User's Manual

TT21/TT22 Transponder Installation Manual 13 July 2009
00560-00 Issue AF
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Trig Avionics Limited Page 31
6.2 Test and Calibration items
6.2.1 Voltage Check
The Voltage Check screen displays the current input voltage received by the
control head from the TT21/TT22. Whilst displaying this screen theTC20 will
exercise certain internal functions in the controller, including periodically
turning on the heater circuit. The purpose of these exercises is to present a
changing load on the power lines from the TT21/TT22. If you suspect a
potential problem with the wiring to the controller, it is worth monitoring the
displayed voltage for several seconds to find the worst-case reading.
The nominal voltage is 6.5 Volts. The displayed value may be lower due to
cable impedance. If the voltage is below 6.0 Volts, then the interface cable has
too much impedance, and you should review the choice of cable.
6.2.2 Altitude Encoder Calibration
The Altitude Encoder Calibration allows you to adjust the built-in altitude
encoder to ensure that the altitude transmitted by the transponder corresponds
to the altitude seen by the pilot on the primary altimeter. This process is
normally carried out every 24 months, as part of the altimeter checks on the
aircraft. The maximum allowed difference between the primary altimeter and
the altitude encoder is 125 feet in ETSO C88a and TSO C88b. The altitude
encoder in the TC20 is accurately calibrated during manufacture to be within
50 feet of the applied pressure altitude at all altitudes, whereas the allowed
error in the primary altimeter increases with altitude, and above 18,000 feet the
altimeter error alone may exceed 125 feet. It is therefore possible that the
combination of the allowed errors in the encoder and the primary altimeter may
exceed 125 feet, in which case the altitude encoder must be adjusted to
correspond to the primary altimeter.
Note: The purpose of calibrating the encoder is to make the output
correspond to the primary altimeter. The encoder calibration
procedure must therefore only be undertaken after the primary
altimeter has been tested and found to comply with the relevant
standards.