AC 43-6D

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

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;

• 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

3 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 Mod e 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.

4 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.

N ote 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

5 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.

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:

psi = inHg2.036

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 Step 1.

25.39 inHg

2.036 = 12.47 psi.

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).

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.

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

9 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.

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.

11 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 M AINT ENANCE 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. ________________

12 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 A ccidental 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.