Aircraft Certification Policy Letter (ACPL)

ACPL No.: 11
Issue No.:  1
Issue Date:  1998-03-17
AARD File:  5009-0-11

Aircraft Certification Policy Letter (ACPL)

Subject: Selection and Annunciation of Multiple Navigation Sources

1.0 Background

1.1 Navigation Systems incorporating multiple navigation sources may display guidance information on a Course Deviation Indicator (CDI) or a Horizontal Situation Indicator (HSI) and/or provide guidance information to a Flight Director/Auto-pilot system. It has become apparent that some installations incorporate a feature in which the selection of an ILS frequency on the VHF NAV control causes the previously selected navigation source to be replaced by ILS guidance. The rationale for automatically selecting ILS as the navigation source concurrent with the selection of the ILS frequency is to preclude the pilot forgetting to make this navigation source selection for the approach phase of flight. This feature is called ILS Reversion.

1.2 Paragraph .1301 of Airworthiness Manual (AWM) Chapters 523, 525, 527, and 529, pertaining to the function and installation of equipment, states that "each item of installed equipment must be of a kind and of a design appropriate to its intended function". It is not expected that the intended function of a frequency selector is to affect the display of previously selected guidance information or the previously selected navigation mode for a Flight Director/Auto-pilot. When multiple navigation sources exist, they should be pilot selectable through an appropriate selector switch. The active navigation source must be clearly annunciated in the pilot's primary field of view. (In front of both pilots in the case of a two crew aircraft).

1.3 Accepted cockpit management procedures are predicated on the pilot's ability to pre-tune and identify an approach facility, prior to actually selecting the facility as guidance, to allow a smooth transition from the Terminal to the Approach phase. Pre-selection of an ILS frequency cannot be done with a navigation system incorporating the automatic reversion feature, without interrupting a previously selected navigation source. With the increasing use of GPS for approach, cockpit procedures must emphasize the need to ensure that the navigation source for the approach is correct. Finally, during the installation approvals, failure analysis must address misleading annunciations, and the flight manual supplement should include adequate cautions and procedures.

1.4 ILS Reversion is in conflict with accepted cockpit management procedures and violates cockpit human factors principles. Accordingly, installations of this feature are not recommended for approval, particularly on approach capable installations.

2.0 Purpose

The purpose of this ACPL is to provide guidance for the airworthiness approval of navigation system designs incorporating multiple navigation sources.

3.0 Transport Canada Policy

3.1 Navigation systems incorporating multiple sources for display and/or guidance should provide a dedicated navigation source selector as the sole means of selection

3.2 Where an additional navigation source selector is installed, the flight manual supplement must clearly point out to the pilot the action which will cause replacement of the previously selected navigation guidance. A suitable cockpit placard identifying this feature should also be provided.

3.3 Whatever the configuration is, the navigation system must:

  1. clearly annunaciate, the selected navigation source, in the pilots' primary fields of view;
  2. display guidance information appropriate to the selected and annunciated navigation source;
  3. provide guidance information to a Flight Director / Auto-pilot appropriate to the selected and
    annunciated navigation source.

4.0 Termination

Once this information is incorporated in an appropriate Airworthiness Manual Advisory, this ACPL will then be rescinded.

Original signed by:

K. Mansfield
Director, Aircraft Certification


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