5.2 14 CFR 91.411 and 91.413 Mandatory 24-Month Testing

14 CFR 91.411 and 91.413 Mandatory 24-Month Testing

Quick Answer: The Federal Aviation Administration mandates recurring 24-calendar-month inspections for instrument flight safety. Under 14 CFR 91.411, no aircraft may be operated in controlled airspace under Instrument Flight Rules (IFR) unless its static pressure system, altimeters, and altitude reporting equipment have been tested per Part 43 Appendix E. Appendix E now requires a static-system proof test acceptable to the Administrator (Part 25 airplanes use the § 25.1325 limits). When the manufacturer gives no procedure, AC 43.13-1B paragraph 12-58 applies a vacuum equal to 1,000 ft and allows no more than 100 ft of leakage in 1 minute. Under 14 CFR 91.413, no person may use an ATC transponder unless it has been tested to Part 43 Appendix F within the preceding 24 calendar months: a reply frequency of $1090 \pm 3\ \text{MHz}$, minimum peak power of 125 W for A-class or 70 W for B-class units, side-lobe suppression, and receiver sensitivity. Appendix E paragraph (c) limits the difference between the altimeter and the reported altitude to 125 feet.

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Regulatory Scope and Authorizations

Safe separation in National Airspace System (NAS) radar environments relies entirely on accurate barometric altitude reporting. The FAA enforces strict testing cycles and technician qualifications for these systems.

+-------------------------------------------------------------------------+
|                   24-CALENDAR-MONTH INSPECTION MANDATES                 |
+-----------------------+------------------------+------------------------+
| Regulation            | 14 CFR 91.411          | 14 CFR 91.413          |
+-----------------------+------------------------+------------------------+
| Equipment Tested      | Static System,         | ATC Transponders       |
|                       | Altimeters, Encoders   | (Mode A, C, and S)     |
| Airspace Trigger      | Controlled Airspace    | Any use of a           |
|                       | under IFR              | transponder            |
| Test Standard         | Part 43 Appendix E     | Part 43 Appendix F     |
| Recurring Interval    | 24 Calendar Months     | 24 Calendar Months     |
+-----------------------+------------------------+------------------------+

The "Calendar Month" Rule

The certification expiration date falls on the last day of the 24th month following the inspection month. For example, if an aircraft's static system and altimeter inspection is completed on October 12, 2024, the aircraft remains airworthy for IFR operations until midnight on October 31, 2026.

Personnel Authorization Limitations

Not all certificated maintenance personnel hold legal authority to sign off every portion of 91.411 and 91.413 inspections:

1. Certificated repair stations: For § 91.411, a properly equipped repair station holding an instrument rating (Class I), a limited instrument rating for that make and model, a limited rating for the test, or an airframe rating appropriate to the aircraft. For § 91.413, a repair station holding a radio rating (Class III), a limited radio rating for that make and model of transponder, or a limited rating for the test.
2. Aircraft manufacturer: For § 91.411, the manufacturer of the airplane or helicopter being tested. For § 91.413, the manufacturer of the aircraft, but only if it installed the transponder.
3. Certificated mechanic with an airframe rating: May perform the static pressure system tests and inspections only (§ 91.411(b)(3)). The altimeter and altitude-reporting tests, and all § 91.413 transponder tests, belong to the persons above.
4. Air carriers (§ 91.413 only): The holder of a continuous airworthiness maintenance program under Part 121 or § 135.411(a)(2).

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Part 43 Appendix E: Static System Leakage Testing

Static system integrity tests verify that plumbing fittings, moisture traps, flexible tubing, and instrument cases are free of leaks that would corrupt barometric altitude readings.

[ Pitot-Static Test Set ]
         |
         +==== Vacuum Hose ====> [ Fuselage Static Port Adapter / Pad ]
                                              |
                                              v
                                   [ Aircraft Static Manifold ]
                                              |
                                +-------------+-------------+
                                |             |             |
                                v             v             v
                              [ASI]       [Alt Case]    [VSI Case]

What the Rule Requires Now

Part 43 Appendix E paragraph (a) requires the person testing to:

1. Ensure freedom from entrapped moisture and restrictions.
2. Perform a proof test that demonstrates the integrity of the static pressure system in a manner acceptable to the Administrator. For Part 25 airplanes, leakage must be within the § 25.1325 tolerances.
3. Determine that any static port heater works.
4. Ensure no airframe alterations or deformations affect the relationship between static-system pressure and true ambient pressure.

Since a 2016 amendment, the appendix no longer prints its own leak numbers, so follow the aircraft manufacturer's procedure first.

Unpressurized Aircraft (AC 43.13-1B Paragraph 12-58)

If the manufacturer has not issued static-system test instructions:

1. Connect the test equipment directly to the static ports if practicable; otherwise connect at a static drain or tee and seal off the static ports.
2. Apply a vacuum equivalent to 1,000 feet of altitude (a differential of about 1.07 in Hg, or 14.5 inches of water) and hold.
3. After 1 minute, the leak must not exceed the equivalent of 100 feet of altitude (about 0.0105 in Hg, or 1.43 inches of water).

The § 25.1325 proof test for unpressurized Part 25 airplanes uses the same values: about 1 in Hg (1,000 ft above airplane elevation) and no more than 100 ft lost in 1 minute.

Pressurized Airplanes (§ 25.1325)

A pressurized cabin is held above outside pressure, so a static-line leak inside the cabin makes the altimeter read lower than true altitude.

1. Evacuate the static system to a differential equal to the maximum cabin pressure differential for which the airplane is type certificated.
2. Hold for 1 minute without additional pumping.
3. 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.

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Part 43 Appendix E: Altimeter Calibration and Tolerances

Altimeters tested under Part 43 Appendix E undergo comprehensive bench testing inside an environmental vacuum chamber to ensure accuracy across their entire certified altitude envelope.

1. Scale Error Test

With the barometric scale set to 29.92 in Hg, the altimeter is tested at the Table I altitudes from $-1{,}000\ \text{ft}$ up to the maximum altitude normally expected for the airplane it serves. Pressure is reduced at no more than 20,000 ft/min to within about 2,000 ft of each test point, and the altimeter is held at each point for 1 to 10 minutes before the reading. At each cardinal test point, the indicated altitude is compared against a NIST-traceable digital air data standard:

2. Friction Test

The altimeter is subjected to a steady pressure decrease approximating 750 ft/min. At each altitude in Appendix E Table III, the change in pointer reading after vibration must not exceed the tolerance: ±70 ft at 1,000 through 5,000 ft, ±80 ft at 10,000 ft, ±90 ft at 15,000 ft, and ±100 ft at 20,000 ft, rising to ±250 ft at 50,000 ft.

3. Case Leak Test

The altimeter is evacuated to an indicated altitude of $18{,}000\ \text{ft}$. After isolating the vacuum source, the leakage rate of the sealed instrument case must not exceed $100\ \text{ft}$ in 1 minute.

4. Hysteresis and After-Effect

• Hysteresis: Metal fatigue and elastic memory in the beryllium-copper aneroid capsule cause slight delays during pressure reversals. After dwelling at maximum test altitude, the altimeter is ramped down. Readings taken at the $50\%$ and $40\%$ test points during descent must not diverge from ascending readings by more than $75\ \text{feet}$.
• After-Effect: Within 5 minutes following the completion of the full test cycle back at ambient sea-level pressure, the altimeter must return to within $\pm 30\ \text{feet}$ of its original reading.

5. Barometric Scale (Kollsman) Error

At constant pressure, the barometric scale is set to each Table IV pressure ($28.10$, $28.50$, $29.00$, $29.50$, $29.92$, $30.50$, $30.90$, and $30.99\ \text{in Hg}$). The pointer must move by the listed altitude difference (for example, $-1{,}727$ ft at 28.10 and $+974$ ft at 30.99) within $\pm 25\ \text{feet}$.

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Part 43 Appendix F: ATC Transponder Testing Standards

Transponders operate as airborne secondary surveillance radar (SSR) beacons, receiving ground interrogations at $1030\ \text{MHz}$ and replying at $1090\ \text{MHz}$. 14 CFR Part 43 Appendix F establishes RF performance criteria:

                  P1               P2               P3
                  ||              ||               ||
                  ||              ||               ||
  Interrogation:  ||<-- 2.0 us -->||<-- 6.0 us --->||
                  +---------------------------------+
                        Mode A (8 us) / Mode C (21 us)

1. Reply Frequency Tolerance

The carrier frequency of all pulsed replies from the transponder must remain strictly within:

$f_{\text{reply}} = 1090 \pm 3\ \text{MHz}$

Mode S Classes 1A, 2A, 3A, and 4 must stay within the tighter 1090 ± 1 MHz (Appendix F(a)(4)). A reply outside its window may not be decoded reliably by ground interrogators or TCAS.

2. Peak RF Output Power

Peak pulse power is measured at the antenna end of the transmission line, at the transponder antenna terminal with a correction for line loss, or by radiated signal (Appendix F(d)), using a ramp test set such as the IFR 6000. Appendix F sets the minimum by class letter, not by the class number:

3. Receiver Sensitivity

The receiver minimum triggering level (MTL) must be −73 ± 4 dBm for ATCRBS transponders, or −74 ± 3 dBm for Mode S format interrogations, and Mode 3/A and Mode C sensitivity may differ by no more than 1 dB. Portable test equipment is allowed an extra 3 dB for antenna-coupling error in this measurement, and it is run at a nominal 235 ATCRBS interrogations per second (50 per second for Mode S) to avoid interference.

4. Side-Lobe Suppression (SLS) Verification

Ground radar antennas radiate a narrow main beam flanked by low-power side lobes. To prevent aircraft from answering false interrogations radiated by side lobes, interrogations include a control pulse ($P_2$) transmitted omnidirectionally:

• $P_2$ is transmitted precisely $2.0\ \mu\text{s}$ after the initial framing pulse $P_1$.
• Suppression Requirement: When $P_2$ amplitude is received at equal to or greater than $P_1$ ($P_2 \ge P_1$), the transponder must suppress replies, maintaining a reply rate of less than $1\%$.
• Interrogation Acceptance: When $P_2$ is received at $9\ \text{dB}$ or more below $P_1$ ($P_2 \le P_1 - 9\ \text{dB}$), the aircraft is in the main radar beam and must reply with a response rate greater than $90\%$.

5. Altitude Encoder Data Correlation Standard

Under Part 43 Appendix E paragraph (c) and Appendix F, the digitizer output (whether legacy Gillham parallel gray-code or digital ARINC 429 databus) sent from the altitude encoder to the transponder must be checked across the full operational range:

$\left| \text{Altimeter Indication} - \text{Encoded Transponder Broadcast} \right| \le 125\ \text{feet}$

If the primary altimeter displays $8{,}000\ \text{ft}$ (at $29.92\ \text{in Hg}$), the transponder altitude data stream received by ATC must report between $7{,}875\ \text{ft}$ and $8{,}125\ \text{ft}$.

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6. Mode S Checks (Appendix F(e) through (j))

• Diversity isolation: On diversity transponders, the selected antenna's peak power must exceed the non-selected antenna's by at least 20 dB.
• Address: The transponder must reply only to its assigned 24-bit address; test with the correct address and at least two incorrect ones.
• Formats: Surveillance interrogations UF 4 and UF 5 must return the same altitude and identity as valid Mode C and Mode 3/A replies; test UF 20, 21, and 24 if equipped.
• All-calls: A Mode S-only all-call (UF 11) must return the correct address and capability (DF 11), and an ATCRBS-only all-call (0.8 µs P4) must produce no reply.
• Squitter: The transponder must send a correct acquisition squitter about once per second.

Records and Limits

• Record the § 91.411 and § 91.413 tests under § 43.9 (Appendix E(d) and Appendix F(k)). The person testing an altimeter also marks on it the test date and the maximum altitude to which it was tested.
• No person may operate under IFR in controlled airspace above the maximum altitude to which all altimeters and the altitude reporting system have been tested (§ 91.411(d)).
• Altimeters and altitude reporting equipment approved under a TSO are considered tested as of their date of manufacture (§ 91.411(c)).
• Any opening and closing of the static system (other than the drains and alternate static valve) requires a new Appendix E(a) static-system test, and any installation or maintenance on the transponder or altitude reporting system that could introduce a correspondence error requires the Appendix E(c) test (§ 91.411(a)(2) and (3); § 91.413(b)).

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Troubleshooting and Documentation