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Guidance concerning Air Navigation in and above the NAT MNPSA


aeroplane. See also Chapter 12, paragraph 12.5.) As indicated below, contact with ATC is not required.

  • e)

    Pilots may apply an offset outbound at the oceanic entry point and must return to centreline prior to the oceanic exit point.

  • f)

    Aircraft transiting ATS Surveillance-controlled airspace mid-ocean should remain on their already established offset positions.

  • g)

    There is no ATC clearance required for this procedure and it is not necessary that ATC be advised.

  • h)

    Voice Position reports should be based on the waypoints of the current ATC clearance and not the offset positions.

      • 8.5.4

        SLOP has been implemented as a standard operating procedure in the NAT Region since

  • 2004.

    An indication of the proportion of pilots adopting a SLOP offset here is obtained through study of

ADS-C position reports. Such study has shown that during 2009 most aircraft flying an offset in the NAT Region 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 the centre-line, 1 and 2 NM Right options. As proposed in Paragraph 8.5.3 b) above, Pilots should attempt to determine the offsets (if any) being flown by aircraft immediately ahead on the same track one flight level above and one flight level below. And then select an offset which differs from those. If this is not possible or practical, then pilots should randomly choose one of the three flight path options.


The previously mentioned study of ADS-C position reports has also shown that some aircraft

continue to adopt an offset LEFT of cleared track centre-line. The standard SLOP procedures are designed to provide safety enhancements for both uni-directional and bi-directional flows. On bi-directional routes a LEFT offset will INCREASE collision risk rather than decrease it. There are areas in the NAT Region where bi-directional traffic flows are routinely used. And there are times when opposite direction traffic may be encountered in any part of the Region. Pilots must therefore recognise that LEFT offsets from the cleared track centre-line must not be adopted. After the introduction of RVSM and before the adoption of SLOP, a NAT offsetting procedure was promulgated for wake-turbulence avoidance. This procedure allowed both right and left offsets to be flown. The procedure was developed primarily with a view to the unique traffic flows of the NAT OTS, where uni-directional traffic occupied every flight level from FL310 to FL390. That procedure is no longer in place. The avoidance of wake turbulence (even in the OTS) can be accomplished 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 offsets

from within the allowed three options, in order to mitigate any wake-turbulence issue.

Monitoring during Distractions from Routine


Training and drills should ensure that minor emergencies or interruptions to normal routine

are not allowed to distract the crew to the extent that the navigation system is mishandled.


If during flight the autopilot is disconnected (e.g. because of turbulence), care must be taken

when the navigation steering is re-engaged to ensure that the correct procedure is followed. If the system in use sets specific limits on automatic capture, the across-track indications should be monitored to ensure proper recapture of the programmed flight path/profile.


Where crews have set low angles of bank, perhaps 10° or less, say for passenger comfort

considerations, it is essential to be particularly alert to possible imperceptible departures from cleared track.

NAT Doc 007


Edition 2010

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