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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)
e) entering the NAT MNPSA at a level different from that contained in the received Oceanic
Clearance.
e.g. 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
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 13
NAT MNPS 82 Edition 2009
continue into the NAT MNPSA at FL310. Separation was immediately lost with a preceeding
aircraft at that flight level.
Crews are responsible for requesting and obtaining any domestic ATC clearance necessary to climb
(or descend) to the initial flight level specified in their received Oceanic Clearance, prior to reaching
the oceanic boundary. While adjacent ACCs generally use their best endeavours to get an aircraft to
it's oceanic level before the boundary, it must be recognized that entry into NAT MNPSA at the
cleared oceanic level is entirely the responsibility of the crew. It does appear from the relative
frequency of this type of error that this is not widely understood. It should also be appreciated that
such requests must be made sufficiently early to allow the domestic ATC unit time to
respond.
f) An occasional error is to fly at one (uncleared) level and report at the (different) cleared level !
e.g. the crew of a major airline reported at FL360 (the cleared level), all the way across the
　
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