TT31 Transponder Installation Manual 1 February 2010
00455-00 Issue AJ
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7. Post Installation Checks
Post installation checks should be carried out in accordance with your certification requirements. These
checks should include:
Mode S interrogations to verify correct address programming.
Verification of the reported altitude using a static tester. For aircraft using parallel Gray code
encoders, the test should include a range of altitudes up to 6,800 feet, 14,800 feet or 30,800
feet, depending on the service ceiling of the aircraft – these altitudes correspond to code
changes which are not otherwise tested at lower altitudes.
Where installed, verification of correct squat switch ground/airborne indications. In an aircraft
with a squat switch, setting the Mode switch to ALT when the aircraft is on the ground should
leave the transponder in GND mode; when the aircraft becomes airborne, the mode should
switch automatically to ALT.
Interrogations to verify the receiver sensitivity. A Mode S transponder should have a
minimum triggering level (MTL) of between -77 dBm and -71 dBm. Failure to meet this
requirement usually indicates antenna or coaxial cable problems.
Interrogations to verify the transmitted power. A Class 1 installation should have no less than
125 Watts at the antenna (and no more than 500 Watts). Failure to meet this requirement is
also generally due to antenna or wiring issues.
Where installed, verification of the GPS position source and ADS-B outputs. In an aircraft
with a configured GPS, pressing the FUNC button on the transponder front panel in normal
operation will display the ADS position monitor. With the aircraft outside the hangar (for
good GPS reception) the aircraft position should be displayed on the transponder. If the
position indications are all dashes then either the GPS position is not valid or the GPS interface
is not correctly configured. Whenever a valid position is received by the transponder and the
transponder is in any mode other than Standby, ADS-B Extended Squitters should be observed
on the transponder test set.
Where installed, verification of the TIS output. A Mode S test set with TIS capability should
be used; with the transponder in ALT mode traffic should be shown on the attached display.
Page 22 Trig Avionics Limited
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