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8.5.8 Crews who decide to check or update their LRNSs by reference to VORs should remember
that in the Canadian Northern Domestic Airspace these may be oriented with reference to true north, rather
than magnetic north.
Navigation in the Area of Compass Unreliability
8.5.9 As aircraft move towards the Earth‟s North magnetic pole the horizontal field strength
reduces and the ability of the compass to accurately sense magnetic North is reduced. It is generally
recognised that when the horizontal magnetic field strength falls below 6000 nanotesla, the magnetic
compass can no longer be considered to be reliable. Moreover, when the horizontal magnetic field strength
falls below 3000 nanotesla, the magnetic compass is considered to be unuseable. Within MNPS airspace the
North West of Greenland is an area of Compass Unreliability and adjoining areas of Canadian airspace
include areas where the magnetic Compass is Unuseable. En route charts for the North Atlantic and North
Polar areas show the areas where the compass is either unreliable or unuseable.
8.5.10 In areas where the compass is unreliable or unuseable, basic inertial navigation requires no
special procedures. Different manufacturers may offer their known solutions to the special problems existing
in such areas. However, such solutions should not involve the use of charts and manual measurement of
direction.
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 8
NAT MNPS 53 Edition 2009
8.5.11 Furthermore, Operators/Pilots are reminded that before operating in an area of
Compass Unreliability they are responsible for checking with their State Authorities whether specific
regulatory approval or training is required.
Deliberate Deviation from Track
8.5.12 Deliberate temporary deviations from track are sometimes necessary, usually to avoid severe
weather; whenever possible, prior ATC approval should be obtained (See para 11.4). Such deviations have
often been the source of gross errors as a consequence of failing to re-engage the autopilot with the
navigation system. It should also be noted that selection of the 'turbulence' mode of the autopilot on some
aircraft may have the effect of disengaging it from the aircraft navigation system. After use of the turbulence
mode, extra care should be taken to ensure that the desired track is recaptured by the steering navigation
system.
8.6 POST-FLIGHT PROCEDURES
Inertial Navigation System Accuracy Check
8.6.1 At the end of each flight, an evaluation of accuracy of the aircraft's navigation systems
should be carried out. Equipment operating manuals specify maxima for radial errors before a system is
considered to be unserviceable. For early gimballed-platform inertial systems these are in the order of 2 NM
per hour. One method used to determine radial error is to input the shutdown ramp position; in other systems
error messages are output giving differences between raw inertial reference positions and computed radio
navigation updated positions. Whatever method is used, a record should be kept of the performance of each
INS.
8.7 HORIZONTAL NAVIGATION PERFORMANCE MONITORING
8.7.1 The navigation performance of operators within NAT MNPS Airspace is monitored on a
continual basis. The navigation accuracy achieved by NAT MNPS aircraft is periodically measured and
additionally all identified instances of significant deviation from cleared track are subject to thorough
investigation by the NAT Central Monitoring Agency (CMA), currently operated on behalf of ICAO by the
UK National Air Traffic Services Limited. The CMA also maintains a data base of all NAT MNPS
Approvals. The CMA runs a continuous monitoring process to compare this Approvals list with the records
of all aircraft flying in the NAT MNPS Airspace. The Approval status of any aircraft involved in a track
deviation is specifically checked against the data base and in any cases of doubt the State of Registry is
contacted.
8.7.2 When a navigation error is identified, follow-up action after flight is taken, both with the
operator and, where the deviation is 25 NM or more (i.e. a GNE), the State of Registry of the aircraft
involved, to establish the circumstances and contributory factors. The format of the (navigation) Error
Investigation Form used for follow-up action is as shown at Attachment 1. Operational errors can have a
significant effect on the assessment of risk in the system. For their safety and the safety of other users, crews
are reminded of the importance of co-operating with the reporting OAC in the provision of incident
information.
8.7.3 The overall navigation performance of all aircraft in the MNPS Airspace is continually
assessed and compared to the standards established for the Region, to ensure that the TLS is being
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