CAT.IDE.H.145 Radio altimeters

AUDIO WARNING DEVICE

(a)The audio warning should be a voice warning.

(b)The audio warning may be provided by a helicopter terrain awareness and warning system (HTAWS).

RADIO ALTIMETER DISPLAY The radio altimeter should be of an analogue type display presentation that requires minimal interpretation for both an instantaneous impression of absolute height and rate of change of height.

AUDIO-VOICE-ALERTING DEVICE

(a)To be effective, the voice warning alert should be distinguishable from other warnings and should contain a clear and concise voice message.

(b)The warning format should meet the following conditions:

(1)the warning should be unique (i.e. voice);

(2)it should not be inhibited by any other audio warnings, except by higher priority alerts such as helicopter terrain awareness and warning system (HTAWS); and

(3)the urgency of the warning should be adequate to draw attention but not such as to cause undue annoyance during deliberate descents through the datum height.

(c)The criteria above can be satisfactorily met if the warning format incorporates all of the following features:

(1)a unique tone should precede the voice message; a further tone after the voice may enhance uniqueness and attract more attention without causing undue annoyance;

(2)the perceived tone and voice should be moderately urgent;

(3)the message should be compact as opposed to lengthy provided that the meaning is not compromised, e.g. ‘One fifty feet’ as opposed to ‘One hundred and fifty feet’;

(4)an information message is preferable (e.g. ‘One hundred feet’); messages such as ‘Low height’ do not convey the correct impression during deliberate descents through the datum height;

(5)command messages (e.g. ‘Pull up, pull up’) should not be used unless they relate specifically to height monitoring (e.g. ‘Check height’); and

(6)the volume of the warning should be adequate and not variable below an acceptable minimum value.

(d)Every effort should be made to prevent spurious warnings.

(e)The height at which the audio warning is triggered by the radio altimeter should be such as to provide adequate warning for the pilot to take corrective action. It is envisaged that most installations will adopt a height in the range of 100–160 ft. The datum should not be adjustable in flight.

(f)The preset datum height should not be set in a way that it coincides with commonly used instrument approach minima (i.e. 200 ft). Once triggered, the message should sound within 0.5 sec.

(g)The voice warning should be triggered only whilst descending through the preset datum height and be inhibited whilst ascending.

RADIO ALTIMETER DISPLAY An analogue type display presentation may be, for example, a representation of a dial, ribbon or bar, but not a display that provides numbers only. An analogue type display may be embedded into an electronic flight instrument system (EFIS).