TT31 Transponder Installation Manual 1 February 2010
00455-00 Issue AJ
______________________
Equipment Transmitted Integrity Level
VFR only GPS or uncertified installation Low
GPS installation certified for en-route and terminal IFR
navigation
Medium
GPS installation certified with augmentation, such as
WAAS or LAAS
High
6.1.11 Aircraft Length and Width
On the ground, ADS-B transmits encoded aircraft size information which is used by ATC to identify
taxiing routes and potential conflicts. When configured for ADS-B, the TT31 will ask for the aircraft
length and width (wingspan), in metres, and will calculate the appropriate size code for transmission.
6.1.12 ADS-B Test Message Options
Because of limitations in the ATC radar computer systems, in some airspace an ADS-B equipped
aircraft is required to periodically transmit the 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.
Because different regulatory authorities have not agreed on the behaviour of the TEST message, the
installer must select the local preferred behaviour. For aircraft registered in the USA, or otherwise
relying on the FAA TSO approval for the TT31, the TEST message should always be transmitted.
For other airworthiness approval authorities, the preferred behaviour is to apply a geographical filter so
that these messages are only sent when the aircraft is visible to US radar systems. Because it has a GPS
position input, the transponder already knows the geographic position of the aircraft, and therefore in
this case, the recommended option setting for these messages is “Only in America”.
6.1.13 Audio Volume
The altitude alert function includes an audio alert. This configuration item lets you adjust the audio
volume output from the transponder using the rotary knob. Whilst you are turning the volume control,
the transponder will periodically output a test signal to verify the settings.
6.1.14 LCD Dim Point
The LCD backlight illumination is controlled automatically by the ambient light sensor. Depending on
the amount of light spill in the cockpit, and the brightness of other adjacent avionics displays, it may be
necessary to adjust the darkest setting of the backlight to best match other equipment and to improve the
cockpit appearance.
Note – it is only practical to do this in pitch darkness, since that is the in-flight environment that you are
trying to reproduce. If you are working in a hangar with any other lighting it may be better to leave the
setting in the mid-range.
6.1.15 LCD Brightness
The actual maximum brightness of the LCD cannot be increased with this control. What it controls is
the rate at which the lighting increases in brightness as the ambient light increases. This allows the
brightness to be matched to other avionics displays during light level changes as far as possible.
Page 20 Trig Avionics Limited
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