World Air Ops | NAT | Doc 007 MNPS Guidance

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Guidance concerning Air Navigation in and above the NAT MNPSA CHAPTER 88.5 SPECIAL IN-FLIGHT PROCEDURESStrategic Lateral Offset Procedures (SLOP)8.5.1 ATC clearances are designed to ensure that separation standards are continually maintainedfor all traffic. However, the chain of clearance definition, delivery and execution involves a series oftechnical system processes and human actions. Errors are very rare but they do occur. Neither pilots norcontrollers are infallible. Gross Navigation Errors (usually involving whole latitude degree mistakes in routewaypoints) are made, and aircraft are sometimes flown at flight levels other than those expected by thecontroller. When such errors are made, ironically, the extreme accuracies of modern navigation and heightkeeping systems themselves increase the risk of an actual collision. Within an SSR environment thecontroller is alerted to such errors and can, using VHF voice communications, intervene in a timely fashion.This is not the case in Oceanic airspace, such as the North Atlantic, where the controller’s awareness of thedisposition of a significant proportion of the traffic is reliant largely upon pilot position reports throughcommunication links utilising HF or SATCOM Voice via third party radio operators. And furthermore, evenamong that proportion of traffic utilising datalink for automated position reporting, and perhaps ATScommunications, navigation errors continue to occur. Consequently, it has been determined that allowingaircraft conducting oceanic flight to fly self-selected lateral offsets will provide an additional safety marginand mitigate the risk of traffic conflict when non-normal events such as aircraft navigation errors, heightdeviation errors and turbulence induced altitude-keeping errors do occur. Collision risk is significantlyreduced by application of these offsets. These procedures are known as “Strategic Lateral Offset Procedures(SLOP)”.8.5.2 This procedure provides for offsets within the following guidelines:a) along a route or track there will be three positions that an aircraft may fly: centreline or oneor two miles right;b) offsets will not exceed 2 NM right of centreline; andc) offsets left of centreline must not be made.8.5.3 Distributing aircraft laterally and equally across the three available positions adds anadditional safety margin and reduces collision risk. This is now a standard operating procedure for theentire NAT Region and pilots are required to adopt this procedure as is appropriate. In this connection, itshould be noted that:a) Aircraft without automatic offset programming capability must fly the centreline.b) It is recommended that pilots of aircraft capable of programming automatic offsets elect tofly an offset one or two nautical miles to the right of the centre line in order to obtain lateralspacing from nearby aircraft (ie those immediately above and/or below). Pilots should usewhatever means are available (e.g. TCAS, communications, visual acquisition, GPWS) todetermine the best flight path to fly.c) An aircraft overtaking another aircraft should offset within the confines of this procedure, ifcapable, so as to create the least amount of wake turbulence for the aircraft being overtaken.d) For wake turbulence purposes, pilots should fly one of the three positions shown above.Pilots should not offset to the left of centreline nor offset more than 2 NM right of centreline.Pilots may contact other aircraft on the air-to-air channel, 123.45 MHz, as necessary; to coordinatethe best wake turbulence mutual offset option. (Note. It is recognised that the pilotwill use his/her judgement to determine the action most appropriate to any given situationand that the pilot has the final authority and responsibility for the safe operations of theNAT Doc 007 56 Edition 2010
Guidance concerning Air Navigation in and above the NAT MNPSA CHAPTER 8aeroplane. See also Chapter 12, paragraph 12.5.) As indicated below, contact with ATC isnot required.e) Pilots may apply an offset outbound at the oceanic entry point and must return to centrelineprior to the oceanic exit point.f) Aircraft transiting ATS Surveillance-controlled airspace mid-ocean should remain on theiralready established offset positions.g) There is no ATC clearance required for this procedure and it is not necessary that ATC beadvised.h) Voice Position reports should be based on the waypoints of the current ATC clearance andnot the offset positions.8.5.4 SLOP has been implemented as a standard operating procedure in the NAT Region since2004. An indication of the proportion of pilots adopting a SLOP offset here is obtained through study ofADS-C position reports. Such study has shown that during 2009 most aircraft flying an offset in the NATRegion selected the 1NM Right option. Only a few chose the 2NM Right option. As indicated above,system safety would be further enhanced if offsetting aircraft were more evenly distributed between thecentre-line, 1 and 2 NM Right options. As proposed in Paragraph 8.5.3 b) above, Pilots should attempt todetermine the offsets (if any) being flown by aircraft immediately ahead on the same track one flight levelabove and one flight level below. And then select an offset which differs from those. If this is not possibleor practical, then pilots should randomly choose one of the three flight path options.8.5.5 The previously mentioned study of ADS-C position reports has also shown that some aircraftcontinue to adopt an offset LEFT of cleared track centre-line. The standard SLOP procedures are designedto provide safety enhancements for both uni-directional and bi-directional flows. On bi-directional routes aLEFT offset will INCREASE collision risk rather than decrease it. There are areas in the NAT Regionwhere bi-directional traffic flows are routinely used. And there are times when opposite direction traffic maybe encountered in any part of the Region. Pilots must therefore recognise that LEFT offsets from thecleared track centre-line must not be adopted. After the introduction of RVSM and before the adoptionof SLOP, a NAT offsetting procedure was promulgated for wake-turbulence avoidance. This procedureallowed both right and left offsets to be flown. The procedure was developed primarily with a view to theunique traffic flows of the NAT OTS, where uni-directional traffic occupied every flight level from FL310 toFL390. That procedure is no longer in place. The avoidance of wake turbulence (even in the OTS) can beaccomplished effectively within the confines of the SLOP procedures, as specified in paragraph 8.5.3 d)above. Pilots should communicate with the other aircraft involved to co-ordinate a pair of mutual offsetsfrom within the allowed three options, in order to mitigate any wake-turbulence issue.Monitoring during Distractions from Routine8.5.6 Training and drills should ensure that minor emergencies or interruptions to normal routineare not allowed to distract the crew to the extent that the navigation system is mishandled.8.5.7 If during flight the autopilot is disconnected (e.g. because of turbulence), care must be takenwhen the navigation steering is re-engaged to ensure that the correct procedure is followed. If the system inuse sets specific limits on automatic capture, the across-track indications should be monitored to ensureproper recapture of the programmed flight path/profile.8.5.8 Where crews have set low angles of bank, perhaps 10° or less, say for passenger comfortconsiderations, it is essential to be particularly alert to possible imperceptible departures from cleared track.NAT Doc 007 57 Edition 2010
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