User's Manual

TT21 Transponder Installation Manual 19 January 2009
00560-00 Issue AA
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6.1.11 ADS-B Test Message Option
Because of limitations in the ATC radar computer systems, in US airspace the FAA require ADS-B
equipped aircraft to periodically transmit their Mode A squawk code in an ADS-B squitter message.
This allows the computer system to reconcile conventional radar traces with ADS-B information in a
mixed surveillance environment. Because the Mode A code does not form part of the base ADS-B
message set, the code is sent in a “TEST” message.
These special messages only need to be sent when the aircraft is visible to US radar systems. The
transponder already knows the geographic position of the aircraft, and therefore the recommended
option setting for these messages is “Only in America”.
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. 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. 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.
6.2.2.1 Calibration Equipment
To calibrate the encoder you will need to be able to power up the transponder subsystem, and you will
need a pitot-static test set with the appropriate adapters to connect to the static port on the aircraft. The
pitot-static test set should be able to drive the altitude down to sea level, and above the service ceiling
of the aircraft.
No transponder test set is required – the calibration procedure displays all the information you need on
the screen of the TC20
6.2.2.2 Calibration Procedure
There are two adjustment points on the altitude encoder, a low altitude adjustment point, and a high
altitude adjustment point. The low altitude point adjusts the correspondence at sea level, and the high
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