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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.
• 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 is adopted prior
to commencing any descent.
• 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
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. Such requests must be made sufficiently early to allow the domestic ATC unit
to respond.
• An occasional error is to fly at one (uncleared) level and report at the (different) cleared level !
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 13
NAT MNPS 70 Edition 2005
e.g. the crew of a major airline reported at FL360 (the cleared level), all the way across the
ocean but were in fact flying at FL350!! They had been cleared to cross 40°W at FL360 and
correctly entered the cleared level into the FMC but did not execute the command prior to
40°W. During position reporting the aircraft level was reported by reference to the FMC
altitude hold box.
Without SSR ATC must rely upon crew position report data to plan for the safe separation of all
traffic. If any such data is in error actual separations can be compromised.
13.3 LATERAL NAVIGATION ERRORS
More Common Causes Of Lateral Navigation Errors
13.3.1 The most common causes of GNEs, in approximate order of frequency, have been as
follows:
• having already inserted the filed flight plan route co-ordinates into the navigation computers, the
crew have been re-cleared by ATC, or have asked for and obtained a re-clearance, but have then
omitted to re-program the navigation system(s), amend the Master Document or update the plotting
chart accordingly.
• a mistake of one degree of latitude has been made in inserting a forward waypoint. There seems to
be a greater tendency for this error to be made when a track, after passing through the same latitude
at several waypoints (e.g. 57°N 50°W, 57°N 40°W, 57°N 30°W) then changes by one degree of
latitude (e.g. 56°N 20°W). Other circumstances which can lead to this mistake being made include
receiving a re-clearance in flight.
• the autopilot has been inadvertently left in the heading or de-coupled mode after avoiding weather,
or left in the VOR position after leaving the last domestic airspace VOR. In some cases, the mistake
has arisen during distraction caused by SELCAL or by some flight deck warning indication.
• an error has arisen in the ATC Controller/Pilot communications loop, so that the controller and the
crew have had different understandings of the clearance. In some cases, the pilot has heard not what
was said, but what he/she was expecting to hear.
Rare Causes Of Lateral Navigation Errors
13.3.2 To illustrate the surprising nature of things which can go wrong, the following are examples
of some extremely rare faults which have occurred:
• the lat/long co-ordinates displayed near the gate position at one international airport were wrong.
• because of a defective component in one of the INS systems on an aircraft, although the correct
forward waypoint latitude was inserted by the crew (51°) it subsequently jumped by one degree (to
52°).
• the aircraft was equipped with an advanced system with all the co-ordinates of the waypoints of the
intended route already in a database; the crew assumed that these co-ordinates were correct, but one
was not.
• when crossing longitude 40°W westbound the Captain asked what co-ordinates he should insert for
the 50°W waypoint and was told 48 50. He wrongly assumed this to mean 48°50'N at 50°00W
(when it really meant 48°N 50°W ) and as a result deviated 50 nm from track.
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