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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. Furthermore, an Oceanic Errors Safety Bulletin is available for downloading from the ICAO EUR/NAT website (http://www.paris.icao.int/ )and is updated every 6 months. This is currently augmented by a NAT Safety Alert. It is suggested that pilots consult these sources regularly.
13.2 OPERATIONAL HEIGHT ERRORS
13.2.1 The most common height errors are caused by:
a) 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 of 15 NM should be flown.
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 13
NAT MNPS 81 Edition 2009
b) misinterpreting an ATC acknowledgement of a request as a clearance
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.
c) 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 has done so immediately or has been cleared to change level immediately and had not done so until a later time. Both cases can very easily result in the loss of safe separation with other traffic. Such instances are often, but by no means exclusively, associated with misinterpretation of CPDLC message sets (a crew training/familiarity issue) whereby the words AT or BY are interpreted differently from their intended meaning. This is a problem particularly (but not exclusively) with crew members whose first language is not English. It is compounded in the cases of languages which have no directly equivalent words to differentiate between AT or BY, or perhaps use the same word for each (this is apparently true of a number of european languages, for example). The dangers associated with misinterpretation of conditional clearances must be appreciated. If an aircraft climbs or descends too soon or too late it is almost inevitable that it will lose separation with the other traffic, that was the reason for the condition being applied by ATC.
d) not following the correct contingency procedures
e.g. following an engine failure a crew descended the aircraft on track rather than carrying out the correct contingency procedures (see Chapter 11).
Particularly when flying in the OTS, crews must appreciate that there is a significant likelihood of conflict with other aircraft at lower levels unless the appropriate contingency offset procedure is adopted. (See paragraph 11.3.4)
　
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