FLYING QUALITIES OF PILOTED AIRCRAFT - CAFE Foundation

MIL–STD–1797AAPPENDIX A4.1.7.2 Aircraft Special Failure States. Certain components, systems, or combinations thereof may haveextremely remote probabilities of failure during a given flight. The failures may, in turn, be very difficult topredict with any degree of accuracy. Special Failure States of this type need not be considered in complyingwith the requirements of this standard, if justification for considering them as Special Failure States issubmitted by the contractor and approved by the procuring activity.REQUIREMENT RATIONALE (4.1.7.1 – 4.1.7.2)Perfection is not a realistic expectation. This requirement is to determine the practical limits in each case.The related MIL–F–8785C paragraph is 3.1.6.2.1.REQUIREMENT GUIDANCEIn most cases, a considerable amount of engineering judgment will influence the procuring activity’s decisionto allow or disallow a proposed Aircraft Special Failure State. Probabilities that are extremely remote areexceptionally difficult to predict accurately. Judgments will weigh consequences against feasibility ofimprovement or alternatives, and against projected ability to keep high standards throughout design,qualification, production, use, and maintenance. Meeting other pertinent requirements––MIL–F–87242,AFGS–87221, etc.––should be considered, as should experience with similar items. Generally, SpecialFailure States should be brought to the attention of those concerned with flight safety.Note that the required approval of Aircraft Special Failure States, in conjunction with Level 3 floors and certainrequirements that must be met regardless of component or equipment status, can be used as desired: forexample to require a level of stability for the basic airframe, limit use of stick pushers to alleviate pitch–up,disallow rudder–pedal shakers for stall warning, rule out fly–by–wire control systems, require an auxiliarypower source, force consideration of vulnerability, etc. The procuring activity should state thoseconsiderations they wish to impose, as completely as they can, at the outset; but it is evident that manydecisions must be made subjectively and many will be influenced by the specific design.Several categories of Special Failure States can be distinguished. Certain items might be approved more orless categorically:* Control–stick fracture.* Basic airframe or control–surface structural failure.* Dual mechanical failures in general.Regardless of the degree of redundancy, there remains a finite probability that all redundant paths will fail. Apoint of diminishing returns will be reached, beyond which the gains of additional channels are not worth theassociated penalties:* Complete failure of hydraulic or electrical, etc., systems.* Complete or critical partial failure of stability augmentation that has been accepted asnecessary to meet Level 3.Some items might be excepted, if special requirements are met. For example, some limited control shouldremain after failure of all engines, provided by accumulators or an auxiliary power source as appropriate.In the last analysis the procuring activity is responsible for approving design tradeoffs that bear upon safety.Rather than inhibiting imaginative design, then, this paragraph should be construed as forcing examination offailure possibilities as they affect flight safety through deterioration of flying qualities. The present state of the112