This AMC addresses the accuracy of stabilised magnetic heading systems, required for safe operation of the aeroplane. These systems include means to compensate or correct for errors induced by stable magnetic effects in the aeroplane. Additional effects due to electromagnetic transients and configuration changes are not normally “compensated” by the magnetic heading system and are also included in this AMC. Should the correction become unavailable (either intentionally or unintentionally), the effects of the resulting heading indication should be considered for safe operation of the aeroplane. This AMC addresses the condition where correction is available and the condition where correction is not available (or failed). In most circumstances, heading information is not directly used as the primary means of navigation. This condition should permit the applicant to show that the accuracy adequate for the safe operation of the aeroplane may be different than what is defined in this AMC.
1.After correction the cumulative deviation on any heading should not exceed 5°, based on the following: a. A change in deviation due to the equipment of the heading system components, the total of which should not exceed 2°. b. A change in deviation due to the current flow in any item of electrical equipment and its associated wiring is permissible, but should not exceed 1°. The total cumulative effect for all combinations of equipment, with all combinations of electrical load, should not exceed 2°. c. A change in deviation due to the movement of any component, (e.g. controls or undercarriage) in normal flight is permissible, but should not exceed 1°.
2.If correction fails or is not available, the change in deviation due to the proximity of all equipment containing magnetic material should not exceed 2°.
3.For magnetic heading indications obtained via geographic (true) heading, the accuracy of the heading indication should account for the accuracy of the magnetic variation data based on geographic position. This variation may change over time. Acceptable accuracy values have been found to be: 2 degrees (Latitudes between 50°S and 50°N) 3 degrees (Latitudes between 50°N and 73°N) 3 degrees (Latitudes between 50°S and 60°S) 5 degrees (Latitudes between 73°N and 79°N) 8 degrees (Latitudes between 79°N and 82°N) The applicant may propose different accuracy values after consultation with the EASA. In areas of known magnetic unreliability (e.g. the magnetic poles), the magnetic variation error can be very large, so the magnetic heading indications (if output) should not be relied upon.
4.For geographic (true) heading indications (such as those provided by Inertial Reference Units), the accuracy should be better or equal to 1°. Note: On aeroplanes with a short cruising range, the above limits may be extended after consultation with EASA. For aeroplanes that do not depend on direction or heading information for navigation (VOR, ILS, FMS, GPS), the above limits may be extended after consultation with EASA.
[Amdt 25/21]