World Air Ops | NAT | Doc 007 MNPS Guidance

Guidance concerning Air Navigation in and above the NAT MNPSA CHAPTER 14b) misinterpreting an ATC acknowledgement of a request as a clearancee.g. a crew requested a step climb from Shanwick OAC using HF Voice through the Shannonaeradio station. The radio operator acknowledged the request to the aircraft and forwarded itto the Shanwick controller for review and action. The crew interpreted the radio operator’sacknowledgement as an approval of the request and immediately executed the step climb. Thecontroller subsequently denied the request due to conflicting traffic with inadequatelongitudinal separation at the requested higher level. The requesting aircraft had reached thenew level and therefore violated separation minima before receiving the denial. Similarincidents have occurred during NAT CPDLC trials when crews have misinterpreted atechnical acknowledgement of a datalink request for an ATC approval.When DCPC is unavailable and air/ground ATS communications are via a third party (whether radiooperator or datalink service provider) crews must be aware that acknowledgements of requests donot constitute approval.c) not climbing or descending as clearede.g. a crew was cleared for a climb to cross 4030W at FL350. The crew mis-interpreted theclearance 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 querythe clearance if in any doubt. Crews should be aware of the risks of non-compliance with aclearance, or with a restriction within a clearance. A significant number of height deviations havebeen reported where an aircraft had been cleared to change level after the next route waypoint andhas done so immediately or has been cleared to change level immediately and had not done so until alater time. Both cases can very easily result in the loss of safe separation with other traffic. Suchinstances are often, but by no means exclusively, associated with misinterpretation of CPDLCmessage sets (a crew training/familiarity issue) whereby the words AT or BY are interpreteddifferently from their intended meaning. This is a problem particularly (but not exclusively) withcrew members whose first language is not English. It is compounded in the cases of languageswhich have no directly equivalent words to differentiate between AT or BY, or perhaps use the sameword for each (this is apparently true of a number of european languages, for example). The dangersassociated with misinterpretation of conditional clearances must be appreciated. If an aircraft climbsor descends too soon or too late it is almost inevitable that it will lose separation with the othertraffic, that was the reason for the condition being applied by ATC.d) not following the correct contingency procedurese.g. following an engine failure a crew descended the aircraft on track rather than carryingout the correct contingency procedures (see Chapter 12).Particularly when flying in the OTS, crews must appreciate that there is a significant likelihood ofconflict with other aircraft at lower levels unless the appropriate contingency offset procedure isadopted. (See paragraph 12.3.4)e) entering the NAT MNPSA at a level different from that contained in the received OceanicClearance.e.g. a crew flying through Brest FIR at FL310 en route to the Shanwick OCA boundaryreceived an oceanic clearance for FL330. The crew requested a climb from Brest but it hadnot been received when the aircraft reached the Shanwick boundary. The crew elected toNAT Doc 007 85 Edition 2010