Annex A: Avionics Requirements and Guidance on the Intended Function

This requirement applies to primary and standby heading displays if the standby heading systems can display True heading. If applicable, the same switch should drive both the primary and standby heading indications.

For True heading indication, slaving to a non-Magnetic heading determination system (e.g., gyrocompassing) is preferred, however Directional Gyro (DG) mode is satisfactory provided acceptable crew procedures are documented in the FM/AOI (see Annex C of this TAA Advisory).

For aircraft approved to conduct Performance Based Navigation operations, the following guidance of FAA AC 20-138D paragraph 14-7 should be followed: “If a magnetic/true heading reference switch is installed in the aircraft, the positioning and navigation equipment should be driven by the same switch to maintain consistency in the displays and operation for both manual and automatic heading reference changes.” The same means of switching between True and Magnetic heading should, therefore, also switch the navigation equipment mode. For more information related to PBN operations, see TAA Advisory 2019-05.

In some implementations, the switching occurs automatically based on an established criterion, for example when reaching a certain latitude. In those cases, the applicant must substantiate how the switching criteria aligns with the boundaries of the NDA. If there are significant differences, crew procedures may be considered as mitigation. 

The availability and performance of all alignment modes as a function of latitude must be determined on the ground and in-air (if applicable). Examples of modes include Gyrocompass (GC), Stored Heading (SH), Best Available True Heading (BATH), and Shipboard Inertial Navigation System (SINS).

The TAA does not set a maximum alignment time. For example, a gyrocompassing Attitude and Heading Reference System (AHRS) certified to TSO-C201 by the FAA must be demonstrated to have a heading alignment time of 10 minutes or less below 60 degrees North, but the alignment time above that latitude need only be specified by the manufacturer. Historically, the TAA has found an alignment time of 30 minutes or less to be appropriate. 

Sufficient annunciation must be provided to alert crews of degraded heading system performances. 

Unless it can be demonstrated that heading performance is unaffected in the polar regions, an alert must be triggered when the heading system is providing degraded performance. For slaved systems, this is generally the case when the heading is erroneous by more than 6 degrees.

This requirement has historically been met by comparing different heading sources. Most modern aircraft designs that are equipped with two or more gyroscopically stabilized heading systems include a comparator that will provide an alert if both systems diverge significantly. A threshold of 6 degrees in level flight is often used, in accordance with the FAA guidance of AC 23-17C, however tighter tolerances exist.

Another option to meet this requirement is using an Original Equipment Manufacturer (OEM) degraded mode function (e.g., see RTCA DO-334).

Additional demonstration of compliance of the alert is not required if compliance was assessed as part of the initial airworthiness, or subsequent design change, provided the results are shown to be applicable to operations in the NDA.

In the southernmost regions of the NDA, a standby heading instrument in magnetic mode may still provide valuable indication to crews. If intended to be used operationally, the performance of the heading instrument in terms of accuracy must be assessed.

The drift rate of standby heading instruments in DG mode must be determined. Should the drift rate change compared to operations outside of the NDA, sufficient information must be included in the FM/AOI (see Annex C of this TAA Advisory). The TAA does not define a maximum drift rate, provided that acceptable crew procedures can be defined to adjust the heading value operationally. 

A standby heading instrument with a heading that can be sourced by using a non-magnetic heading determination (e.g., Embedded GPS/INS (EGI)) must be capable of accepting such true heading in the NDA.