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provided a good indication both of the frequency with which navigation errors occur and their causes. Their
frequency is low: only one flight in around ten thousand commits a serious navigation error. However
because of the accuracy and reliability of modern navigation systems, the errors which do occur are most
often seen to be as a result of aircrew error.
13.1.2 Operational errors in the vertical plane also occur. Aircraft are sometimes flown at levels
other than those for which ATC clearance has been issued. In preparation for the introduction of RVSM in
the NAT Region (1997) a comprehensive data collection programme for vertical deviations was
implemented, together with an annual assessment of the resulting collision risks. As in the horizontal plane,
the frequency of vertical errors is low. However, the potential risk of even a single incidence of flying at an
uncleared level can be very significant. Currently, the NAT MNPSA risk estimates in the vertical plane, as a
result of operational errors or uncleared departures from flight level, exceed those arising from lateral gross
navigation errors.
13.1.3 It is therefore essential that crews do not take modern technology for granted. They should
at all times, especially during periods of low workload, guard against complacency and over-confidence, by
adhering rigidly to approved cockpit/flight deck procedures which have been formulated over many years, in
order to help stop operational errors from being an inevitability.
13.1.4 This chapter lists some of the errors that have been recorded in the NAT during recent years.
Also the NATSPG commissioned the UK National Air Traffic Services to produce an interactive DVD
ROM, “On the Right Track”, which highlights many of the common errors and discusses their causes. The
DVD ROM additionally contains general information on Air Traffic Control in the North Atlantic Region.
The DVD ROM, like this Manual, is aimed at pilots, dispatchers and others concerned in operations on the
North Atlantic. It is available at no charge to bona fide operators on application to:
customerhelp@nats.co.uk.
13.2 OPERATIONAL HEIGHT ERRORS
13.2.1 The most common height errors are caused by:
• executing an uncleared climb.
e.g. the crew of an aircraft entering Reykjavik OCA from Edmonton FIR encountered HF
Blackout conditions prior to reaching the Reykjavik OCA boundary and before receiving an
Oceanic Clearance. During the subsequent more than two hours of flight in the MNPSA, the
crew executed two step climbs before re-establishing contact with ATC.
Aircraft following an ATC clearance are assured of separation from other potentially conflicting
traffic. In HF Blackout conditions if an aircraft unilaterally changes level, ATC has no means to
advise or intervene with other traffic and separation can no longer be assured. In such a
circumstance, if a climb without ATC clearance is imperative then this should be treated as a
contingency and the appropriate track offset should be flown.
• misinterpreting an ATC acknowledgement of a request as a clearance
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 13
NAT MNPS 69 Edition 2005
e.g. a crew requested a step climb from Shanwick OAC using HF Voice through the Shannon
aeradio station. The radio operator acknowledged the request to the aircraft and forwarded it
to the Shanwick controller for review and action. The crew interpreted the radio operator’s
acknowledgement as an approval of the request and immediately executed the step climb. The
controller subsequently denied the request due to conflicting traffic with inadequate
longitudinal separation at the requested higher level. The requesting aircraft had reached the
new level and therefore violated separation minima before receiving the denial. Similar
incidents have occurred during NAT CPDLC trials when crews have misinterpreted a
technical acknowledgement of a datalink request for an ATC approval.
When DCPC is unavailable and air/ground ATS communications are via a third party (whether radio
operator or datalink service provider) crews must be aware that acknowledgements of requests do
not constitute approval.
• not climbing or descending as cleared
e.g. a crew was cleared for a climb to cross 4030W at FL350. The crew mis-interpreted the
clearance and took it to mean climb to cross 40°N 30°W (instead of 40° 30'W) at FL350.
While this was caused by a seemingly ambiguous clearance, crews must be on their guard and query
the clearance if in any doubt. Crews should be aware of the risks of non-compliance with a
clearance, or with a restriction within a clearance. A significant number of height deviations have
been reported where an aircraft had been cleared to change level after the next route waypoint and
　
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