User's Manual

(usually to GND mode) before taxiing to the runway, to ensure that the

sensor is operating before you become airborne.

General Low Temperature Operation

The TT21 is certified to operate correctly down to -25C, but at low

temperatures the display may be impaired. On a cold day you may

need to wait for the cockpit to warm up to ensure normal operation.

Warning Messages

If the transponder detects a problem, the screen will indicate

WARNING and a brief statement of the problem. Depending on the

nature of the problem, your transponder may not be replying to

interrogations. Note the message on the screen and pass that

information to your avionics maintenance organisation. Press ENT to

clear the message; if the fault is still present the message will

reappear.

Fault Annunciation

If the transponder detects a catastrophic internal failure, the screen

will indicate FAULT and a brief statement of the problem. No replies

will be made to interrogations when a fault has been detected.

Some FAULT indications can be recovered by switching the

transponder off and back on again, although in all cases a FAULT

code implies that there is a fault with the transponder or the

installation. Note the FAULT message at the bottom of the screen

and pass that information to your avionics maintenance organisation.