46] suggests that for the landing approach task, t γ should not exceed 1.5 secondsgenerally and 1.0 seconds for precision flight path control.Figure 10: Generic step responses showing therelationship between pitch attitude and flight path angle.3.6.2 Criterion EvaluationFigure 11 presents the evaluation of the original Gibson time domain dropback criterion withthe three databases presented in Table 2, clarifying the correlation between the criterionparameters and PIO ratings obtained within the experiments.Figure 11: Application of Gibson Dropback criterion to thelanding databases LAHOS, HAVE PIO, HAVE CONTROL

Version: 3- 26 - Date: October 15 th , 1999GARTEUR/TP-120-01In Table 8 a summary of the results of the application of the Dropback criterion to the threelanding databases is presented. For this purpose, a criterion mapping in the satisfactory region(Figure 8) is considered to indicate no PIO susceptibility. A mapping outside this region isconsidered to indicate PIO susceptibility.Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by NO PIO 19 (B) 16 (A)Gibson dropbackPIO 13 (C) 28 (D)Table 8: PIO prediction with Gibson dropback criterion.From the numbers in Table 8 it is possible to evaluate the value of the three effectivenessindices introduced above:I1) Global success rate (BD/ABCD) = 47/76 = 61.8%I2) Index of conservatism (D/CD) = 28/41 = 68.3%I3) Safety index (D/AD) = 28/44 = 63.6%Figure 12 presents the evaluation of the modified dropback criterion as defined in ref. [27] usingthe same data, clarifying the correlation between the criterion parameters and PIO ratingsobtained within the experiments.Figure 12: Application of the modified dropback criterion tothe landing databases LAHOS, HAVE PIO, HAVE CONTROL

Version: 3- 27 - Date: October 15 th , 1999GARTEUR/TP-120-01In Table 9 a summary of the results of the application of the modified Dropback criterion byMitchell and Hoh to the three landing databases is presented.Number of casesFlight test PIO(Mean PIOR>2)NO PIO PIOPIO prediction by NO PIO 26 (B) 30 (A)modified dropback PIO 6 (C) 14 (D)Table 9: PIO prediction with modified dropback criterion.From the numbers in Table 9 it is possible to evaluate the value of the three effectivenessindices introduced above:I1) Global success rate (BD/ABCD) = 47/76 = 52.6%I2) Index of conservatism (D/CD) = 14/20 = 70%I3) Safety index (D/AD) = 14/44 = 31.8%This performance is not very good. However, the test should be used to complement thebandwidth criterion according to ref. [27]. The evaluation presented here is merely to show theeffect of the modified definition of dropback on the criterion results.3.7 Discussion of Category I PIO CriteriaThe criteria presented above are discussed with respect to their effectiveness in Category I PIOprediction and the gaps in the criteria and possible extensions. A general comment is thatCategory I PIO is adequately predicted by the available criteria. Really good configurations arerated good by all criteria and vice versa. But there are significant differences between thecriteria and open questions to be discussed.3.7.1 Criterion EffectivenessThe first four criteria presented in Table 10 are suitable for predicting category I PIO problems,but with different effectiveness with relation to the three indices. The Neal-Smith, Bandwidth-Phase Delay and Smith-Geddes criteria have the highest global success rate, about 81% successcases. The Bandwidth-Phase Delay is also the less conservative criterion, with 89.5%.With a lower global success rate, about 73% success cases, follow the Phase Rate and theGibson Frequency Domain template. These criteria, in addition to the Neal-Smith criteria, are

Version: 3- 28 - Date: October 15 th , 1999GARTEUR/TP-120-01Global success Conservatism SafetyPIO criterionrate[%][%][%]1. Neal-Smith 81.6 60 97.32. Bandwidth-Phase Delay 81.6 89.5 77.33. Smith-Geddes 80.3 82.2 84.14. Average Phase Rate + Gain-Phase 72.4 68.3 97.75. Gibson frequency domain template 73.7 69.4 97.76. Dropback 61.8 68.3 63.67. Modified dropback 52.6 70 31.8Table 10: Performance indices of Category I PIO prediction criteria.all characterised by a high value of the safety index, above 90%. The two Gibson criteria, thatare the only criteria that include a bound for the gain of the transfer function, share an almostcomplete success rate of 97.7% for the safety index. Therefore it is suggested to include thesecriteria in a PIO analysis. The Neal-Smith criterion appears to be the most effective PIOcriterion in the pitch axis since it predicts PIO and bobbling tendencies.Apart from PIO, also other phenomena such as bobbling tendencies, although not reallyconsidered as PIO, can be annoying for the pilot. In order to show the capability of detectingbobbling tendencies, one specific configuration, LAHOS 5_1, is considered more in detail, andcan be identified in the criterion graphics. This configuration was rated with PIOR 3 after tworuns with the following typical pilot comments:Tendency to bobble, low frequency PIO during landing.The Smith-Geddes criterion does not represent these pilot ratings, the configuration is predictedto be PIO free. The bandwidth/phase delay also does not predict the PIO potential of thisconfiguration, but the bobbling is indicated due to the excessive dropback, which has beensuggested as a complementary criterion. The Neal-Smith criterion also indicates the bobblingtendency. The phase rate criterion predicts the PIO potential since the gain-phase templaterequirement is not satisfied. The Gibson frequency domain criterion also predicts unsatisfactorybehaviour from both the low gain margin and the high attitude dropback.

Version: 3- 29 - Date: October 15 th , 1999GARTEUR/TP-120-013.7.2 Criterion Gaps and ExtensionsAll the presented criteria address only PIO caused by pilot control of aircraft attitude, while it isgenerally accepted that acceleration cues are also important for the pilot. The type I PIOcriterion by R. Smith assumes that the PIO is triggered by switching from attitude toacceleration control. The computations required for application of this criterion are much morecomplex than those of the very simple type III PIO criterion presented here.Some criteria address the gain of the attitude transfer function. This is done either directly as abound for the gain at the -180° frequency, as in the two Gibson criteria in the frequency domain.It is worth noting that these criteria are also those with the highest values of the safety index, allabove 90%.Regarding practical applications of the bandwidth/phase delay criterion to flight test dataproblems can arise, since the measured frequency response data might be doubtful in the highfrequency range of 2ω180[24]. The computation of the average magnitude slope for the Smith-Geddes criterion can be no longer meaningful for configurations with low damped modes, suchas flexible modes, within the frequency range of interest, 1 to 6 rad/sec. Indeed, these cases canshow a significant variation of the magnitude slope with respect to the average value, in therange of the low damped modes.

Version: 3- 30 - Date: October 15 th , 1999GARTEUR/TP-120-01

Version: 3- 31 - Date: October 15 th , 1999GARTEUR/TP-120-014. Category II PIO Criteria Evaluation4.1 GeneralThis part provides a description of the evaluation of the most prominent Category II PIOsusceptibility criteria as defined in chapter 2. The criteria will be highlighted by their theoreticalbackground followed by an evaluation of their effectiveness to predict PIO (4.2, 4.3).4.2 Time Domain Neal-Smith Criterion4.2.1 Criterion BackgroundThe Time Domain Neal-Smith Criterion was presented in “A Quantitative Criterion for Pilot-Induced Oscillations: Time Domain Neal-Smith Criterion” by Bailey and Bidlack [45]. Thecriterion is based on the Neal-Smith handling qualities criterion in the frequency domain (e.g.ref. [8]). For the extension to evaluation of PIO tendencies, the time domain was chosen withthe intention to allow evaluation of nonlinear effects in exactly the same way as a linear system.A valuable contribution to the further development of the Time Domain Neal-Smith Criterion isgiven in ref. [39], “An analysis of the Time-Domain Neal-Smith Criterion” by Foringer andLeggett.The criterion uses simulation with a pilot model performing a closed-loop tracking task.θ cθePilotmodelδpAircraft+controlsystemθFigure 13: The closed-loop tracking task.A 5° step in commanded pitch attitudeθ cis issued 0.25 s into the simulation. The pilot modelmust acquire the commanded pitch angle at exactly the time D s after the start of the simulation.The angle is considered as acquired at the first time when the pitch attitude error is less than1/40 of the step command, i.e. 0.125°. The commanded pitch angle must then be tracked asclosely as possible. The Root Mean Square of the pitch tracking error, rmsθ , is calculatedfrom the time of acquisition until 5 s from the simulation start.e

Version: 3- 32 - Date: October 15 th , 1999GARTEUR/TP-120-01The performance of the pilot model must be optimized using two constraints:1. The commanded angle must be acquired at D s after the simulation start. A low value of Dforces the pilot to acquire the target quickly.2. The rmsθemust be minimised. This requires a minimum of overshoot and oscillation.The parameter D, the acquisition time, is related to the bandwidth frequencyfrequency domain Neal-Smith criterion in the following way:ω BWin theωBW= −( 1/( D − 0.25)) *ln(1/ 40)For example, a smaller D corresponds to a higher bandwidth frequency and thus a moredemanding task.The optimisation is accomplished by changing two parameters in the pilot model, the gain K pand the pilot compensation parameter T L . Positive values of T L correspond to pilot leadcompensation, and the pilot model will look as follows:δpτs= e− ⋅ Kp( TLs+ 1)⋅θeNegative values of T L represent pilot lead-lag compensation, which will make the pilot modellook like this:δps + 1⋅θτsp= e− 1⋅ Kptp2s+ 1tewhere t = (1/ ω − T ) , t = (1/ ω ) and ω = −( 1/( D − 0.25)) ⋅ ln(1/ 40).p2 BW L2p1 t p 2 BWBWThe pilot time delay τ is always 300 ms.A mechanisation of the aircraft model, pilot model and tracking task can be made in Simulinkas in Figure 14. This Simulink system can then be called by a Matlab program that searches forthe optimum values of K p and T L for each value of D. The data that are then used in theevaluation are rmsθeand the pilot lead or lag compensation angle, ∠pc. The compensationangle is calculated from T L and ωBW.

Version: 3- 33 - Date: October 15 th , 1999GARTEUR/TP-120-01−1∠ = 57.3tan ( T ω ) for T L > 0.pcLBW−1−1∠pc= 57.3tan( tp1ωBW) − 57.3tan ( tp2ωBW) for T L < 0.Figure 14: A typical Simulink system for use of the criterion. 1An extra 50 ms has been added to the pilot time delay in Figure 14 to account for the time delayof FFA’s flight simulator FOSIM. The AIAA paper mentioned is ref. [39]. A number ofsimulations are needed to find the optimum for each value of D, and if many configurations ormany points in the envelope are to be checked, use of the criterion is very time consuming. Apowerful computer and a fast search/optimisation method are needed (FFA’s current set-up isnot the ultimate here.)An alternative strategy for finding the optimum values, also using Simulink, is described by vander Weerd in ref. [53].1 Note that the location of the elevator rate limiter and elevator position limiter should be switched.

Version: 3- 34 - Date: October 15 th , 1999GARTEUR/TP-120-017Typical Time History6Pitch Command and Attitude, deg5432100 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Time, sFigure 15: A typical time history, for the Neal-Smith configuration 1c.In the diagram above it can be seen that the 5° step command comes in at 0.25 s, and that ittakes a little while for the pilot model to react. The angle is acquired at 1.05 s, and rmsθ iscalculated to 0.8224° for the remaining time which means that the compensation is fairly closeto the optimum for this configuration (see Figure 16).eThe criterion can be used for both handling qualities and PIO susceptibility analysis. Forhandling qualities, Bailey and Bidlack used the Neal-Smith database to come up with thefollowing diagram:Figure 16: Level 1, 2 and 3 HQ limits from ref. [45].

Version: 3- 35 - Date: October 15 th , 1999GARTEUR/TP-120-01The points in the diagram correspond to D = 1.05 s. It should be noted that all theconfigurations in the diagram above are entirely linear. Configurations that have rate or positionlimits on their control surfaces may not be able to attain a D of 1.05 s regardless of pilot gainand compensation. How this affects the possibility to use the criterion for HQ evaluation willnot be entered into here.For the prediction of PIO susceptibility, the criterion is based on the assumption that when D islowered for a PIO prone configuration, there will be a sudden and dramatic change in howtracking performance deteriorates with respect to a change in D. At higher values of D, rmsθwill smoothly and gradually increase when D is lowered. Then at some smaller D, rmsθ willstart to increase much more quickly for a decrease in D. This would correspond to a large value2∂ rmsθ of the second derivative of rmsθewith respect to D, i.e. e2 . Bailey and Bidlack had∂Donly linear databases available, and from these deduced that a second derivative of 100 deg/s 2 atsome value of D ≥ 0.9 s would indicate that a configuration was PIO prone, while aconfiguration would be PIO resistant if the second derivative did not exceed 100 deg/s 2 .eeBailey and Bidlack also used their criterion on two hypothetical configurations with rate limitedelevators, one of them also degraded by a 200 ms time delay. It was clearly shown that the rate2∂ rmsθlimit created a handling qualities cliff, a large eat some D. In connection with this it2∂Dwas suggested that the value of D at which the handling qualities cliff occurs should beincorporated as a parameter for the criterion. As will be explained below, it has later been foundthat this can not be done in a simple way.In ref. [39], the criterion is compared to the HAVE LIMITS database, which was not availablewhen the criterion was first conceived. The HAVE LIMITS database is a result of an in-flightsimulation campaign at the USAF Test Pilot School in 1997, sponsored by the WrightLaboratory. Three basic configurations were flown in the Calspan NT-33. One solid Level 1aircraft (designated 2D), one Level 2 aircraft (2P) and an unstable aircraft augmented to Level 1(2DU). All were flown with elevator rate limits of 10, 20, 30, 40, 50, 60 and 157 deg/s. Cooper-Harper and PIO ratings from the performance of a discrete tracking task in the HUD were usedfor comparison with the criterion. It was shown that the metric proposed by Bailey and Bidlackwas not satisfactory for nonlinear configurations. By reducing D it was possible to make2∂ rmsθ e2 exceed 100 deg/s 2 even for the PIO resistant configurations. When a limit for D was∂Dadded to the criterion as proposed by Bailey and Bidlack, it was found that this limit had to beset differently for the three baseline configurations. Therefore a limit like this was not very

Version: 3- 36 - Date: October 15 th , 1999GARTEUR/TP-120-01useful. Finally, based on the HAVE LIMITS database, Foringer and Legget in ref. [39]proposed a number of constraints for rmsθeat the smallest feasible D vs. D, maximum2∂rmsθe∂ rmsθvs. D, maximum evs. D and rmsθ2eat the smallest feasible D vs. D for∂D∂Dcommanded pitch angles θc= 2.5° and θc= 10°. These are shown in the diagrams below. Theystate, however, that more data is needed to determine if the changes are sufficient to make thecriterion satisfactory for the prediction of PIO.45rms_theta_e at Smallest Feasible D vs. D40rms_theta_e at Smallest Feasible D353025201510PIONo PIO500.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4D, sFigure 17: Constraints for rmsθeat the smallest feasible D vs. D, θc= 5°.0Min 1st Derivative rms theta e vs. D−200No PIOMin 1st Derivative rms theta e−400−600−800−1000PIO−1200−14000.8 1 1.2 1.4 1.6 1.8 2D, sFigure 18: Constraints for minimum∂rmsθ e vs. D, θc= 5°.∂D

Version: 3- 37 - Date: October 15 th , 1999GARTEUR/TP-120-01Max 2nd Derivative rms_theta_e vs. D6 x 104 No PIO5Max 2nd Derivative rms_theta_e4321PIO00.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4D, sFigure 19: Constraints for maximumrms e∂D2∂ θ2 vs. D,cθ = 5°.45rms_theta_e at Smallest Feasible D vs. D, 2.5 Deg Command40rms_theta_e at Smallest Feasible D353025201510PIONo PIO500.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4D, sFigure 20: Constraints for rmsθeat the smallest feasible D vs. D, θc= 2.5°.

Version: 3- 38 - Date: October 15 th , 1999GARTEUR/TP-120-01120rms_theta_e at Smallest Feasible D vs. D, 10 Deg Command100rms_theta_e at Smallest Feasible D80604020PIONo PIO01 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3D, sFigure 21: Constraints for rmsθeat the smallest feasible D vs. D, θc= 10°.4.2.2 Criterion EvaluationAccording to available documentation, i.e. ref. [45] and ref. [39], the original criterion workswell for entirely linear configurations. However, it was the wish to handle nonlinearconfigurations that was the motive to develop the criterion. A step in the right direction hasbeen taken by the work presented in ref. [39]. More nonlinear data, preferably from in-flightsimulation or flight tests, is needed for the validation of the criterion.Below are a few results of the application of the criterion on nonlinear configurations tested inFFA’s FOSIM. Some data needed for the constraints suggested in ref. [39] have not beenplotted here, but the ones that are shown will be discussed.The nonlinearities of the configurations are rate limits of 565, 35 and 15 deg/s respectively, andelevator deflection limits of ± 25° approximately. The linear dynamics are Level 1 according toGibson’s Phase Rate Criterion and according to the Time Domain Neal-Smith Criterion.However, the elevator rate and deflection limits in combination with a not very effectiveelevator mean that a very high K p is needed to acquire the tracking command for the lowervalues of D, even for the highest rate limit. This way, rmsθ is pushed up.e

Version: 3- 39 - Date: October 15 th , 1999GARTEUR/TP-120-01Lead Comp, degPlot for PIO assessment, Neal−Smith in time domain806040200−201 1.2 1.4 1.6 1.8 2Required acquire time D, sRMS of tracking error, from D to 5 s15105L1 basic system, 565 deg/s rate limitPIONo PIO01 1.2 1.4 1.6 1.8 2Required acquire time D, s0150001st derivative of RMS curve−100−200−300−400No PIOPIO−5001 1.2 1.4 1.6 1.8 2Required acquire time D, s2nd derivative of RMS curve1000050000PIONo PIO−50001 1.2 1.4 1.6 1.8 2Required acquire time D, sFigure 22: 565 deg/s rate limit.2∂ rmsθ As can be seen, this system exceeds the e2 of 100 deg/s 2 . The main cause here is that K p∂Dis raised to give very abrupt pilot response, in order to compensate for the elevator deflectionlimits and ineffective elevator. This system was given Level 1 to Level 2 ratings in the2∂ rmsθ simulator and was given PIO ratings of 1 and 2. According to the limits for e2 and∂Drmsθefrom ref. [39], this configuration is not PIO prone.

Version: 3- 40 - Date: October 15 th , 1999GARTEUR/TP-120-01Lead Comp, degPlot for PIO assessment, Neal−Smith in time domain100806040200−201.4 1.6 1.8 2Required acquire time D, sRMS of tracking error, from D to 5 s15105L1 basic system, 35 deg/s rate limitPIONo PIO01 1.2 1.4 1.6 1.8 2Required acquire time D, s0150001st derivative of RMS curve−100−200−300−400No PIOPIO−5001 1.2 1.4 1.6 1.8 2Required acquire time D, s2nd derivative of RMS curve1000050000PIONo PIO−50001 1.2 1.4 1.6 1.8 2Required acquire time D, sFigure 23: 35 deg/s rate limit.2∂ rmsθ For this configuration e2 clearly exceeds 100 deg/s 2 . It was evaluated against the OLOP∂Dcriterion and was found not to be PIO prone. It was given PIO ratings of 1 to 3 in the simulator,2∂ rmsθ with a mean of a little more than 2. According to the limits for e2 and rmsθefrom ref.∂D[39] the configuration is not PIO prone.

Version: 3- 41 - Date: October 15 th , 1999GARTEUR/TP-120-01Plot for PIO assessment, Neal−Smith in time domain70Lead Comp, deg60504030201001.75 1.8 1.85 1.9 1.95 2Required acquire time D, sRMS of tracking error, from D to 5 s15105L1 basic system, 15 deg/s rate limitPIONo PIO01 1.2 1.4 1.6 1.8 2Required acquire time D, s0150001st derivative of RMS curve−100−200−300−400No PIOPIO−5001 1.2 1.4 1.6 1.8 2Required acquire time D, s2nd derivative of RMS curve100005000PIONo PIO01 1.2 1.4 1.6 1.8 2Required acquire time D, sFigure 24: 15 deg/s rate limit.2∂ rmsθThis configuration is PIO prone according to the limits for eand rmsθ2efrom ref.∂D[39], and was also pointed out as PIO prone by the OLOP criterion. It got a mean PIO rating of3.5 in the simulator, with all pilots giving it either a 3 or a 4.One weakness of the Time Domain Neal-Smith Criterion is that any small fluctuation in the22∂ rmsθ rmsθecurve results in a very shaky e∂ rmsθ2 curve. Another weakness is that the e2∂D∂Dcurve can be changed substantially by changing the step in D. In ref. [39] a step in D of 0.025 sis mentioned as appropriate. A fixed step in D should probably be defined as a part of thecriterion. The recommended step of 0.025 s has been used here.It is clear that the work by Foringer and Legget has improved the accuracy of the criterion fornonlinear configurations. Using their constraints has resulted in a fairly good agreement bothwith the OLOP criterion and the simulator results in the examples above. The state of thecriterion is probably still such that the use of it is not a way to insure oneself against PIO. Useof the criterion should be seen as a way to increase the knowledge about the configuration thatis being studied, and as a step on the way to improve and develop the criterion further. With thatin mind, it is strongly advised to use the criterion.

Version: 3- 42 - Date: October 15 th , 1999GARTEUR/TP-120-014.3 Open Loop Onset Point Criterion (OLOP)4.3.1 Criterion BackgroundThe OLOP criterion is a new Category II PIO prediction criterion being developed at DLR [13,47, 48, 49]. OLOP means open-loop onset point and it refers to the onset point of a rate limitingelement displayed in a Nichols chart. Currently, OLOP is the only validated Category II PIOcriterion.The development of the OLOP criterion is based on the describing function technique. Thedescribing function of an isolated rate limiting element has been developed using a Fourierseries for the fully developed rate limiting situation (pure triangle output function) [50]. Thedescribing function is dependent on frequency and input amplitude u rle, while the amplitudedependence is included in the onset frequency ω = R / u . The latter is defined as theonsetfrequency at which the rate limiter is activated for the first time.rleFor Category II PIO prediction the rate limiting effects in a closed control loop have to beanalysed. Therefore, a method has been developed to calculate the describing function of a ratelimited closed-loop system. The application of this method to a highly augmented aircraft with arate limiter in the feedback loop is presented in Figure 25. The closed-loop system describingfunction is characterised by a jump phenomenon after rate limiting onset, which can berecognized in a Nichols chart as a significant phase jump. In the presented example the phasejump leads to a dramatic loss of phase and amplitude margin indicating the potential for aninstability of the closed-loop system. This instability was verified by a nonlinear simulation inthe time domain [48].

Version: 3- 43 - Date: October 15 th , 1999GARTEUR/TP-120-01Titel:Autor:ErstellDatum :Figure 25: Jump phenomenon after rate limiting onset.In that Nichols chart, the open-loop onset point (OLOP) can be identified as the point where thephase jump starts. Within further studies, the OLOP parameters of a great number of aircraftsystems have been determined, indicating that the severity of the jump phenomena in thefrequency domain and the corresponding destabilization observed in the time domain are highlycorrelated with the OLOP location in a Nichols chart.The OLOP location in a Nichols chart is a measure of the magnitude of the additional timedelay due to rate limiting onset. It has been shown by the describing function analysis that theprimary effect caused by the activation of a rate limiter is a strong increase in phase delay and aslight decrease in amplitude [49]. If the OLOP is located at high amplitudes the additional phasedelay causes an increase in the closed-loop amplitude as demonstrated in the Nichols chart ofFigure 26. This increase in closed-loop amplitude provokes a stronger rate saturation and,therefore, further increasing phase delay. This mechanism can lead to a closed-loop instability.For an OLOP located clearly below 0 dB the increasing phase delay does not cause an increasein closed-loop amplitude, so the rate limiting effects are less dramatic.

Version: 3- 44 - Date: October 15 th , 1999GARTEUR/TP-120-01Tite l:Autor:ErstellDatum :Figure 26: Physical significance of the OLOP parameter.4.3.2 Criterion EvaluationFor the application of the OLOP criterion the use of the describing function technique is notrequired. A linear model of the aircraft including the flight control system, the positions of therelevant rate limiter, and the information about maximum stick deflections and maximum ratesmust be available. The procedure for the evaluation of the OLOP criterion is summarized below[49]:1. Definition of a simple (high) gain pilot model based on the linear aircraft dynamics.2. Calculation of the linear closed-loop frequency response from the stick input to the input ofthe rate limiter.3. Determination of the closed-loop onset frequency ωˆ onsetconsidering stick and controlsurface deflection limits.4. Calculation of the linear open-loop frequency response F OLOP(jw) and separation intoamplitude A0 ( ω)and phase angle Φ0( ω).5. OLOP = [ Φ ( ˆ ω ) , A ( ˆ ω ) ].0 onset0 onsetThe pilot model has to be adjusted to the linear aircraft model, which means that the pilot hasadapted himself to an aircraft behavior without rate saturation. It is assumed that in a timeperiod after rate limiting onset the pilot dynamics remain those adapted to the linear aircraftbehavior (post transition retention) [32]. The sudden change in closed-loop aircraft behaviormay lead to a strong misadaptation of the pilot which can cause an instability of the closed-loopaircraft-pilot system (= PIO).

Version: 3- 45 - Date: October 15 th , 1999GARTEUR/TP-120-01It is recommended that simple gain pilot models be used since the pilot usually reacts as asimple gain during a fully developed PIO (synchronous precognitive behaviour) [32]. The pilotgain Kpil( Φcr) has to be adjusted based upon the linear crossover phase angle of the open-loopaircraft-pilot system Φcr. It is recommended that a gain spectrum from Φ cr= −120deg(lowpilot gain) up to Φ cr= −160deg(high pilot gain) should be applied. This gain spectrumshould be used to assess the pilot model gain sensitivity of the aircraft.The linear open-loop frequency responseF OLOP(jw) is determined by cutting the system at therate limiter and treating the system with rate limiter removed: the output of the rate limiter isdefined as the input of the open-loop system uOLOP; the input of the rate limiter is defined asthe output of the open-loop system y OLOP. More details on the application of the OLOPcriterion are available in references [13, 48, 49]. The introduced procedure is applicable to thepitch and roll axis.For verification of the OLOP criterion boundary, an off-line analysis of flight experiments wasconducted. This analysis was performed using hand aircraft models based on the three lateraldatabases in Table 9 [49]. Available aircraft (flight control system) models of Category II PIOprone configurations of X-15, YF-16, YF-12 were also used. All these investigations confirmedthe OLOP criterion being well suited for Category II PIO prediction in roll, pitch and yaw axes.These evaluations have shown that for the configurations with forward path and feedback looprate limiters the OLOP location is highly correlated with PIO susceptibility. This indicates thatthe OLOP criterion is applicable to both forward path and feedback loop rate limiters using thesame PIO boundary.LATHOS(LATeral High Order System): In-flight simulationprogram on the NT-33 to study the effects of time delay andprefilter lag in the lateral flight control system [51].F-18 In-flight simulation program on the NT-33 to identifyhandling qualities problems of the F-18A prior to its firstflight [52].YF-16The famous first flight PIO incident of the YF-16 aircraftincluding the flight control system modifications [31].Table 11: Lateral databases for PIO research.

Version: 3- 46 - Date: October 15 th , 1999GARTEUR/TP-120-01After these promising results, the final validation of the OLOP criterion has been performed byevaluating new flight simulator experiments which where especially designed to get a wideOLOP spectrum. This resulted into the FOSIM data [49]. Figure 27 presents the final results ofthese investigations. In that figure, the differences between the PIO ratings of nonlinear andlinear runs (DPIOR = PIOR non-linear - PIOR linear ) were compared with the OLOP boundary. Thiswas possible since all configurations were tested with and without rate limiting.amplitude [dB]151050-5-10-15LATHOS F-18DPIOR < 1DPIOR 1 - 2DPIOR 2 - 3DPIOR 3 - 4YF-16-180 -160 -140 -120 -100phase [deg]Figure 27: Validation of the OLOP criterion using the FOSIM data.It has been shown that the activation of rate limiters in the feedback loop provides a very strongCategory II PIO potential, especially for high flight control system feedback loop gains. Furtherinvestigations have shown that Category I and II PIO are not correlated, which means the OLOPcriterion is not correlated with the linear PIO criteria [49].

Version: 3- 47 - Date: October 15 th , 1999GARTEUR/TP-120-015. ConclusionsIn this report prominent PIO susceptibility criteria for the prediction of Category I and II PIOshave been described and evaluated. The criteria that have been assessed are:Category I PIO1. Neal-Smith2. Bandwidth-Phase Delay.3. Smith-Geddes.4. Gibson Average Phase Rate/ω 180 + Gain/Phase Template.5. Gibson Time Domain DropbackCategory II PIO1. Time Domain Neal-Smith2. Open Loop Onset PointThe theoretical background has been described first, and an evaluation of the effectiveness ofthe presented criteria to predict PIO are presented. For Category I PIOs, the evaluation wasperformed by applying them to three PIO databases.The results of the Category I PIO criteria evaluation can be summarized as follows:Neal-SmithBandwidth/Phase DelayPhase Rate (local)Gibson• Most effective PIO indicator, bandwidth sensitivity important• Modified criterion available in the roll axis.• Effective PIO indicator (high frequency phase rolloff).• Problems when applied to flight test data.• Effective PIO indicator (high frequency phase rolloff).• Applicable to the roll axis as well.• Effective PIO indicator, especially for safety indexThe results of the Category II PIO criteria evaluation can be summarized as follows:Time Domain Neal-SmithOpen Loop Onset Point• Still under development• Currently used to increase knowledge about the system• Promising effective PIO indicatorThe prominent PIO criteria show a high degree of success in predicting PIOs for the casesevaluated. As such, all presented criteria may be applied for PIO prediction analysis.

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Version: 3- 49 - Date: October 15 th , 1999GARTEUR/TP-120-01Industrial ViewThe following presents the views of the industrial participants in the project consortium withregard to the contents of this report.DASAReport commentsThe report is written well understandable. From an industry point of view one comment thoughis to be made.Other than in a research environment sometimes detailed information about criteria is notreadily available in an industry environment. For this reason, it would increase readability of thedocument if more information about how to apply these criteria was included. At this point, thedocument does not give a guideline for the application of these criteria. This applies also to thedescription of the physical background, which would help to understand the criterion and itsresults better.A deeper description would increase the value in a sense that all relevant information is readilyavailable. Possibly this comment applies better for the envisaged handbook.Experience on PIO criteriaIn present projects, DASA Military Aircraft applies the following PIO criteria:• Phase rate criterion (average phase rate)• Absolute amplitude criterion• Pitch attitude frequency response criterion• Gibson dropback criterion• Neal-Smith criterion• Rögers phase and gain margin criterionThe last three criteria in the list above are used to backup the results of the first four criteria.

Version: 3- 50 - Date: October 15 th , 1999GARTEUR/TP-120-01Experience with real PIO/handling deficiencies (flight test) exists only with regard to the Neal-Smith and the Röger Criterium. In present projects, the use of the above mentioned criteriaduring design allowed to exclude PIO tendencies - at least none have been found up to now.Rögers phase and gain margin criterionThe Röger criterion was successfully applied to a tracking problem of a former combat aircraftand allowed to solve the problem.It was further on applied for approach and landing of an experimental aircraft, where pilotscomplained about handling especially during flare, when the normal control laws with attitudefeedback on were used. When in two flights the attitude feedback was switched off excellentpilot ratings were obtained.When other criteria as the Neal-Smith, the Smith-Geddes and the bandwidth criterion ofMitchell/Hoh were applied they failed to identify the difference in behaviour with the twocontrol laws. Whereas the Röger criterion clearly delivered different ratings for the two casesand allowed to successfully design the command filter, which improved landing control to asatisfactory status -overcontrol tendencies were gone.Manned simulation study at DASA/DornierSome years ago, a manned simulation study on handling qualities criteria for modern combataircraft at DASA/Dornier with five testpilots showed that the Röger criterion can identify PIOtendencies. This study also showed that the Neal-Smith criterion and the phase rate criterion areindeed able to identify PIO tendencies. Furthermore, Gibson’s time domain criteria were ingood agreement with pilot ratings and comments.SAABReport commentsThe report should be more self-contained. This means that the different criteria should beexplained in such detail that an engineer should be able to implement the method withoutlooking for details in other references. A typical example illustrating the method is also of greatvalue.Experience on PIO criteriaCurrently, Saab is using 3 criteria for testing a new FCS edition for the Saab Gripen.

Version: 3- 51 - Date: October 15 th , 1999GARTEUR/TP-120-01• Neal-Smith• Smith-Geddes• Gibson (phase rate, amplitude margin, and dropback)When using the Neal-Smith criterion Saab deviates slightly from the description in Section3.2.1. For Category C we use 3.0 rad/s bandwidth, and the pilot time delay is 0.25s. We use theold boundaries.For Gibson's phase rate we use the limits dPHI/df > -70 deg/Hz, frequency at -180 deg shouldbe >= 1 Hz. For the Gain-Phase template we use the gain margin 16 dB.The criteria are evaluated for a light aircraft (20% fuel) at low altitude, for a few selectedspeeds, and for both the single-seater and two-seater versions.In general, the different PIO criteria may give contradicting results for a flight condition. Forexample: in one particular flight condition the Neal-Smith criterion may give Level 1, while theGibson phase rate is in Level 2. And vice versa in another flight condition.For the latest FCS editions, the PIO criteria have mainly been checked for differences betweenthe new edition and the previous flying edition. Since the earlier editions have been flight testedand were found to be free from PIO proneness, you can rely on similarity between the editions.If PIO criteria results for a new edition are essentially the same as for the previous flyingedition, they are accepted. But if results abruptly changes into Level 3 or are much worse thanbefore, this is taken as an alarm bell.One other important check is the Klonk-simulations, which is a test on abrupt manoeuveringwhere rate limiting takes place. If the simulations stay inside a defined angle-of-attackenvelope, we conclude that even if the pilot gets into a PIO this will not cause loss of control ofthe aircraft.Looking back into the early design stages of the Gripen FCS, a number of PIO criteria havealways been used in order to check that the basic design is reasonably good with respect to PIO.After the Stockholm Water Festival crash (August 1993) around 10 PIO criteria were used inorder to evaluate the PIO proneness of the unfortunate FCS edition. The results were OK for allthe used criteria, since they were all linear criteria. This indicates the need for nonlinear PIOcriteria in order to catch PIO caused by rate limiting.

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Version: 3- 53 - Date: October 15 th , 1999GARTEUR/TP-120-01References[1] GARTEUR Exploratory Group FM/EG18, Terms of Reference for GARTEUR FlightMechanics Action Group FM/AG12 on Pilot-in-the-Loop Oscillations Analysis and TestTechniques for their Prevention, GARTEUR FM/EG18-M-1, February 1999.[2] GARTEUR Action Group FM/AG12, Evaluation of Prominent PIO SusceptibilityCriteria, GARTEUR/TP-120-1, July 1999.[3] Scala S., Verde L., Amato F., Iervolino R., Analysis and Synthesis of PIO PredictionCriteria, CIRA-TR-SIV-99-075, April 1999.[4] AGARD Working Group 23, Flight Control Design- Best Practices, September 1999.[5] McRuer, D., et al.: AVIATION SAFETY AND PILOT CONTROL. Understanding andPreventing Unfavorable Pilot-Vehicle Interactions, ASEB National Research Council,National Academy Press, Washington D.C., 1997.[6] N.N.: Military Specification: Flying Qualities of Piloted Airplanes, MIL-F-8785 C,Wright Paterson AFB, 1980.[7] N.N. : Military Standard: Flying Qualities of Piloted Aircraft, MIL-STD-1797 A, WrightPaterson AFB, 1990.[8] Neal, T.P., Smith, R.E.: An In-Flight Investigation to Develop Control System DesignCriteria for Fighter Airplanes, AFFDL-TR-70-74, Vol. I & II, Wright Paterson AFB,1970.[9] Smith, R.E.: Effects of Control System Dynamics on Fighter Approach and LandingLongitudinal Flying Qualities, AFFDL-TR-87-122, Buffalo, 1978.[10] Bjorkman, E.A.: Flight Test Evaluation of Techniques to Predict Longitudinal Pilot-Induced Oscillations, AFIT-GAE-AA-86J-1, Wright Paterson AFB, 1986.[11] Lindsey, S.W.: Prediction of Longitudinal Pilot Induced Oscillations Using the OptimalControl Model, AFIT-GAE-ENY/90M-1, Wright Paterson AFB, 1989.[12] Kish, B, et al: A Limited Flight Test Investigation of Pilot-Induced Oscillation Due toRate Limiting, AIAA New Orleans, 1997.[13] Duda, H., Duus, G., New Handling Qualities Database on PIO due to Rate Saturation,DLR-FB 97/53, Braunschweig, 1997[14] Neal, T.P., Smith, R.E.: A Flying Qualities Criterion for the Design of Fighter FlightControl Systems, Journal of Aircraft, Volume 8, Number 10, Pages 803-809, 1971.[15] Hoh, R.E., Hodgkinson, J.: Bandwidth - A Criterion for Highly Augmented Airplanes,AGARD-CP-333, Paper 9, Fort Worth, April 1982.[16] Mitchell, D.G., Hoh, R.H., Aponso, B.L., Klyde, D.H.: The Measurement and Predictionof Pilot-in-the-Loop Oscillations, AIAA-Paper 94-3670, Scottsdale, 1994.[17] Smith, R.H.: A Theory for Longitudinal Short-Period Pilot-Induced Oscillations,AFFDL-TR-77-57, Dayton, 1977.

Version: 3- 54 - Date: October 15 th , 1999GARTEUR/TP-120-01[18] Smith, R.H., Geddes, N.D.: Handling Quality Requirements for Advanced AircraftDesign: Longitudinal Mode, AFFDL-TR-78-154, Dayton, 1978.[19] Smith, R.H.: The Smith-Geddes Handling Qualities Criteria, 1994 SAE AerospaceAtlantic Conference, Dayton, 1994.[20] Gibson, J.C.: Piloted Handling Qualities Design Criteria for High Order Flight ControlSystems, AGARD-CP-333, Paper 4, Fort Worth, April 1982.[21] Gibson, J.C.: Evaluation of Alternate Criteria for FBW Handling Qualities, AGARD-CP-508, Paper9, Quebec, October 1990.[22] Gibson, J.C.: The Prevention of PIO by Design, AGARD FMP Symposium on ActiveControl Technology, Turin, 1994.[23] Gibson, J.C.: Looking for a simple PIO model, AGARD-AR-335, Paper 5, Turin, 1994.[24] Koehler, R.: X-31A Flying Qualities Results using Frequency Response Flight TestEvaluation Techniques, DLR-IB 111-96/11, Braunschweig, 1996.[25] McRuer, D., Graham, D., Krendel, E., Reisener, W.: Human Pilot Dynamics inCompensatory Systems, AFFDL-TR 65-15, Wright Paterson AFB, 1965.[26] Duda, H., Minutes on a Workshop on Pilot-in-the-Loop Oscillations held at DLR,Braunschweig on June 12-13th, 1997, DLR IB 111-97/25, Braunschweig, 1997.[27] Mitchell, D., Hoh, R., Aponso, B., Klyde, D., Proposed Incorporation of Mission-Oriented Flying Qualities into MIL STD-1797A. WL-TR-95-3049, Wright Laboratory,Wright-Patterson Air Force Base, Ohio, 1994.[28] Klyde, D.H., McRuer, D.T., Myers, T.T.: Unified Pilot-Induced Oscillation Theory,Volume I: PIO Analysis with Linear and Nonlinear Effective Vehicle Characteristics,including Rate Limiting, WL-TR-96-3028, Wright Laboratory, Wright-Patterson AirForce Base, Ohio, 1995.[29] Mc.Ruer, D., Klyde, D.H.: Pilot-Induced Oscillations: Theory, Analysis & FlightResearch, An AIAA Professional Development One-Day Tutorial Short Course, AIAA,August 9, 1998.[30] Duda, H., Duus, G.: New Flight Simulator Experiments on Pilot-in-the-Loop Oscillationsdue to Rate Saturation, DLR-IB 111-97/35.[31] Smith, J.W.: Analysis of a Lateral Pilot-Induced Oscillation Experienced on the firstFlight of the YF-16 Aircraft, NASA TM 72867, 1979.[32] McRuer, D.: Pilot-Induced Oscillations and Human Dynamic Behaviour, NASA CR4683, Hawthorne 1995.[33] Powers, B.G.: An adaptive Stick-Gain to Reduce PIO Tendencies, Journal of Guidance,Control and Dynamics Vol.5 No.2, 1982.[34] Bailey, R.E., Bidlack, T.J.: Unified Pilot-Induced Oscillation Theory, Volume IV: TimeDomain Neal-Smith Criterion, WL-TR-96-3031, Dec 1995.

Version: 3- 55 - Date: October 15 th , 1999GARTEUR/TP-120-01[35] Mitchell, D.G., Hoh, R.H.: Development of a Unified Method to Predict Tendencies forPilot-Induced Oscillations, WL-TR-95-3049, June 1995.[36] Anderson, M.R., Page, A.B.: Unified Pilot-Induced Oscillation Theory Volume III: PIOAnalysis Using Multivariable Methods, WL-TR-96-3030, Dec 1995.[37] Mitchell, D.G., Klyde, D.H.: A Critical Examination of PIO Prediction Criteria, AIAAAtmospheric Flight Mechanics Conference and Exhibit, Boston, MA, Aug 1998.[38] Mitchell, D.G., Kish, B.A., Seo, J.S., Mosle III, W.B.: A Flight Investigation of Pilot-Induced Oscillation due to Rate Limiting, IEEE 1998 Aerospace ConferenceProceedings, Paper 270, Snowmass, CO, March 1998.[39] Foringer, L.A., Leggett, D.B.: An Analysis of the Time-Domain Neal-Smith Criterion,AIAA Atmospheric Flight Mechanics Conference and Exhibit, Boston, MA, Aug 1998.[40] Klyde, H.K., Mitchell, D.G.: Extraction of Pilot-Vehicle Characteristics In the Presenceof Rate Limiting, AIAA Paper 99-1069, 37t h AIAA Aerospace Sciences Meeting andExhibit, Reno, NV, Jan 11-14, 1999.[41] McRuer, D., Klyde, D.H., Myers, T.T.: PIO Analysis with Actuator Rate Limiting,AIAA-96-3432-CP, Hawthorne, CA.[42] Wilmes, T., Duda, H.: Investigation of Electronic Filters to Prevent Aircraft-PilotCoupling, DLR-IB 111-98/39, Braunschweig, Nov 1998.[43] Ezelius, K.: Pilot Induced Oscillations, FFA TN 1995-25, The Aeronautical ResearchInstitute of Sweden 1995.[44] Hess, R.A., Stout, P.W.: Assessing Aircraft Susceptibility to Non-Linear Aircraft-PilotCoupling/Pilot-Induced Oscillations, Journal of Guidance, Control and Dynamics, Vol.21No.6, Nov-Dec 1998.[45] Bailey, R.E., Bidlack, T.J.: A Quantitative Criterion for Pilot-Induced Oscillations: TimeDomain Neal-Smith Criterion, AIAA-96-3434-CP.[46] Gibson, J.C.: Development of a methodology for excellence in handling qualities designfor fly-by-wire aircraft, Ph.D. Thesis, Delft University of Technology, Delft UniversityPress, 1999.[47] Duda, H.: Frequency Domain Analysis of Rate Limiting Elements in Flight ControlSystems, DLR-FB-94-16, Braunschweig, 1994 (in German).[48] Duda, H.: Effects of Rate Limiting Elements in Flight Control Systems-A new PIOCriterion, AIAA-Paper-95-3340, August 1995.[49] Duda, H.: Flying Qualities Criteria Considering Rate Limiting, DLR-FB 97-15,Braunschweig, 1997 (in German).[50] Duda, H.: Prediction of Pilot-in-the-Loop Oscillations due to Rate Saturation, Journal ofGuidance, Navigation and Control, Vol.20, No.3, May-June 1997.[51] Monagan, S.J., Smith, R.E., Randall, E.B.: Lateral Flying Qualities of Highly AugmentedFighter Aircraft, AFWAL-TR-81-3171, Vol. I&II, 1982.

Version: 3- 56 - Date: October 15 th , 1999GARTEUR/TP-120-01[52] Smith, R.E.: Evaluation of F-18 Approach and Landing Flying Qualities Using an In-Flight Simulator, Calspan Report No. 6241-F-1, 1979.[53] Van der Weerd, R.: Pilot-Induced Oscillation Suppression Methods and Their Effects onLarge Transport Aircraft Handling Qualities, Master’s Thesis, Delft University ofTechnology, 1999.[54] Field, E.J.: Flying Qualities of Transport Aircraft: Precognitive or Compensatory?, Ph.D.Thesis, Cranfield University, College of Aeronautics, June 1995.[55] Mooij, H.A.: Low Speed Longitudinal Flying Qualities of Modern Transport Aircraft,AGARD-LS-157, May 1988.[56] Röger, W.; Beh, H.: Gain and Phase Margin as a Basis of Longitudinal Flying QualitiesEvaluation, AGARD-CP-333.

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