Trig Avionics TT31 User manual

Generic TT31 Minor Modification
21 February 2008
Document Control
Title: Generic TT31 Minor Modification
Identifier: SUP/TT31/001
Issue: Issue 1.6
Issue Date: 21 February 2008
Author: Andy Davis
Authority: Martin Gray
CCB Category: 2
File Name: SUP TT31 001 1.6 Generic TT31
Minor Mod.DOC
Printed on: 28/02/2008 16:05:00

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CONTENTS
1. PREFACE.......................................................................................................................................1
1.1 PURPOSE....................................................................................................................................1
1.2 SCOPE........................................................................................................................................1
1.3 CHANGES FROM PREVIOUS ISSUE ..............................................................................................1
1.4 CHANGES FORECAST.................................................................................................................1
1.5 DOCUMENT CROSS-REFERENCES ..............................................................................................1
1.5.1 Internal Documents ..........................................................................................................1
1.5.2 External Documents .........................................................................................................1
1.6 ABBREVIATION AND ACRONYMS...............................................................................................1
2. INTRODUCTION..........................................................................................................................3
3. MODIFICATION DETAILS........................................................................................................4
3.1 DESCRIPTION OF MODIFICATION ...............................................................................................4
3.2 MECHANICAL DETAILS..............................................................................................................4
3.3 CONTINUED AIRWORTHINESS INSTRUCTIONS............................................................................4
3.4 INSTALLED EQUIPMENT SUITABILITY........................................................................................5
3.4.1 ETSO.................................................................................................................................5
3.4.2 Deviations.........................................................................................................................5
3.4.3 Environmental ..................................................................................................................5
3.5 WIRING DIAGRAM.....................................................................................................................6
3.5.1 General Wiring Arrangement...........................................................................................6
3.5.2 Voltage Conversion in 28 Volt Aircraft............................................................................7
3.6 ELECTRICAL LOAD ANALYSIS...................................................................................................7
3.7 TESTING DETAILS......................................................................................................................7
3.8 FLIGHT MANUAL/POH AMENDMENTS.......................................................................................7
3.9 RADIO STATION LICENCE..........................................................................................................7
3.10 MODE SADDRESS .....................................................................................................................7
4. ACCOMPLISHMENT INSTRUCTIONS...................................................................................8
4.1 EQUIPMENT AND TOOLS REQUIRED............................................................................................8
4.2 PREPARATION............................................................................................................................8
4.3 PRE-TEST EXISTING INSTALLATION...........................................................................................8
4.4 PROCESS....................................................................................................................................8
4.4.1 Verify Circuit Breaker Status............................................................................................8
4.4.2 Verify Antenna Status .......................................................................................................8

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4.4.3 Remove Existing Transponder..........................................................................................9
4.4.4 Inspect Wiring ..................................................................................................................9
4.4.5 Remove Voltage Converter (28V Only)............................................................................9
4.4.6 KT76A Style Replacement ................................................................................................9
4.4.7 Non KT76A Replacement................................................................................................10
4.4.8 Install TT31 ....................................................................................................................10
4.4.9 Commission Transponder...............................................................................................10
4.5 POST-INSTALLATION TEST.......................................................................................................10
4.5.1 Equipment Function .......................................................................................................10
4.5.2 Interference Effects.........................................................................................................11
4.5.3 Ramp Test.......................................................................................................................11
5. COMPLIANCE STATEMENT ..................................................................................................12
THIS DOCUMENT IS CONFIDENTIAL AND CONTAINS COMMERCIALLY SENSITIVE
INFORMATION TO TRIG AVIONICS LTD AND PARTNER COMPANIES. THE
CONTENTS OF THIS DOCUMENT ARE TO BE KEPT CONFIDENTIAL AND ARE NOT
TO BE DISCLOSED TO THIRD PARTIES WITHOUT THE PRIOR WRITTEN CONSENT
OF TRIG AVIONICS LTD.

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1. Preface
1.1 Purpose
To document the minor modification to replace a Mode A/C transponder with a Trig Avionics TT31
Mode S transponder.
1.2 Scope
This minor modification applies to unpressurised single engine piston aeroplanes with fixed landing
gear and 14 Volt or 28 Volt DC electrical systems, having an existing certified Mode A/C transponder
installation, and an existing altitude encoder. It also applies to simpler retractable aircraft which lack
switchgear required to derive an accurate “on the ground” indication, and which are therefore treated
for the purposes of this installation as fixed gear aircraft.
1.3 Changes from Previous Issue
Section Change
Section 1.5, Document Cross-
References Added Annex I Type Minor Change approval list.
Section 3.5, Wiring Diagram Corrected error in pin-out numbering.
1.4 Changes Forecast
None.
1.5 Document Cross-References
1.5.1 Internal Documents
00455-00 TT31 Installation Manual Issue AC
SUP/TT31/006 Type approved list for TT31 Minor Change approval Issue 2.0
DEV/TT31/199 Declaration of Design Performance for TT31 Mode S
Transponder Issue 5.0
1.5.2 External Documents
CS-23 Certification Specifications for Normal, Utility, Aerobatic,
and Commuter Category Aeroplanes EASA
CAP747 Mandatory Requirements for Airworthiness CAA
ED-73B MOPS for SSR Mode S Transponders Eurocae
TGL 13 Certification of Mode S Transponder Systems for
Elementary Surveillance JAA
1.6 Abbreviation and Acronyms
The following abbreviations and acronyms are used in this document:
AFM Aircraft Flight Manual

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DC Direct Current
DDP Declaration of Design Performance
EASA European Aviation Safety Agency
ETSO European Technical Standards Order
MOPS Minimum Operational Performance Standard
POH Pilots Operating Handbook

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2. Introduction
The TT31 Mode S panel mount transponder is an ED-73B Class 1 compliant Mode S level 2 datalink
transponder, with support for extended squitter, which also meets the relevant environmental
requirements of ED-14D. The TT31 meets the European requirements for Mode S Elementary
Surveillance and is certified to ETSO 2C112b.
The TT31 is targeted at the light aircraft retrofit market, and is designed to be compatible with the
existing wiring and installation provisions of current Mode A/C transponders. In addition to this
general compatibility, the TT31 is physically directly compatible with the Bendix/King KT76A and
KT78A Mode A/C transponder. Replacement of a KT76A or KT78A may therefore be able to reuse
some or all of the existing mounting provisions. Replacement of other transponders is expected to re-
use the existing antenna, altitude encoder, and circuit breaker, and may also use some or all of the
existing wiring harness.
This minor modification describes the process of upgrading an existing Mode A/C transponder to a
TT31.

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3. Modification Details
3.1 Description of Modification
This modification involves removing a Mode A/C transponder, and replacing it with a Trig Avionics
TT31 Mode S transponder.
The TT31 is designed to be plug compatible with the KT76A and KT78A; it fits in the same mounting
tray and uses the same connectors and connector pin assignments. Both transponders use the same
quick-release latch to hold them in the mounting tray, and therefore the physical upgrade can be as
simple as unlatching the KT76A or KT78A, sliding it out of the tray, sliding in a TT31, and latching it
into the tray.
If the existing transponder is another model (including the Bendix/King KT76) then the mounting tray
and connectors will need to be replaced. This will involve fitting the new tray as described below, and
rewiring the connectors to the transponder in accordance with the wiring diagram in section 3.5.
In all cases the existing antenna, altitude encoder, and circuit breaker will be re-used.
The upgrade is predicated on the suitability of the existing installation, and the compatibility of the
new transponder with the existing installation. The activities involved in the upgrade therefore include
pre-testing of the installation; verification of the suitability of the existing mechanical arrangements;
verification of the suitability of the existing power and signal wiring; transponder commissioning; and
post-installation testing. These activities are described in detail in the Accomplishment Instructions in
this document.
3.2 Mechanical Details
The TT31 is directly compatible with any approved antenna used with a Mode A/C transponder and
subject to this being correctly sited and in good condition the same antenna will be used.
The TT31 chassis is mechanically identical to the KT76A and KT78A, and fits into the same
instrument panel mounting tray. The TT31 is slightly lighter (by about 100g) than earlier models of
KT76A, but is slightly heavier (also by about 100g) than later models. The difference in weight has no
material effect on the mechanical mounting.
For replacement of other models of transponder the mounting tray must be replaced. The TT31
mounting tray complies with the “Mark width” de facto industry standard and the existing tray
mounting position will be appropriate for the TT31 tray. Note however that it is unlikely that the
existing screw holes for the old tray will be in the correct location, and new mounting provisions may
be required.
The TT31 uses a combination of knobs and press buttons to set transponder codes and control the
functions of the unit. The operating mode, squawk code and altitude are displayed on an LCD. It is
important that the knobs and buttons be reasonably accessible to the pilot, and that the screen be
visible, although the transponder is obviously not a primary flight control or instrument. Since the
switch positions and display location are very similar to the legacy Mode A/C transponders, a panel
location that was appropriate to the Mode A/C transponder will also be appropriate to the TT31.
The knobs on the TT31 protrude slightly further than some older transponders (up to 5mm). This is
not a significant factor in conventional panel mount radio locations.
3.3 Continued Airworthiness Instructions
The system described here is a Mode S transponder installation utilising a Gilham code altitude input,
and therefore EASA AD 2006-0265 will apply. The initial test described in section 4.5.3.2 of this
document satisfies the requirements of the AD. Subsequent functional checks shall be carried out at

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intervals of not more than 24 months.
Other than for periodic functional checks required by the regulations, the TT31 Mode S transponder
has been designed and manufactured to allow “on condition maintenance”. This means that there are
no periodic service requirements necessary to maintain continued airworthiness, and no maintenance is
required until the equipment does not properly perform its intended function. When service is
required, a complete performance test should be accomplished following any maintenance action.
3.4 Installed Equipment Suitability
3.4.1 ETSO
The TT31 is certified by EASA to ETSO 2C112b under ETSOA EASA.21O.643.
3.4.2 Deviations
A single deviation is noted on the DDP for the TT31 which affects the behaviour of the transponder
when recovering from All-Call interrogations which have been locked out. This deviation has
negligible effect on this installation.
3.4.3 Environmental
The environmental testing conducted for the TT31 is appropriate for this installation. Key aspects of
the environmental qualification are summarised here:
DO-160D reference Qualification Applicability
Temperature & Altitude Category C1 Equipment intended for installation in a non-
pressurised but controlled temperature location.
Loss of Cooling +70C without cooling air Forced air cooling not required.
Temperature Variation Category C Temperature controlled internal section of the
aircraft.
Humidity Category A Standard humidity environment.
Operational Shock &
Crash Safety Category B Equipment generally installed in fixed-wing aircraft or helicopters, tested for standard
operational shock and crash safety.
Vibration Aircraft zone 2; type 5 to
category S level M Single engine fixed wing reciprocating or turboprop, fitted to instrument panel, console or
equipment rack.
Magnetic Effect Category Z Equipment may be mounted within 0.3m of
magnetic compass.
Power Input Category B DC equipment intended for use on aircraft
electrical system supplied by engine driven
alternator or generator, where a battery of
significant capacity is on the DC bus at all times.
Voltage Spike Category B Installation where a lower standard of protection is
acceptable.
Audio Susceptibility Category B DC equipment intended for use on aircraft
electrical system supplied by engine drive
alternator or generator, where a battery of

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significant capacity is on the DC bus at all times.
Induced Signal
Susceptibility Category A Equipment intended for operation where interference-free operation is desirable.
RF Susceptibility Category T Specified in the HIRF rules; representative of the
internal EMI environment from aircraft equipment.
RF Emission Category B Basic emission control.
In each case the environmental qualification is appropriate to the installation in the instrument panel of
a light piston engine aircraft with a DC electrical system.
3.5 Wiring Diagram
3.5.1 General Wiring Arrangement
The wiring diagram is the same in 14V and 28V aircraft, with the exception of the lighting bus input.

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3.5.2 Voltage Conversion in 28 Volt Aircraft
Some existing Mode A/C transponders, including the KT76A and KT78A, are 14 Volt only devices.
When installed in a 28 Volt aircraft, these transponders will be fitted with a voltage converter. This is
typically a passive resistive dropper, but may be an active voltage regulator such as the KA39. The
TT31 will NOT meet the certification low voltage requirements when installed with the dropper
resistor in place, and the resistor should therefore be removed or bypassed. Active voltage regulators
need not be removed.
Although not required for correct operation of this minor modification, if the modification identified
and removed a resistive dropper on a 28 Volt aircraft with a KT76A it is good practice to replace the
KT76A tray with a Trig TT31 tray, and install a connector – even if it is empty – in the TT31 tray
secondary connector position. This makes it impossible to put a KT76A back into the tray.
In aircraft with other models of transponder the tray will be changed anyway, so avoiding this problem.
3.6 Electrical Load Analysis
Existing Mode A/C transponders draw typically 1.1 Amp from the DC power supply, with currents of
up to 1.9 Amp during high activity.
The TT31 draws typically 0.25 Amp from a 14V DC power supply on standby, with currents of around
0.45 Amp during high activity. On 28V supplies the currents are lower.
Since the current taken by the TT31 is less than half that of the transponder it is replacing, any systems
that were properly sized to support an existing transponder will be adequate to support the TT31.
On the same basis, it can be concluded that the 30 minute battery requirement of CAP747 GR6 will
also be satisfied.
3.7 Testing Details
The test procedure is based on the installation test guidelines in ED-73B, the MOPS for SSR Mode S
Transponders.
3.8 Flight manual/POH Amendments
No AFM amendments are required as part of this minor modification.
A pilots operating booklet is provided with the TT31 and this should be made available to the flight
crew.
3.9 Radio Station Licence
No change is required to the radio station licence for the aircraft. The TT31 operates in the same band
at similar nominal power levels as the transponder being replaced.
3.10 Mode S Address
Installation of the TT31 transponder requires allocation of an Airframe Address from the national
authority of aircraft registration for the aircraft.
In the case of UK aircraft, Mode S addresses have been allocated to all aircraft, and can be obtained
directly from the CAA web site G-INFO database.