Guidance concerning Air Navigation in and above the NAT MNPSA
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 new level and therefore
the requested higher level. The requesting aircraft had reached the
have occurred during NAT acknowledgement of a datalink
CPDLC trials when crews 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
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.
not following the correct contingency procedures
g. following an engine failure a crew descended the aircraft on track rather than carrying
out the correct contingency procedures (see Chapter 12).
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 12.3.4)
entering the Clearance.
. a crew flying through Brest FIR at FL310 en route to the Shanwick OCA boundary
received an oceanic clearance for FL330. The crew requested a climb from Brest but it had not been received when the aircraft reached the Shanwick boundary. The crew elected to
NAT Doc 007