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.

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4. Accomplishment Instructions
4.1 Equipment and tools required
You will need a Mode S transponder ramp test set, a pitot/static system test set, an inspection lamp or
torch, and a 3/8th inch Allen key. Depending on the accessibility of the mounting tray and rear
connector wiring, you may also need screwdrivers – including a long screwdriver – to inspect the
mounting brackets and connectors.
4.2 Preparation
During the installation you will need to program the unique Mode S airframe address into the
transponder. Allocation of Mode S addresses comes from the appropriate national authority of aircraft
registration; ensure that you have applied for and been issued with a Mode S address before you start.
4.3 Pre-test Existing Installation
This step is optional, but may assist in fault finding if a problem is found later in the process. Pre-
testing will not be possible if the reason you are replacing the Mode A/C transponder is because the
transponder itself is faulty.
The pre-test activities involve testing the existing installation and noting in particular:
•Transponder receiver sensitivity – Minimum Triggering Level or MTL. The existing
transponder MTL should have an MTL between -71 dBm and -77 dBm. Sensitivity below
this range may indicate a problem with the antenna or antenna cable, although could also be
an indication of a fault in the existing transponder.
•Transponder transmitted power. The existing transponder should provide not less than 125
Watts (Class 1) or 70 Watts (Class 2) at the antenna. Power levels below this may also
indicate a problem with the antenna or antenna cable, although could also be an indication of
a fault in the existing transponder.
•Altitude reporting. Test the altitude reporting system, ideally over the service ceiling of the
aircraft. A problem with the altitude reporting may indicate a fault in the altitude encoder or
wiring between the encoder and the transponder tray, although could also be an indication of a
fault in the existing transponder.
If a fault is identified in the pre-testing, you will need to trace the fault cause. If the fault is in the
transponder to be replaced, then the upgrade process described here would clear the fault. If the fault
is in the existing installation however, upgrading the transponder will not fix it.
4.4 Process
4.4.1 Verify Circuit Breaker Status
Trace and identify the existing transponder circuit breaker. Verify that the circuit breaker is in
satisfactory condition and is of the correct type and rating. The circuit breaker for the TT31 should
rated between 3 Amps and 5 Amps.
4.4.2 Verify Antenna Status
Trace and identify the existing transponder antenna. The transponder antenna will be a small stub or
blade antenna on the bottom of the aircraft. Note that on an aircraft with DME the antenna for the
DME will look similar to the transponder antenna; ensure you are looking at the right one. Check the

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condition of the antenna, including the attachment to the airframe. It is important that the ground plane
of the antenna is correctly bonded to the aircraft skin. The antenna should be in a vertical orientation,
as clear as possible from other antennae and from airframe obstacles and protrusions, such as landing
gear.
4.4.3 Remove Existing Transponder
Using the Allen key, remove the existing transponder. Turning the locking screw should initially turn
the camlock and then jack the transponder out of the tray. Check the transponder for mechanical
damage that may indicate the existing tray is inadequately fitted.
4.4.4 Inspect Wiring
Inspect the wiring to the interface connector, check general condition and gauge. The power wires
should be AWG 22 or heavier; the other signal wires carry only light currents and may be any gauge
appropriate to the mechanical environment.
4.4.5 Remove Voltage Converter (28V Only)
If this is a 28 Volt aircraft with a 14 Volt transponder, such as the KT76A, trace the power wire from
the transponder connector to determine the method used for voltage conversion. The voltage reduction
typically uses a resistive dropper attached to the firewall or other metal aircraft structure. Remove or
bypass the resistive dropper; the power supply to the TT31 must come directly from the 28V aircraft
supply.
Consider replacing the transponder mounting tray as discussed in section 3.5.2.
4.4.6 KT76A Style Replacement
4.4.6.1 Inspect/Replace Mounting Tray
Check the security and integrity of the mounting tray. The tray should be securely mounted with the
four screws at the front of the tray and two at the rear.
On 28 Volt aircraft, even where the tray is compatible, consider replacing the mounting tray as
discussed in section 3.5.2.
4.4.6.2 Inspect/Replace Interface Connector
Using an inspection light, examine the interface connector at the rear of the tray. The connector should
be securely fitted with two screws. The connector is numbered on the rear, but since this inspection is
conducted looking into the tray from the front of the tray, the orientation is open to confusion. For the
avoidance of doubt, the contacts populated should be as follows when viewed from the FRONT:
Not all of these contacts need be present. In aircraft flown below 30,000 feet, the D4 signal from the
altitude encoder is not used, and need not be connected. The DME suppress contact may not be

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present in an aircraft without DME. It is also not strictly necessary to connect the lighting bus, and the
previous installation may have omitted these contacts.
The only required contacts for basic functionality are the power contact, the two ground contacts, and
the 9 lower altitude encoder inputs.
Pin N – the lower left pin when viewed from the front – is not normally used in a KT76A or KT78A
installation and should be empty. Pin N is used for bench testing of the TT31 and MUST NOT be
connected in the aircraft – it is marked “Do Not Connect” in the diagram above. Confirm that there is
no connection to pin N before continuing with the installation.
4.4.6.3 Inspect Antenna Connector
Using the inspection light, examine the coaxial connector at the rear of the tray. Check that the
connector is secure and that all the leaf contacts on the outer surface are intact.
4.4.7 Non KT76A Replacement
4.4.7.1 Replace Interface Connector
Before rewiring the connector, establish the relationship between pin positions on the transponder
being removed and the TT31, referring to the wiring diagram in section 3.5.
Rewire the crimp connector in accordance with the TT31 installation manual.
4.4.7.2 Inspect/Replace Antenna Connector
Inspect the antenna connector removed from the previous transponder tray. Although most
transponders use compatible blind-mate BNC connectors it may be necessary to replace the existing
antenna connector if it is damaged or incompatible with the TT31.
4.4.7.3 Replace Mounting Tray
Fit the connectors to the TT31 mounting tray and install the tray in accordance with the TT31
installation manual.
4.4.8 Install TT31
Install the TT31 transponder. Check that the camlock is in the correct orientation before inserting the
TT31 in the tray; tighten with the Allen key. Do not over tighten.
4.4.9 Commission Transponder
Apply power. The TT31 should light up and – assuming this is the first installation – will
automatically start the installation setup process.
Continue with the setup process by entering the Mode S address and other parameters in accordance
with the TT31 Installation Manual.
4.5 Post-installation Test
The following post-installation test procedure shall be followed.
4.5.1 Equipment Function
Verify that the proper mechanical and electrical connections have been made. Exercise the setup and
test software to confirm correct operation of the aircraft lighting bus input (where fitted).

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Operate each of the controls and verify that each performs the intended function.
4.5.2 Interference Effects
With the transponder powered on, operate each of the other electrically operated aircraft systems to
determine that no significant interference effects are present.
4.5.3 Ramp Test
Using the transponder ramp test set, verify the following parameters. Note that actual procedures may
vary according to the test set specific operating instructions; many test sets will execute the tests listed
here in a semi-automated sequence, and will report the answers directly or as a Pass/Fail indication.
4.5.3.1 Reply Frequency
Verify that the reply frequency is 1090 ± 1 MHz.
4.5.3.2 Pressure Altitude Transmission
Verify using the pitot/static system test set that altitudes are correctly reported by the transponder.
Tests for each code segment of the encoder (1000, 2300, 2500, 3800, 4100, 4300, 4800, 6800 and
14800 feet) are sufficient to ensure proper operation of each altitude code input. Verify that the
altitude reported is within +- 125 feet of the supplied altitude.
NOTE: Precautions must be taken during altitude reporting tests to prevent nuisance ACAS Traffic
Advisories and ACAS Resolution Advisories to aircraft flying in the area.
4.5.3.3 Receiver Sensitivity
Verify that for Mode A/C interrogations the receiver sensitivity of the transponder at the antenna is -73
dBm ± 4 dB.
Verify that for Mode S interrogations the sensitivity of the transponder at the antenna is -74 dBm,
±3dB.
4.5.3.4 Transmitter Power Output
Verify that the transponder has a peak pulse power at the antenna of at least +21 dBW (125 Watts).
4.5.3.5 Received Reply
Interrogate the transponder with UF=11 (Mode S Only All-Call) and record the announced address in
the reply. Verify that the address matches the assigned address for this airframe.
4.5.3.6 Airspeed Fixed Field
Interrogate the transponder to confirm the maximum airspeed reported is correctly set.
4.5.3.7 Aircraft Identification
Interrogate the transponder with UF=4 or 5, and correct address, with RR=18. Verify that the
equipment correctly reports the aircraft call sign in the MB field of the reply.

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5. Compliance Statement
CS 23
Para Requirement Compliance References
23.1301 (a) Installed equipment to be of a
design appropriate to its
intended function.
TT31 is approved under
ETSO 2c112b. Review of
certification basis in DDP
completed.
TT31 DDP.
23.1301 (b) Be labeled as to its
identification, function or
operating limitations.
All controls are adequately
labeled. No limitations are
recorded.
ETSO compliance is shown
on the product identification
label.
TT31 installation
manual.
23.1301 (c) Be installed according to
specified limitations Review of environmental testing, deviations and
limitations in DDP completed.
TT31 DDP.
23.1301 (d) Function properly when
installed. System tested by ground tests on completion Section 4.5 of accomplishment
instructions
23.1309 (a) System must not adversely
affect existing systems The system does not interface with any other system except
dedicated altitude encoder.
Installation is physically
separate from other systems.
EMI tests carried out post-
installation.
Section 3.5, Wiring
Diagram.
Section 4.5.2 of
accomplishment
instructions.
23.1351(a) Electrical system capacity Existing 3 A circuit breaker
used supplying nominal 0.45A
load. Wire gauge 20
appropriate.
New equipment replaces load
of 1.1A with load of 0.45A.
Battery endurance increased
as a result.
TT31 Installation
Manual.
23.1357 Circuit Protective Devices Existing circuit breaker used -
inspected as part of
modification.
Section 4.4.1 of
accomplishment
instructions.
23.1431(a) Environmental conditions
must be considered. Section 3.4.3, review of environmental testing. TT31 DDP.

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23.1431(b) Not adversely affect
simultaneous operation of
other radio or electronic
systems or units.
EMI tests carried out post-
installation. Section 4.5.2 of accomplishment
instructions.
23.1529 Instructions for Continued
Airworthiness On condition maintenance used; instructions listed in
installation manual.
Installation Manual
Section 9.
CAP747,
GR6 Battery duration not less than 30 minutes New equipment replaces load of 1.1A with load of 0.45A.
Battery endurance increased
as a result.
TT31 Installation
Manual.
A transponder installation carried out in accordance with this Minor Modification will meet the
requirements of TGL 13 Rev 1 – Certification of Mode S Transponder Systems for Elementary
Surveillance. This compliance is dependent on the performance of the existing altitude encoder, which
must meet the requirements of ICAO Annex 10, Vol IV, 3.1.1.7.12.2.4.
TGL 13 Ref Requirement Compliance
Section 7,
and Table 1. Provide Aircraft Identification, Capability Report, Pressure Altitude
and Flight Status
COMPLIANT
With the exception of Pressure Altitude all these
(including for the avoidance of doubt, the Flight Status
requirement) are provided directly by the TT31
transponder.
Pressure Altitude compliance depends on the
performance of the altitude encoder.
8.1 Mode S Address COMPLIANT
Satisfied by assignment from National Authority of
Aircraft Registration.
8.2 Aircraft >5,700kgs or TAS >250kts
must operate with transponder
antenna diversity
COMPLIANT
Aircraft MTOW less than 5,700kgs and TAS less than
250kts. Antenna diversity is not required.
8.3 Transponder peak pulse power to be
ICAO Annex 10, Volume IV,
Amendment 77 compliant.
COMPLIANT
The T31 output power is in excess of 21.0 dBW and
less than 27.0 dBW.
8.4 Transponder and ACAS antenna
location need to satisfy physical
separation limits
COMPLIANT
Not applicable to this modification.
8.5 Pressure altitude source to be
obtained from a monitored air data
sensor in either databus or synchro
format, ideally the same source as the
pilot’s cockpit display.
COMPLIANT
The Altitude Encoder is fed from the same static source
as the pilot’s altimeter. Encoded altitude readout is
available on Transponder display.
8.6 Where Gillham is used a detection of
source/encoding failure must be COMPLIANT – ALTERNATE MEANS
Only a single Gillham encoded altitude source is

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provided. provided by this installation, so a comparator function
is not available.
Provision is made in the Instructions for Continued
Airworthiness (see section 3.3) of this Minor Change to
ensure periodic checking of the integrity of the Gillham
interface as required by EASA AD 2006-0265.
Encoded altitude is displayed to the pilot by the TT31.
Complete failure of the altitude source is flagged by the
transponder. Erroneous indications can be identified by
the pilot.
8.7 Transponder must indicate the correct
altitude resolution according to the
altitude source.
COMPLIANT
This modification uses a parallel altitude source and
therefore only 100 foot resolution is available. The
TT31 correctly displays and transmits this data.
8.8 Simultaneous operation of both
transponders must be prevented. COMPLIANT
Only a single transponder is installed in this Minor
Change.
9.1 Transponder will meet the minimum
requirements for Elementary
Surveillance (ELS)
COMPLIANT
This is a single transponder installation. The TT31
transponder is ELS compliant.
9.2 Certification standard for transponder
is JTSO-2C112a including SI
functionality as required by ICAO
Annex 10 Amendment 77.
COMPLIANT
The TT31 is certified to ETSO 2C112b, which adopts
ED-73B as a Minimum Operational Performance
Specification and includes compliance to Annex 10
amendment 77.
9.3 The applicant shall submit:
(a) a TGL 13 compliance statement.
(b) a statement showing compliance
with airworthiness requirements for
installation.
(c) safety analysis of transponder
data source interfaces.
COMPLIANT
(a) this document
(b) refer to the airworthiness compliance matrix for
this Minor Change
(c) refer to 23.1309 statement in the compliance matrix
for this Minor Change
9.4 Following Mode S System
functionality must be demonstrated:
System operation
ICAO 24-bit address in transmitted
response
Data in transmitted response
Function of system fault detectors
COMPLIANT
Ground testing is described in section 4.5 above.
12.1 Maintenance of altitude reporting
transponders should be suitably
screened.
COMPLIANT
Testing detailed in section 4.5.3.2 recommends
appropriate precautions to avoid interference.
12.2 Maintenance should include a COMPLIANT

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periodic check of aircraft derived
data including 24-bit address or in the
event of a change of registration of
the aircraft.
Testing detailed in Instructions for Continued
Airworthiness (see 3.3) requires testing, which includes
24-bit address and aircraft derived data, every 24
months.
12.4 Testing of Gillham code data should
be based on the transition points as
defined in Annex 2 of TGL13
COMPLIANT
Encoder testing detailed in Instructions for Continued
Airworthiness (section 3.3) and section 4.5.3.2
complies with Annex 2.