FAR 91.411: Altimeter & Altitude Reporting Tests

FAR 91.411 is a crucial FAA regulation ensuring your aircraft's altimeter and altitude reporting systems are accurate and reliable for safe IFR flight in controlled airspace. Understand the testing requirements, inspection intervals, and compliance tips every pilot and owner must know to stay legal and fly safely.

Quick Overview of FAR 91.411

14 CFR 91.411 demands that the static pressure system, altimeter instruments, and that pesky automatic pressure altitude reporting systems all get a check-up and a fairly recent one at that before an airplane or helicopter can fly IFR in controlled airspace. These inspections help ensure that the required flight instruments specified under FAR 91.205 remain accurate and airworthy for the intended operation.

The test needs to have happened in the last 24 months. And here's the thing - if you can't pass this one, you can't fly IFR in controlled airspace.

Now, it's worth noting that this bit of the FAR connects up with FAR 91.413, which deals with ATC transponder tests they often get done at the same time as an altimeter system check. And then there's that handy appendix e to Part 43 which gives the nitty-gritty on how to test and what to look for.

One thing to bear in mind FAR 91.411 and FAR 91.413 aren't the same animal one covers the altimeter system and the other the transponder even though many shops tackle both during the same visit.

FAR 91.411 is meant to keep altitude info on the up-and-up for collision avoidance and making sure ATC keeps planes apart, and to make sure instrument landings and takeoffs are reliable.

Regulatory Basis: 14 CFR 91.411 and Appendix E

FAR 91.411 sits under 14 CFR Part 91, Subpart E (Maintenance, Preventive Maintenance, and Alterations). Its core language is direct: no person may operate an airplane or helicopter in controlled airspace under IFR unless each static pressure system, altimeter, and automatic altitude reporting system has been tested and inspected within the preceding 24 calendar months.

Altimeter systems must be tested every 24 months, and these tests must comply with appendices E and F of Part 43.

The technical specifications are not found in 91.411 itself but in Appendix E to Part 43, which details the exact static pressure system tests, altimeter scale error checks, hysteresis tests, friction tests, and case leak tests.

The appendix includes several components of testing and specifies tolerances at defined altitude intervals-for example, ±20 ft at 0 ft, ±80 ft at 10,000 ft, and ±280 ft at 50,000 ft.

The regulation references Part 43 to require that these tests and inspections be performed and the aircraft approved for return to service by appropriately certificated persons or repair stations, in accordance with the specifications laid out in Appendix E. The tests are amended by AMDT as the FAA updates tolerances and procedures over time.

Which Aircraft and Operations Must Comply

FAR 91.411 applies to U.S.-registered airplanes and helicopters operating in controlled airspace under IFR, regardless of whether they fly under Part 91, 121, 125, or 135 operations. Typical aircraft categories affected include:

• Single-engine pistons (Cessna 172, Piper PA-28)

• Multi-engine pistons (Beechcraft Baron)

• Turboprops (King Air model variants)

• Business jets (Citation, Learjet)

• Helicopters used for IFR flight or training

VFR-only aircraft that never operate on IFR flight plans are not required to complete these tests, but owners may choose to have them performed for safety, resale value, or future IFR use.

However, if your plane has altitude reporting gear hooked up to a transponder and you're planning to fly IFR in class A, B, C or other controlled airspace then you're required to comply because FAR 91.411 says you can't fly IFR without a current inspection.

Part 135 and Part 121 operators may need to do extra or more frequent checks through their maintenance programs but they still need to meet 91.411 at the very least. And even experimental and light-sport planes that do fly IFR have to meet this rule when they're flying in controlled airspace with an IFR clearance.

Altimeter System and Altitude Reporting Equipment Tests

FAR 91.411 focuses on two main areas: the altimeter system and the altitude reporting equipment. The altimeter system includes the primary altimeter instrument, static ports, static lines, manifolds, fittings, and any components between the exterior static source and the instrument-collectively the static pressure system.

The altitude reporting equipment is typically a blind encoder or integrated Air Data Computer (ADC) that converts static pressure into a digital altitude output for use by the ATC transponder and onboard systems.

Altitude reporting equipment approved under technical standard orders is considered tested upon manufacture, but ongoing periodic checks remain tests required by the regulation.

The tests and inspections verify:

• Altimeter scale error across defined altitude test points

• Hysteresis and after-effect behavior

• Friction in pointer movement

• Case leak integrity

• Static system leakage

• Data correspondence error between indicated altitude and reported pressure altitude. Pilots who want to better understand this measurement should first learn what is pressure altitude and how it differs from indicated altitude.

Tests are normally performed with calibrated test equipment (a pitot-static test set) that simulates pressure altitudes from field elevation up to the aircraft's maximum altitude or limits in Appendix E.

Altitude reporting equipment tests ensure that Mode C or Mode S outputs stay within the ±125-foot correspondence tolerance between the encoded altitude and altimeter indication. The accuracy of altitude reporting affects instrument approaches and departures, making these checks critical.

Altimeters must also be tested after maintenance on the ATC transponder where data correspondence error could be introduced into the integrated system.

The static pressure system must be re-tested any time it has been opened and closed-except for routine use of a system drain or alternate static pressure valves.

Inspection Intervals, "24 Calendar Months," and Common Scenarios

The phrase "within the preceding 24 calendar months" does not mean exactly 730 days. It runs to the end of the 24th calendar month after the month the inspection was completed.

Example: If altimeter and altitude reporting system tests and inspections were signed off on 10 March 2024, they remain valid through 31 March 2026.

The 24-month interval applies equally to the altimeter, static system, and pressure altitude reporting system. An expired test on any single component means the aircraft cannot operate IFR in controlled airspace until re-tested and found in compliance.

Operators are responsible for scheduling and documenting required inspections. Many coordinate 91.411 system tests and inspections with 91.413 transponder checks and annual inspections to reduce downtime and closing costs.

However, if an altimeter, encoder, or static system component is removed, replaced, or disturbed-introducing error could be introduced-post-maintenance testing per Appendix E is inspection required even if the 24-month interval has not elapsed. Even after a successful inspection, pilots should understand the effects of flying from high pressure to low pressure, since proper altimeter settings remain essential for maintaining accurate altitude information in flight.

Set a calendar reminder at least 60 days before your 91.411 due date to avoid scrambling before an IFR trip or checkride.

Who Can Perform 91.411 Tests and How They Are Documented

These tests must be performed by properly authorized persons using calibrated equipment-not by owners doing routine preventive maintenance. Only certificated repair stations with appropriate ratings can conduct the required tests. Acceptable providers include shops holding a limited instrument rating, limited rating for airframe, or radio ratings with pitot-static capabilities.

A certificated mechanic with an airframe rating may perform static pressure system tests in certain cases, but the full scope of Appendix E altimeter and encoder testing generally requires a certificated repair station. The manufacturer of the appliance or aircraft can also perform the work, done in the same manner acceptable to the FAA under Part 43 regulations.

The basic test process involves connecting a pitot-static test set to the aircraft's static ports, applying simulated altitudes, checking altimeter and encoder responses, and recording results against Appendix E tolerances. Upon completion, the authorized person makes a logbook entry that includes:

• Date of test and completion

• Reference to 14 CFR 91.411 and Appendix E

• Identification of equipment tested and inspected (altimeters, static system, altitude reporting equipment)

• Maximum altitude to which the system was tested

• Name, certificate number, and signature for return to service

Many shops also affix a cockpit sticker indicating when the next 91.411 tests are due, helping pilots verify compliance during preflight and aircraft records reviews.

Practical Compliance Tips for Owners and Pilots

Proactive planning makes FAR 91.411 compliance straightforward and helps you avoid last-minute IFR cancellations or enforcement issues.

• Review logbooks annually. Confirm the last 91.411 sign-off date and note the end-of-month expiration on a tracking sheet, digital calendar, or maintenance app.

• Schedule early. Book altitude reporting equipment tests and required inspections several weeks before the due month-especially before busy IFR seasons or training periods.

• Verify before filing IFR. Confirm that both 91.411 and 91.413 are current, particularly after aircraft purchase, lease, or return from major avionics or instrument installation work.

• Request test sheets. Ask your maintenance provider whether tests were performed in accordance with the latest revision of Appendix E using properly calibrated pitot-static test equipment, and keep a copy in your aircraft records.

• Determine your operating ceiling. If you fly a class I altimeter in a high-altitude aircraft, ensure the test covers the maximum altitude you intend to operate at-otherwise you cannot legally fly IFR above the tested altitude.

Static pressure system accuracy is critical for ATC separation and collision avoidance. Accurate altimeter system and altitude reporting equipment tests directly support the safety functions that keep aircraft separated in the IFR environment. Treat 91.411 compliance as essential infrastructure for every IFR flight, not just a paperwork exercise.

Review your aircraft records today, confirm your next due date, and schedule your tests well in advance. Safe flying starts with accurate altitude data.