AMC1 ACNS.D.ELS.025 Altitude source

(a)     Compliance with JAA TGL No 6 is an approved acceptable means of compliance for an altimeter as an altitude source.

(b)     A pressure altitude resolution of less than or equal to 7.62 m (25 ft) is an approved acceptable means of compliance for an altimeter.

Note: An altitude source resolution of less than or equal to 7.62 m (25 ft) is required for aeroplanes intended to be used for international air transport, as defined in ICAO Annex 6, Part 1, Section 6.19.

(c)      A pressure altitude resolution of less than or equal to 30 m (100 ft) and greater than 7.62 m (25 ft) is an approved acceptable means of compliance for an aircraft altimeter, provided that the following conditions are met:

(1)     There is no conversion of the Gillham-encoded data to another format before it is input to the transponder unless failure detection can be provided, and the resolution (quantisation) is set in the transmitted data to indicate 30 m (100 ft).

         Note 1: It is not recommended to install altimeters with a Gillham altitude encoder interface, as it supports a resolution of only 30 m (100 ft).

         Note 2: Losses or errors of pressure altitude have an impact on the provision of separation by the air traffic control (ATC). It is, therefore, important to design the altitude pressure source to minimise the loss of this data or the provision of erroneous data.

         Note 3: Further guidance on altitude measurement and coding systems can be found in EUROCAE Document ED-26.

(2)     Altitude source comparison:

         For aircraft equipped with ACAS II, where the available source of pressure altitude information is only in Gillham-encoded format, the requirement for detection of an altitude source or encoder failure can be satisfied by means of dual independent altitude corrected sensors, together with an altitude data comparator (which may be incorporated and enabled in the transponder). Similar provisions are also acceptable for alternative altitude information sources that do not signal erroneous data.

The flight deck interface should provide a means to inhibit the transmission of pressure altitude information for aircraft equipped with a Gillham-encoded altitude interface.

(d)     If it is impractical to connect the transponder to the altitude source used to fly the aircraft, consistency may be achieved by:

(1)     connecting the pressure altitude source directly (e.g. via a T-junction) to the same pitot/static-pressure line(s) as the altitude source being used to fly the aircraft; and

(2)     ensuring that the pressure altitude source has built-in test equipment (BITE) that permanently or frequently runs an automatic system self-test and triggers a ‘FAIL’ annunciator/indicator (e.g. an amber light) in the pilot’s normal field of view upon detection of a failure; and

          (3)     ensuring that the altitude source meets design and performance standards that achieve an adequate level of integrity of its output, to mitigate the risk of a possible inconsistency between the output of the altitude source and the altimeters used by the flight crew to fly the aircraft.

The altitude source may be integrated into the transponder if the above-mentioned requirements are met.

(e)     The provision of manual or automatic selection of the altitude source is an acceptable means of compliance.

[Issue: CS-ACNS/4]