U.S.

Department
of Transportation
Advisory
Federal Aviation
Administration Circular
Subject: Altitude Reporting Equipment and Date: 2/15/17 AC No: 43-6D
Transponder System Maintenance Initiated by: AFS-300 Change:
and Inspection Practices

1 PURPOSE. This advisory circular (AC) provides information concerning acceptable

methods of testing altimeters, static systems, altitude encoders, and air traffic control
(ATC) transponder systems (ATCTS). This guidance also applies to the above articles,
but does not include all requirements for testing the article, when part of 1090 megahertz
(MHz) Extended Squitter (ES) or Universal Access Transceiver (UAT) Automatic
Dependent Surveillance-Broadcast (ADS-B) systems. Like all advisory material, this AC
is not in itself mandatory and does not constitute a regulation. It provides a means, but not
the only means, of testing at the time of original installation, after performing repairs, or
during scheduled recertification. Where indicated, this AC ensures compliance with
regulatory requirements. Operators may elect to follow an alternative method that the
Federal Aviation Administration (FAA) has found acceptable.

2 CANCELLATION. This AC cancels AC 43-6C CHG 1, Altitude Reporting Equipment

and Transponder System Maintenance and Inspection Practices, dated September 17,
2012.

3 RELATED TITLE 14 OF THE CODE OF FEDERAL REGULATIONS (14 CFR).

• Part 25, § 25.1325;

• Part 27, § 27.1325;
• Part 29, § 29.1325;
• Part 43, §§ 43.3, 43.5, 43.9, and 43.13, and Appendices E and F;
• Part 91, §§ 91.215, 91.217, 91.225, 91.227, 91.411, and 91.413; and
• Part 145, § 145.109.

4 RELATED READING MATERIAL (current editions). You can find this AC on the
FAA website at http://www.faa.gov/regulations_policies/advisory_circulars. You can find
the following Technical Standard Orders (TSOs) on the FAA’s Regulatory and Guidance
Library (RGL) website at http://rgl.faa.gov:

• AC 43-2, Minimum Barometry for Calibration and Test of Atmospheric Pressure

Instruments;
• TSO-C10, Aircraft Altimeter, Pressure Actuated, Sensitive Type;
2/15/17 AC 43-6D

• TSO-C74d, Air Traffic Control Radar Beacon System (ATCRBS) Airborne

Equipment;
• TSO-C88b, Automatic Pressure Altitude Reporting Code-Generating Equipment;
• TSO-C112e, Air Traffic Control Radar Beacon System/Mode Select
(ATCRBS/Mode S) Airborne Equipment;
• TSO-C166b, Extended Squitter Automatic Dependent Surveillance-Broadcast
(ADS-B) and Traffic Information Service-Broadcast (TIS-B) Equipment
Operating on the Radio Frequency of 1090 Megahertz (MHz);
• TSO-C195b, Avionics Supporting Automatic Dependent Surveillance-Broadcast
(ADS-B) Aircraft Surveillance Applications (ASA);
• TSO-C154c, Universal Access Transceiver (UAT) Automatic Dependent
Surveillance-Broadcast (ADS-B) Equipment Operating on Frequency of 978
MHz; and
• TSO-C106, Air Data Computer.

5 BACKGROUND. Altitude reporting equipment and transponder systems are significant

elements for the safe operation of aircraft in the National Airspace System (NAS). ATC
monitors aircraft movements and ensures positive separation with identity and
three-dimensional positional information obtained from individual aircraft altitude
reporting equipment and transponder systems. Traffic Alert and Collision Avoidance
Systems (TCAS), installed in nearly all commercial passenger air carrier aircraft and
many other aircraft, depend on reliable and accurate altitude reporting equipment and
transponder systems in surrounding aircraft. ADS-B systems are dependent on aircraft
being equipped with reliable and accurate position-reporting equipment and transponder
systems.

6 INSTALLATION. Any appropriately rated person (as specified in § 43.3) may perform
an aircraft alteration that consists of installing or reinstalling equipment (following
maintenance). The equipment may consist of an encoding or non-encoding altimeter, air
data system components, remote encoding devices (blind encoder), or transponders.
ADS-B equipment includes an ADS-B transponder or UAT and the associated source
system’s components to broadcast the aircraft’s identity, altitude, velocity, and other
information. Only after the performance of the suitable functional check to determine that
the altered system will perform its intended functions can an appropriately rated person
grant approval of the aircraft for return to service. Additionally, if applicable, test and
inspections requirements described in §§ 91.411 and 91.413 and part 43 appendices E and
F must be satisfied.

Note: For those situations where the operator uses the services of a certificated
repair station (CRS) for the installation of an altitude reporting system or for any
component part of such a system, a repair station should possess either an
Airframe, Radio Class 3 and Instrument Class 1 (both ratings are required for the
installation of a complete system if an Airframe rating is not held), or a Limited

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class rating. The repair station ratings may be limited to specific makes and
models (M/M) of airframes, transponders, altimeters, or blind encoders.

6.1 Installation Preparation. Before attempting the installation or reinstallation of an

aircraft altitude reporting system, the person installing should ensure the following:

1. That required test equipment, technical data, and qualified personnel are
available to perform a static system leak check if the system is opened or
closed, and other testing as required by § 91.411(a)(2) to verify the integrity
of the newly installed or altered system.

Note: Combined probe and sensor assemblies that may be installed without
opening or closing the static system do not require testing under § 91.411(a)(2).

2. That the capability exists to determine the pressure altitude transmitted by the
transponder as referenced against the primary required or pilot’s altimeter
display (refer to § 91.217(a)(2) for altitude reference).

3. That appropriately rated and qualified personnel are available to perform any
necessary structural modifications and appropriate tests and inspections
following maintenance or alterations.

4. That an appropriately rated person returns the aircraft to service following

alteration or maintenance.

5. That data is available to enable return to service of the aircraft following

substitution of an encoding altimeter or remote encoding device for the
equipment currently listed on the aircraft’s approved equipment listing. The
person installing may use equipment that meets the requirements of the
appropriate TSO and is certificated to the maximum operating altitude of the
intended aircraft to replace equipment listed on the aircraft’s approved
equipment list with no further data approval.

6.2 Installation and Inspection Data. Each installation and inspection should be in
accordance with appropriate data and the work that appropriately rated persons performed
or supervised. Data may consist of manufacturer’s drawings or Service Bulletins (SB)
that list acceptable replacements or equipment substitutes if specifically authorized by the
Administrator.

6.3 For Mode Select (Mode S) Transponder and UAT Installation. To facilitate
programming of transponders or UATs which require input of the raw Mode S octal code,
the aircraft identification code may be obtained from the aircraft registration certificate or
from the Aircraft Registration Branch (AFS-750). This information is also available
online at the FAA’s Aircraft Certification Registration Inquiry page or from the Aircraft
Registration Branch (AFS-750), P.O. Box 25504 Oklahoma City, OK 73125-0504,
telephone: (405) 954-3116.

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7 MAINTENANCE. Any appropriately rated person (as specified in § 43.3) may perform
maintenance or preventive maintenance on an aircraft altitude reporting and transponder
system. This work may include the removal of and/or replacement with an identical
system component. An appropriately rated person can approve the aircraft for return to
service only after the performance of a suitable functional check of the entire system (as
installed in the aircraft) to determine that the system, as well as the component, will
perform its intended functions.

Note 1: For ADS-B systems, alteration of the transponder or any of the source
system components may impact the system’s ability to meet the requirements of
§ 91.227. For example, when initially certified, the ADS-B system shows that it
meets the latency requirements. It is impossible to test or measure the latency
requirements and they can only be determined by using technical data from the
ADS-B transponder manufacturer, the position source’s manufacturer, and an
analysis of the system interconnection. A change to any one of these articles
could result in a change in the system data latency. Any alteration of the
transponder, the position source, other source system components, or their
interconnectivity must include an analysis and evaluation showing that the
alteration does not impact the system’s compliance with § 91.227. Refer to
AC 20-165, Airworthiness Approval of Automatic Dependent
Surveillance-Broadcast OUT Systems, for detailed information on certification
requirements of the system.

Note 2: In the instance when the operator uses the services of a CRS for the
maintenance, preventive maintenance, or replacement of an altitude reporting and
transponder system, or for any portion of such a system, the repair station should
be appropriately rated as described in the note in paragraph 6. Repair station
ratings may be limited to specific M/Ms of airframes, transponders, or encoding
altimeters.

8 FUNCTIONAL TESTING. An appropriately rated person (as specified in § 43.3) may

make major repairs (as specified in part 43 appendix A) to component parts of an aircraft
altitude reporting and transponder system. Such repairs require testing and inspection
before return to service. Following reinstallation into the aircraft, an appropriately rated
person will test the entire system for proper function to ensure that it performs its
intended functions. The scope of testing required in determining system functionality is
dependent on the repaired component parts. See Appendix A, Table A-3, Test
Association Matrix, to determine the appropriate tests necessary.

9 RECOMMENDED TEST PROCEDURES. The following test procedures provide one

way, but not the only way, of demonstrating altitude reporting and transponder system
performance and of testing individual components. The test procedures noted below are
adequate to demonstrate compliance with the maintenance requirements of §§ 91.411 and
91.413.

9.1 Static Pressure System Test. Performance of this test on all instruments that rely on
connected static air will ensure component leak integrity and that no leaks occurred while

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making connections to the encoding altimeter, blind encoder, or other instruments. This
procedure is one method of demonstrating compliance with the requirements within
§ 91.411(a)(2). Section 91.411(b) lists the persons authorized to perform this test. While
static systems contained within an article that the authorized person did not open or close
during article installation do not require post installation testing in accordance with
§ 91.411(a)(2), the FAA recommends completion of the items in paragraphs 9.1.1 and
9.1.2.

Note: Damage may occur to other aircraft instruments, such as the vertical speed
indicator (VSI), if the altitude rate changes faster than the limit of the installed
instruments.

9.1.1 Visual Inspection. Visually inspect the ports, tubing, accessories, and instruments
connected to the static system, and repair or replace those parts that are defective
(e.g., broken “B” nuts, cracked flare sleeves, deteriorated flexible tubing, bad valves).
Purge the system, if necessary, to remove foreign matter that may have accumulated in
the tubing.

Caution—Be sure to remove all pitot pressure and static air connections to every
instrument before purging the system tubing.

9.1.2 Static Port Heater. Check the static port heater, if so equipped, to ensure proper operation.
If a manufacturer has not developed a specific procedure, confirm static port heater
operation by noting ammeter current or that the pitot tube or static port becomes hot.

Note: It is unwise to touch an operational pitot probe to verify proper heating

operation. Pitot probes are capable of reaching extremely high temperatures
quickly.

9.1.3 Multiple Static Systems. When an aircraft has more than one static system, separately test
each system that was opened or closed to ensure their independence and that the leak rate
for each system is within tolerances established in Appendix E or § 25.1325, whichever is
applicable. Static systems that the authorized person did not open or close do not require
testing.

9.1.4 Connecting the Test Equipment. Connect the test equipment directly to the static ports, if
practicable. Otherwise, connect to a static system drain or tee connection and seal off the
static ports. If the test equipment connects to the static system at any point other than the
static port, it should be at a point where the authorized person may readily inspect the
connection for system integrity after returning the system to its normal configuration.
Remove all static port seals after completion of the static system test.

9.1.5 Testing the Alternate Static System. Test the alternate static system at field elevation to
ensure the selection valve functions, if installed. If the reading of the altimeter when on
the alternate static pressure system differs from the primary system by more than 50 feet,
provide a correction card for the alternate static system in accordance with §§ 27.1325
and 29.1325.

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9.1.6 Unpressurized Airplanes. For unpressurized airplanes, conduct the static pressure system
proof test to the standards prescribed in Appendix E.1 or § 25.1325(c)(2)(i), as
applicable.

9.1.7 Pressurized Airplanes. For pressurized airplanes, conduct the static pressure system proof
test to the standards prescribed in Appendix E.2 or § 25.1325(c)(2)(ii), as applicable (see
paragraph 13 for precautions).

9.1.7.1 An authorized person may use an accurate vacuum gauge referenced to

atmospheric pressure and connected to the static pressure system to measure
the equivalent cabin differential pressure.

9.1.7.2 An authorized person may use either the altimeter in the aircraft under test or
that in the test equipment as a vacuum gauge, provided that he or she converts
barometric pressure, measured in inches of mercury (inHg), to pressure in
pounds per square inch (psi). A convenient formula for this conversion is:

inHg
psi =
2.036
9.1.7.3 The following steps are the suggested way of using the altimeter as a vacuum
gauge:

1. Step 1. Convert the actual local barometric pressure (not reduced to

sea level) to psi.
2. Step 2. Subtract the approved maximum cabin differential pressure
in psi, located in the applicable aircraft service document, from the
psi value obtained in Step 1.
3. Step 3. Convert the psi value obtained in Step 2 to inHg using the
formula: inHg = psi x 2.036
4. Step 4. The test pressure expressed in inHg can be converted to test
altitude in feet using Table IV, Geopotential Altitude, English
Altitudes, contained in the document titled U.S. Standard
Atmosphere, 1976 (Stock No. 003-017-00323-0), which is
available from Superintendent of Documents, U.S. Government
Printing Office (GPO), Washington, D.C.
psi = ___________________
EXAMPLE:
inHg = 25.39
Approved Maximum Cabin Differential Pressure = 5.3
25.39 inHg
Step 1. = 12.47 psi.
2.036
Step 2. 12.47 psi − 5.3 psi = 7.17 psi.
Step 3. 7.17 psi x 2.036 = 14.60 inHg.
Step 4. 14.60 inHg = 18,600 ft (altitude).

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9.2 Altimeter Certification Test. This test ensures that an altimeter is calibrated and
acceptable for use in the NAS. This procedure is adequate to ensure proper operation, but
may not fulfill all the requirements of a manufacturer’s minimum performance test
required after maintenance of an altimeter.

9.2.1 Persons Authorized to Conduct Altimeter Testing. Section 91.411(b) lists the persons
authorized to conduct the altimeter test. A certificated mechanic with an Airframe rating
is only authorized to perform static leak testing and is not authorized to perform altimeter
testing.

9.2.2 Part 43 Appendix E Test Procedure. Perform the test procedure in part 43 appendix E
paragraph (b). This procedure demonstrates compliance with the maintenance
requirements of § 91.411. An authorized person may test altimeters that are the air data
computer type with associated computing systems (or that incorporate air data correction
internally) in a manner and to specifications developed by the manufacturer that are
acceptable to the Administrator.

9.2.3 Maximum In-Tolerance Altitude. An authorized person should test the altimeter on the
bench to the maximum altitude of its design specification. He or she should record the
date of the actual altimeter test and maximum in-tolerance altitude on the altimeter or on
a label attached to the altimeter. An authorized person may put into service an altimeter
found to have a lower maximum in-tolerance altitude than its design specification
provided that the in-tolerance altitude is at least that of the maximum certificated altitude
of the aircraft it will be installed into, or if there is an operational limitation on the aircraft
and noted by placard.

Note: For altimeters of the air data computer type that consist of multiple
components, an authorized person should mark or attach the label to the air data
computer unless impractical. If an authorized person cannot mark or attach the
label, he or she should provide the information on the accompanying Authorized
Release Certificate, FAA Form 8130-3, Airworthiness Approval Tag. When
testing is performed on the aircraft and entries are made directly into the
maintenance log or permanent records, a label is not required.

9.3 Altimeter Field Elevation Verification. Normal installation of an altimeter or encoding

altimeter should not alter its calibration or certification basis. A field elevation
verification of performance is adequate after installation to ensure safe operation within
the NAS.

9.3.1 Altimeter Field Elevation Verification Observation. Section 91.411(b) lists the persons
authorized to conduct the altimeter test. A certificated mechanic with airframe rating is
only authorized to perform static leak testing and is not authorized to perform altimeter
testing. The altimeter field elevation verification is an observation made at the time of
installation and in the context of § 91.411(b), not a test of the altimeter.

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9.3.2 Comparison of Altitude Displayed. Compare the altitude displayed on the subject
altimeter when referenced to 29.92 inHg (1013.2 millibars) with that of a calibrated
reference altimeter (as described in paragraphs 11.1 or 11.2) and ensure agreement within
± 20 feet.

9.4 Pressure Altitude Correspondence Test. The pressure altitude correspondence test
ensures that the altitude reporting equipment associated with a radar beacon transponder,
1090 MHz ES, or UAT ADS-B system is calibrated to transmit altitude data
corresponding within 125 feet (on a 95-percent probability basis) of the indicated or
calibrated datum of the altimeter normally used to maintain flight altitude, as required by
§ 91.217(a)(2). This procedure is adequate to ensure proper operation of a pressure
altitude encoding device installed in a transponder system but may not fulfill all the
requirements of a manufacturer’s minimum performance test required after maintenance
of an encoder. The following test procedure (as specified in part 43 appendix E paragraph
(c)) demonstrates compliance with the maintenance requirements of § 91.411.

9.4.1 Persons Authorized to Conduct Altimeter Testing. Section 91.411(b) lists the persons
authorized to conduct the altimeter test. A certificated mechanic with airframe rating is
only authorized to perform static leak testing and is not authorized to perform altimeter or
encoder testing.

9.4.2 Procedures:

1. Connect the transponder test set so as not to radiate an interfering signal. To

accomplish this, connect directly to the transponder antenna terminal or to the
antenna end of the system transmission line, or use a test apparatus to shield
the antenna.

Note: It also may be necessary to isolate or shield the antennas of a diversity

transponder that are not under test to prevent interference.

2. Check to ensure that the equipment transmits only the framing pulses (F1 and
F2) in response to Mode C interrogations when the altitude reporting feature
is turned off.

3. Alternately interrogate the transponder on Mode 3/A and Mode C while

observing the pulse train output, or the decoded altitude display on those test
sets capable of decoding the pulse train.

4. Set the altimeter normally used to maintain flight altitude to 29.92 inHg
(1013.2 millibars). Verify that the ATC transponder altitude output and the
altimeter displayed altitude are within ± 125 feet.

5. For Gillham code altimeters, apply suction to the static system or directly to
the altimeter and compare transponder altitude output with
altimeter-displayed altitude at the test points in Appendix A, Table A-1,
Abbreviated Correspondence Test Altitude Information Pulse Positions, or
Appendix A, Table A-2, Correspondence Test Altitude Information Pulse

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Positions. Test each point while increasing altitude and while decreasing
altitude. If separate static systems serve altimeters and digitizers,
simultaneously apply identical pressures to each. Approach each test point
slowly, decreasing pressure for increasing altitude and vice versa, until a
transition to the test point value occurs in the digital output. Apply vibration
to a pneumatically driven altimeter when taking readings to reduce any error
due to friction. Record the altimeter reading at the instant of digital code
transition. Appendix A, Table A-3 provides a test association matrix to assist
in the determination of the appropriate test to perform.

Note: Item 5 under paragraph 9.4.2 is not required if a data bus provides altitude
information to the transponder in a digital form. The instructions in item 4 under
paragraph 9.4.2 ensure data integrity.

9.4.3 Encoding Digitizers. An authorized person should check encoding digitizers, which are
separate units (blind encoders) that have their own individual pressure sensor, against the
pilot’s altimeter upon installation to ensure that they meet the overall system accuracy. In
addition, where an installation allows for the blind encoder to be connected to a static
source other than the static source connected to the altimeter normally used to maintain
flight altitude, an authorized person should apply the following corrections during
certification of compliance to § 91.411(a):

9.4.3.1 An authorized person should determine and record the difference between
both static sources. (This information may be available from the original
aircraft certification data).

9.4.3.2 An authorized person should use the differences determined in

paragraph 9.4.3.1 (static source errors) as a correction factor when checking
for compliance with § 91.217(a)(2).

9.4.4 Altitude Encoding Devices Not Connected to an ATC Transponder or UAT. Altitude
encoding devices not connected to an ATC transponder or UAT do not require testing
under § 91.411; however, due to the importance of ensuring correspondence between all
pressure altitude systems, the FAA recommends testing in accordance with items 4 and 5
under paragraph 9.4.2, using Appendix A, Table A-1. An authorized person should test
units not equipped with a display or means of monitoring altitude input to ensure data
integrity, when possible.

9.5 Test and Inspection Requirements of Aircraft Certified for Operation Where
Reduced Vertical Separation Minimum (RVSM) is Applied. An appropriately rated
person(s) must follow the altimeter and static system testing requirements listed in the
manufacturer’s recommended aircraft maintenance program or instructions for continued
airworthiness (ICA) if applicable. The required test and inspections may not ensure
compliance with § 91.411, and he or she may need to complete the additional tests or
inspections listed in part 43 appendix E.

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9.6 Transponder Test and Inspection. This test ensures the proper operation of a
transponder in the NAS. This procedure is adequate to ensure proper operation of a
transponder system but may not fulfill all the requirements of a manufacturer’s minimum
performance test required after maintenance of a transponder. This procedure
demonstrates compliance with the maintenance requirements of § 91.413.

9.6.1 Persons Authorized to Conduct Transponder Testing. Section 91.413(c) lists the persons
authorized to conduct transponder tests and inspections.

9.6.2 Procedures:

1. Perform the test procedure in part 43 appendix F. For TSO-C112d and later
transponders, see Appendix C to determine the appropriate class reference.
Aircraft altitude information transmitted during testing may cause
interference to ATC facilities or other TCAS equipped aircraft. Take
appropriate measures to ensure interference is not generated.

2. Perform an ATC radar beacon system (ATCRBS) all-call test. Interrogate the
transponder with an ATCRBS (Mode A and C) all-call interrogation signal at
a nominal repetition rate of 235 interrogations per second and at a signal
level 3 dBm above receiver minimum trigger level. Adjust P4 pulse equal in
amplitude to P3 pulse and verify that the reply rate is equal to or greater than
223 replies per second. Verify proper response. An ATCRBS transponder
should reply to an ATCRBS all-call interrogation, a Mode S transponder
should not reply.

9.6.3 Bench-Testing Transponders. An authorized person may bench-test transponders to

part 43 appendix F specifications for compliance with § 91.413 and functionally check
them after installation in the aircraft, provided that during the bench check the
transponder operates into an antenna system presenting the same Voltage Standing Wave
Ratio (VSWR) characteristics and cable attenuation as that in the aircraft.

9.6.4 Removal and Replacement of Transponder Units. The accomplishment of the removal
and replacement of transponder units without repeating § 91.413 testing is possible;
however, the next 24-month test and inspection date will be relative to the replacement
transponder’s previous § 91.413(a) test and inspection. An authorized person will perform
a manufacturer’s minimum performance test before return to service. Any time an
authorized person removes and reconnects the aircraft connections to a transponder, he or
she must test each altitude reporting Gillham code line or the digital data bus (if
equipped) for integrity of connection. Successful reporting of a single altitude may verify
the integrity of connection for systems using a digital data bus to convey altitude
information to a transponder. Performing an abbreviated correspondence test at the test
points of Appendix A, Table A-1 using the installed automatic pressure altitude encoding
device or through the use of an encoder substation test unit capable of simulation of the
Appendix A, Table A-1 altitudes may verify the integrity of connection for systems using
Gillham code connections.

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10 SECTIONS 91.411 AND 91.413 COMPLIANCE FLOWCHART. See the compliance

flowchart, Sections 91.411 and 91.413 Compliance Flowchart, provided in Appendix D
to determine compliance with §§ 91.411 and 91.413. Since both sections contain multiple
requirements, compliance is not ensured until all decision points have been satisfied.

11 ALTIMETER TEST EQUIPMENT. The following test equipment is acceptable for

testing altimeters:

11.1 Barometers. Mercurial, aneroid, or digital barometers with accuracy specified in and in
accordance with AC 43-2 and newer equipment with accuracies that meet RVSM
tolerances.

11.2 Portable Test Equipment. High accuracy portable test equipment (with correction card,
if appropriate) maintained in accordance with § 145.109(b). Calibration checks of the test
equipment in accordance with the following schedule provide a satisfactory level of
performance:

1. Every 30 days, after initial calibration, the repair station should check the
equipment for accuracy against:

• A barometer described in paragraph 11.1; or

• An altimeter (with appropriate correction card) that the repair station has
calibrated within the past 30 days against a barometer described in
paragraph 11.1.
2. Before use, the repair station should check the equipment for proper
operations within calibration limits at station pressure using a digital,
aneroid, or mercurial barometer, or in accordance with paragraph 11.1.

3. It is possible to extend the 30-day calibration period in paragraph 11.2.1,

provided the calibration records of the individual test equipment reflect
continued accuracy, as specified by technical information or
recommendations of the equipment manufacturer.

12 MAINTENANCE RECORD ENTRY. The following example of a permanent

maintenance record entry will be satisfactory for compliance with § 43.9:

EXAMPLE:

I certify that the altimeter and static system tests required by 14 CFR part 91, § 91.411
and transponder tests, including data correspondence, required by § 91.413, have been
performed and found to comply with 14 CFR part 43, appendices E and F.

Altimeter Model ___________________ Serial No. ________________

Transponder Model ________________ Serial No. ________________
Encoder Model ____________________ Serial No. ________________

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The altimeter was tested to______feet on (date of altimeter test).

The transponder was tested on (date of transponder test).
Encoder data correspondence was tested to______feet on (date of correspondence test).
Static system leak tested on (date of static test).
Signature _______________________
Certificate Number _______________

13 TESTING PRECAUTIONS. This paragraph contains precautions that technicians

should take during tests to avoid damage to the sensitive instruments connected to static
systems.

13.1 Determining Design Limits. Before testing any static system, determine that the
instruments attached to it will not exceed design limits during the test. To determine this,
locate and identify all instruments attached to the system. In addition to the altimeter,
airspeed, and rate of climb, many aircraft use static pressure for the operation of
autopilots, flight recorders, air data computers, altitude reporting digitizers, etc. The use
of a static system diagram of the aircraft involved may be helpful in locating all of the
instruments. If a diagram is not available, locate the instruments by tracing the physical
installation.

13.2 Static System and Pitot System. Damage can occur to instruments that are connected to
both the static system and pitot system when only the static system is evacuated. These
instruments may exceed the maximum design differential pressure. One method to
prevent this type of damage is to tie both the pitot and static systems together when
conducting static system checks. This should result in zero differential pressure
regardless of the degree of static system evacuation. Note that a leak in either system will
affect the test process.

13.3 Accidental Disconnection. Technicians should take safeguards to prevent accidental

disconnection of the test equipment plumbing from the aircraft or the test equipment
while evacuating the static system. The resultant sudden pressure change may damage
both the test instruments and the aircraft instruments. The aircraft static system should be
returned to ambient pressures before disconnecting static test equipment from the system.

13.4 Marking Blockage Devices. If a technician covers the static ports for the purpose of
testing, it is recommended that he or she attach a bright-colored tape (red or orange) or
similar method of warning to the blockage device.

13.5 Barometric Correlation Adjustment. Technicians should not perform a barometer

correlation adjustment of an altimeter in the field; changing this adjustment will nullify
the correspondence between the altimeter and its encoding digitizer or the associated
blind encoder. Barometric correlation adjustment of an altimeter is a repair action and
technicians should not perform this action without an appropriate instrument or repair
station limited rating.

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13.6 Altimeter Jerkiness. Some altimeters may exhibit a tendency toward jerkiness (when not
under vibration). If the jerkiness appears excessive, then conduct a friction test as
described in part 43 appendix E.

13.7 Encoder Errors. Some encoders may exhibit errors and excessive drift during initial
warmup. Lack of correspondence within 125 feet between altimeter and encoder, if
observed 5 minutes after initial power turn on or later, should be considered failure to
meet the standards of § 91.411(a). Only after the repair of the encoder should subsequent
retesting occur.

13.8 Encoder Calibration. Adjustment of encoder calibration, including high or low

reference settings, is considered a repair action not part of certification and requires an
appropriate instrument or limited rating. Limited ratings authorizing the test of encoders
only do not include authorization for this adjustment.

Note: No field adjustments are allowed unless the repair station is rated for this
adjustment.

14 INSTALLATION AND TESTING CONCERNS.

14.1 Altitude Encoding Capability. The blind encoder, altimeter, air data system
components, and encoding altimeter should have an altitude encoding capability of at
least the service ceiling or maximum certificated altitude of the aircraft. If the altitude
reporting system will not function throughout the aircraft operational envelope (up to the
aircraft maximum operating altitude), install a placard stating the aircraft altitude
limitation.

14.2 Installation Location. The altimeter and encoder must be installed in the same
environmental location unless the person performing the installation obtains specific FAA
engineering approval to deviate from this requirement.

14.3 Shop Testing and Functional Tests. Altitude reporting system installations (either blind
encoder or encoding altimeter types) may be tested in the shop by a technician for
correspondence (using the transponder decoded output) and then functionally checked
after installation in the aircraft, provided the transponder encoding digitizer, altimeter,
wiring harness, and coaxial cable are either installed in the aircraft or accurately
compensated for.

14.4 Deterioration and Repairs. Transponder antenna systems may deteriorate to the point
that they pull transponder frequency out of tolerance. If this occurs, then the repair station
should notify the operator that the antenna system requires repair. The practice of
offsetting transponder output frequency to compensate for antenna system pull is not
acceptable.

14.5 Purging. Whenever there is reason to suspect that the static lines are blocked, purge them
before performing the static pressure system test. Besides the obvious benefits of
removing foreign objects from the lines, purging may keep such objects from entering the

13
2/15/17 AC 43-6D

test equipment. Since purging applies positive pressure to lines, take the following
precautions:

1. Disconnect all instruments and air data sensors;

2. Cap off those lines not being purged;

3. Restrain hoses that can whip due to purge pressure;

4. Ensure that lines are clear by feeling discharge pressure at ports; and

5. Clean system drains and traps after purging, since they can act as a sump for
foreign material.

14.6 Quick Disconnect Static Line Ports. When opening quick disconnect static line ports,
you should accomplish a visual inspection or static pressure system test.

John S. Duncan
Director, Flight Standards Service

14
 2/15/17 AC 43-6D
Appendix A
APPENDIX A. TABLES

TABLE A-1. ABBREVIATED CORRESPONDENCE TEST ALTITUDE

INFORMATION PULSE POSITION

RANGE PULSE POSITION

(0 or I in a pulse position indicates absence or presence of a pulse, respectively)

INCREMENTS
(FEET) D4 A1 A2 A4 B1 B2 B4 C1 C2 C4

-1000 0 0 0 0 0 0 0 0 1 0
-900 0 0 0 0 0 0 0 1 1 0
-700 0 0 0 0 0 0 1 1 0 0
-400 0 0 0 0 0 0 1 0 1 1
-200 0 0 0 0 0 1 1 0 0 1
800 0 0 0 0 1 1 0 0 0 1
2800 0 0 0 1 1 0 0 0 0 1
6800 0 0 1 1 0 0 0 0 0 1
14800 0 1 1 0 0 0 0 0 0 1
30800 1 1 0 0 0 0 0 0 0 1
Integrity of code lines is ensured through testing at these increments.

A-1
 2/15/17 AC 43-6D
Appendix A
TABLE A-2. CORRESPONDENCE TEST ALTITUDE INFORMATION
PULSE POSITIONS
RANGE PULSE POSITION
(0 or 1 in a pulse position indicates absence or presence of a pulse respectively)
INCREMENTS
(FEET) D4 A1 A2 A4 B1 B2 B4 C1 C2 C4
- 1050 to - 950 0 0 0 0 0 0 0 0 1 0
- 50 to + 50 0 0 0 0 0 1 1 0 1 0
450 to 550 0 0 0 0 0 1 0 0 1 0
950 to 1050 0 0 0 0 1 1 0 0 1 0
1050 to 1150 0 0 0 0 1 1 0 1 1 0
1250 to 1350 0 0 0 0 1 1 1 1 0 0
1450 to 1550 0 0 0 0 1 1 1 0 1 0
1750 to 1850 0 0 0 0 1 0 1 0 0 1
1950 to 2050 0 0 0 0 1 0 1 0 1 0
2550 to 2650 0 0 0 0 1 0 0 0 1 1
2650 to 2750 0 0 0 0 1 0 0 0 0 1
2950 to 3050 0 0 0 1 1 0 0 0 1 0
3950 to 4050 0 0 0 1 1 1 1 0 1 0
5950 to 6050 0 0 0 1 0 0 1 0 1 0
6750 to 6850 0 0 1 1 0 0 0 0 0 1
7950 to 8050 0 0 1 1 0 1 1 0 1 0
9950 to 10050 0 0 1 1 1 0 1 0 1 0
11950 to 12050 0 0 1 0 1 1 1 0 1 0
13950 to 14050 0 0 1 0 0 0 1 0 1 0
14750 to 14850 0 1 1 0 0 0 0 0 0 1
15950 to 16050 0 1 1 0 0 1 1 0 1 0
17950 to 18050 0 1 1 0 1 0 1 0 1 0
19950 to 20050 0 1 1 1 1 1 1 0 1 0
21950 to 22050 0 1 1 1 0 0 1 0 1 0
24950 to 25050 0 1 0 1 1 1 0 0 1 0
29950 to 30050 0 1 0 0 0 0 1 0 1 0
30750 to 30850 1 1 0 0 0 0 0 0 0 1
34950 to 35050 1 1 0 1 1 0 0 0 1 0
39950 to 40050 1 1 1 1 0 1 1 0 1 0
44950 to 45050 1 1 1 0 0 1 0 0 1 0
49950 to 50050 1 0 1 0 1 0 1 0 1 0

A-2
 2/15/17 AC 43-6D
Appendix A
TABLE A-3. TEST ASSOCIATION MATRIX

The following table lists appropriate tests to assure system functionality after removal,
replacement, or installation of altitude reporting equipment and transponder system components.

COMPONENT DESCRIPTOR TESTs

Altimeter Pilot reference. 1. Field elevation verification.
2. Correspondence test.
3. Static leak test.
Altimeter Pilot reference. 1. Field elevation verification.
Matched to encoder prior to installation. 2. Abbreviated correspondence test.
3. Static leak test.
Altimeter Other than pilot reference. 1. Field elevation verification.
2. Static leak test.
Encoding altimeter Pilot reference. 1. Field elevation verification.
2. Abbreviated correspondence test.
3. Static leak test.
Encoding altimeter Other than pilot reference. 1. Field elevation verification.
2. Correspondence test.
3. Static leak test.
Encoding altimeter Other than pilot reference. 1. Field elevation verification.
Matched to pilot reference altimeter prior 2. Abbreviated correspondence test.
to installation. 3. Static leak test.
Blind altitude encoder Connected to transponder. 1. Correspondence test.
2. Static leak test.
Blind altitude encoder Connected to transponder. 1. Abbreviated correspondence test.
Matched to pilot reference altimeter prior 2. Static leak test.
to installation.

Blind altitude encoder Not connected to transponder. 1. Abbreviated correspondence test

(modified).
2. Static leak test.
Combined static probe/air Installed as single component without 1. Single indicated altitude and
data computer opening static system. transponder pressure altitude
correspondence verification.
Transponder High reliability style connector system. 1. Transponder test and inspection.

Transponder Non-high reliability style connector 1. Transponder test and inspection.

system. 2. Abbreviated correspondence test.

A-3
 2/15/17 AC 43-6D
Appendix B
APPENDIX B. DEFINITIONS

1. Approved. Unless used with reference to another person, means approved by the Administrator.

2. Blind Encoder (Digitizer). An altitude reporting encoder that is pressure operated, having no
altitude display; is not part of a pressure/altitude indicating device or system; does not contain an
external means for barometric setting; and may supply altitude reporting information to the air
traffic control (ATC) transponder, Global Positioning System (GPS), or other onboard system.

3. Calibrated Datum of the Altimeter. The correction applied via a specific calibration card
applicable to a specific altimeter to correct for instrument error (scale error) only.

4. Correspondence. The maximum absolute difference between altimeter display and encoder
output for a constant encoder output. The altimeter’s displayed pressure/altitude (referenced to
29.92) compared to encoded altitude output from the blind encoder or encoding altimeter for the
entire period (from the moment that the code output changes to a value to the moment the code
output changes to the next value while the pressure/altitude is changing).

5. Data. Drawings, sketches, stress analyses, reports, operating limitations, or photographs that
support or describe an alteration.

6. Encoding Altimeter (Pressure Altitude). An altitude indicator that displays to the pilot the
pressure/altitude sensed by the device and produces an altitude reporting output.

7. Gillham Encoder. An altitude reporting encoder that provides parallel data output employing
the Gillham (Grey) code.

8. Indicated Datum of the Altimeter. The altitude displayed by the altimeter when an ideal
absolute pressure is applied to the sensing member of the altimeter and not corrected for
instrument error (scale error), nor corrected for static source error.

9. Matched Components. An altimeter and a blind encoder that have been tested and
calibrated together and, as a combination, meet the requirements of 14 CFR part 91,
§ 91.411(a).

10 High Reliability Style Connectors. Connectors designed to or meeting Military or similar

industry standards.

11. Pilot Reference Altimeter. The altimeter normally used to maintain flight altitude.

B-1
 2/15/17 AC 43-6D
Appendix C
APPENDIX C. TSO-C112C MARKING AND PART 43 APPENDIX F CLASS

C.1 The Technical Standard Order (TSO)-C112c, Air Traffic Control Radar Beacon System/Mode
Select (ATCBS/Mode S) Airborne Equipment, labeling scheme does not match the labeling
called out in 14 CFR part 43 appendix F. The new TSO labeling is based on RTCA, Inc. DO-
181D, Minimum Operational Performance Standards for Air Traffic Control Radar Beacon
System/Mode Select (ATCRBS/Mode S) Airborne Equipment, Section 1.4.6, rather than the
labeling that had been defined previously in TSO-C112.

C.2 To assist operators in complying with part 43, the following table shows the relationship
between the TSO-C112c (or later version) labeling scheme and the part 43 appendix F
requirement. Equipment approved under previous versions of TSO-C112 are not affected and
may continue to use the label from the equipment directly. Manufacturers of TSO-C112c
equipment are encouraged to include this cross-reference information in their operating guide
and maintenance instructions.

TABLE C-1. TSO-C112C MARKING AND PART 43 APPENDIX F CLASS

ATC Mode S Transponder Equipment Part 43, Appendix F Class Reference

TSO-C112c (or later version) 14 CFR Part 432, Appendix F class:

Transponder1 marking:
Level 1, Class 1 Class 1A

Level 1, Class 2 Class 1B, with optional

1090 + 1 MHz reply frequency
Level 2, Class 1 Class 2A

Level 2, Class 2 Class 2B, with optional

1090 + 1 MHz reply frequency

Level 3, Class 1 Class 3A

Level 3, Class 2 Class 3B, with optional

1090 + 1 MHz reply frequency
Level 4, Class 1 Class 4

Level 4, Class 2 Class 4, except for RF peak output power and

suppression, which should apply Class 3B
Level 5, Class 1 Class 4

Level 5, Class 2 Class 4, except for RF peak output power and

suppression, which should apply Class 3B
1
Per RTCA Inc., DO-181E, Section 1.4.6—with options noted in Section 1.4.4.
2
The part 43 marking shown here originated in the original version of TSO-C112.

C-1
2/15/17 AC 43-6D
Appendix D
APPENDIX D. SECTIONS 91.411 AND 91.413 COMPLIANCE FLOWCHART

FIGURE D-1. SECTIONS 91.411 AND 91.413 COMPLIANCE FLOWCHART

D-1
2/15/17 AC 43-6D
Appendix E
APPENDIX E. STATIC PRESSURE SYSTEM PROOF TEST

A proof test may be conducted to demonstrate the integrity of the static pressure system in the
following manner:

E.1 Unpressurized Airplanes. Evacuate the static pressure system to a pressure differential
of approximately 1inHg or to a reading on the altimeter, 1,000 feet above the aircraft
elevation at the time of the test. Without additional pumping for a period of 1 minute, the
loss of indicated altitude must not exceed 100 feet on the altimeter.

E.2 Pressurized Airplanes. Evacuate the static pressure system until a pressure differential
equivalent to the maximum cabin pressure differential for which the airplane is type
certificated is achieved. Without additional pumping for a period of 1 minute, the loss of
indicated altitude must not exceed 2 percent of the equivalent altitude of the maximum
cabin differential pressure or 100 feet, whichever is greater.

E-1
 Advisory Circular Feedback Form

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for new items/subjects to be added, you may let us know by contacting the Flight Standards
Directives Management Officer at 9-AWA-AFS-140-Directives@faa.gov.

Subject: AC 43-6D, Altitude Reporting Equipment and Transponder System

Maintenance and Inspection Practices
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