Guidance concerning Air Navigation in and above the NAT MNPSA
Avoiding Confusion between Magnetic and True Track Reference
To cover all navigation requirements, some operators produce flight plans giving both
magnetic and true tracks. However, especially if crews are changing to a new system, there is a risk that at some stage (e.g. during partial system failure, re-clearances, etc.), confusion may arise in selecting the correct values. Operators should therefore devise procedures which will reduce this risk, as well as ensuring
that the subject is covered during training.
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
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.
In areas where the compass is unreliable or unuseable, basic inertial navigation requires no
special procedures. Different manufacturers may offer their own solutions to the special problems existing in such areas. However, such solutions should not involve the use of charts and manual measurement of direction.
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
Deliberate temporary deviations from track are sometimes necessary, usually to avoid severe
weather; whenever possible, prior ATC approval should be obtained (See paragraph 12.4). 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
Inertial Navigation System Accuracy Check
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
NAT Doc 007