Direct Answer — The 24-Calendar-Month Tests and RVSM
Under 14 CFR §91.411, the static pressure system, each altimeter, and the automatic pressure altitude reporting system must be tested and inspected within the preceding 24 calendar months to operate under IFR in controlled airspace. Under 14 CFR §91.413, the ATC transponder must likewise be tested within the preceding 24 calendar months. These are two separate tests with two separate due dates. For an aircraft authorized for Reduced Vertical Separation Minimum (RVSM) operations under 14 CFR §91.180 and appendix G to part 91 — where ATC separates traffic by just 1,000 feet between FL290 and FL410 — the §91.411 altimeter and static system test is the airworthiness foundation of the altitude-keeping accuracy RVSM depends on. A lapsed §91.411 test does not just ground IFR operations; it removes the airworthiness basis for flying RVSM. An FAA inspector who cannot find a current §91.411 or §91.413 record treats the aircraft as ineligible for that operation until a current test is accomplished and recorded.
"24 calendar months" is not a rolling 730 days from the exact date
Both §91.411 and §91.413 use the phrase "within the preceding 24 calendar months." A calendar-month interval runs through the last day of the final month — a test completed any day in March 2026 is valid through March 31, 2028. The two tests share the same interval length but not necessarily the same anchor month, so they can drift apart unless you deliberately re-sync them.
The Two 24-Month Tests — §91.411 and §91.413 Are Not the Same Sign-Off
Operators routinely conflate these because they are usually performed in the same shop visit and recorded near each other in the logbook. But they are independent regulatory requirements with independent appendices, independent authorized signers, and independent due dates. An inspector verifies each one separately.
14 CFR §91.411(a) prohibits operating an airplane or helicopter in controlled airspace under IFR unless, within the preceding 24 calendar months, each of the following has been tested, inspected, and found to comply with the standards in part 43:
- The static pressure system — tested and inspected per appendix E of part 43
- Each altimeter instrument — tested per appendix E of part 43
- The automatic pressure altitude reporting system (Mode C / the encoding side feeding the transponder) — tested per the applicable part 43 appendix provisions
§91.411 also requires that, following any opening and closing of the static pressure system (except the use of system drain and alternate static pressure valves), or any maintenance or alteration affecting the altimeter or altitude reporting system, the affected portions be tested and inspected before they are used again. In other words, certain maintenance events reset the clock or add an interim test requirement — the 24-calendar-month cycle is the floor, not the only trigger.
14 CFR §91.413(a) prohibits using an ATC transponder (one described in §91.215(a), §121.345(c), or §135.143(c)) unless, within the preceding 24 calendar months, it has been tested and inspected and found to comply with appendix F of part 43.
- The test references appendix F of part 43 — a different appendix from the §91.411 altimeter/static tests (appendix E)
- Following any installation or maintenance on the transponder where data correspondence error could be introduced, the integrated system must be tested per the relevant part 43 appendix provisions before use
- The transponder test is what proves the Mode C/Mode S reply and the altitude data being transmitted are correct — distinct from the §91.411 altimeter accuracy test, even though both touch altitude data
Note the relationship with ADS-B: the §91.413 transponder test addresses the transponder itself. ADS-B Out equipment (under §91.225 / §91.227) has its own performance and, where applicable, verification requirements. Keep the transponder test record and any ADS-B documentation distinct in your filing — an inspector looks for the §91.413 sign-off specifically, by date.
Side-by-Side: What Each Test Covers
| Attribute | §91.411 Altimeter/Static | §91.413 Transponder |
|---|---|---|
| Interval | 24 calendar months | 24 calendar months |
| Part 43 appendix | Appendix E (altimeter/static) | Appendix F (transponder) |
| Triggers the requirement | IFR ops in controlled airspace | Use of an ATC transponder |
| What it proves | Altimeter + static system read true | Transponder reply + altitude data correct |
| RVSM relevance | Airworthiness backbone of altitude-keeping | Correct altitude data to ATC |
These tests sit inside the broader maintenance-record framework. The sign-off must be a proper maintenance record entry — see the §43.9 maintenance record entry requirements for what every entry must contain, and the Part 91 aircraft records requirements for how these test records fit alongside the rest of the aircraft's maintenance documentation.
Who Is Authorized to Sign Each Test
This is where operators most often create a defective record without realizing it. The authorized-signer rules differ between the two tests, and a general A&P mechanic does not have blanket authority to sign either one in full.
- 1The manufacturer of the airplane or helicopter on which the tests are performed.
- 2A certificated repair station holding the appropriate rating — an instrument rating Class I, a limited instrument rating appropriate to the appliance, a limited rating appropriate to the test, or an airframe rating appropriate to the aircraft.
- 3A certificated mechanic with an airframe rating — but only for the static pressure system tests and inspections. Not the altimeter or altitude-reporting portions.
- 1A certificated repair station properly equipped to perform the test and holding the appropriate radio ratings.
- 2A holder of a continuous airworthiness maintenance program as specified in part 121 or §135.411(a)(2).
- 3The manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by that manufacturer.
A general A&P certificate alone does not authorize either test in full
For §91.411, a mechanic with an airframe rating may sign only the static system portion — the altimeter and altitude-reporting tests require an appropriately rated repair station or the manufacturer. For §91.413, the transponder test requires a radio-rated repair station, a CAMP holder, or the manufacturer — an A&P certificate by itself is not on the list. A record signed by someone outside the authorized categories is a defective record, even if the equipment is genuinely within tolerance.
For Part 135 operators, the maintenance program and the continuous-airworthiness arrangements under §135.411 govern who performs and signs these tests — see what records a Part 135 operator must keep and Part 145 repair station recordkeeping requirements for the shop side of the sign-off.
Related Aviation Compliance Guides
How RVSM Authorization Depends on the Altimeter Test
14 CFR §91.180(a) states that no person may operate a civil aircraft in airspace designated as RVSM airspace unless two conditions are met: (1) the operator and the operator\'s aircraft comply with the minimum standards of appendix G of part 91, and (2) the operator is authorized by the Administrator or the country of registry to conduct the operation. §91.180(b) allows the Administrator to authorize a deviation from these requirements.
What RVSM Is — and Why Altimetry Accuracy Is Existential
Per appendix G to part 91, within RVSM airspace ATC separates aircraft by a minimum of 1,000 feet vertically between FL290 and FL410 inclusive. Outside RVSM, the standard vertical separation above FL290 is 2,000 feet. Halving the buffer doubles the usable flight levels in that band — but it is only safe if every aircraft\'s altitude-keeping is demonstrably accurate.
That is why appendix G requires RVSM-approved aircraft to have, among other things, two independent altitude-measurement systems, an altitude alerting system, and an automatic altitude control system, all held to defined altimetry-system-error and altitude-keeping tolerances. The hardware that produces those altitude measurements is the same hardware the §91.411 altimeter and static system test verifies every 24 calendar months. The test is the recurring proof that the RVSM-critical altitude data is true.
The RVSM Evidence Chain — Four Layers That Must All Hold
Aircraft RVSM approval / data package
The aircraft must be approved as meeting appendix G — typically established through an approved data package, type-design RVSM compliance, or an STC, retained as part of the aircraft's permanent records. This is the one-time eligibility document for the airframe.
Operator authorization (LOA or OpSpec)
Under §91.180(a)(2) the operator must be authorized by the Administrator. Under Appendix G Section 3 that authorization is a document — commonly a Letter of Authorization for Part 91, or the operations specifications for Part 135 — and it is what an inspector asks for. Since 2019, Appendix G Section 9 provides a second path: an aircraft with qualifying ADS-B Out meeting §91.227, meeting Section 9 standards, with height-keeping performance monitored in a form and manner acceptable to the Administrator, is authorized with no LOA or OpSpec document. Know which path you are on before an inspector asks.
Current §91.411 altimeter / static test
The 24-calendar-month altimeter and static system test keeps the altitude-keeping hardware continuously airworthy. If this test lapses, the airworthiness basis for accurate altitude-keeping lapses with it — and IFR operations generally are prohibited under §91.411(a) regardless of RVSM.
Height-monitoring and continued conformity
RVSM programs include height-monitoring requirements to confirm the aircraft's altitude-keeping performance in service, plus the ongoing maintenance that keeps the RVSM equipment within tolerance. The monitoring results and the maintenance records together demonstrate the aircraft has stayed within the appendix G performance envelope since approval.
A lapsed §91.411 test is a double failure for an RVSM operator
It grounds IFR operations under §91.411(a), and it simultaneously removes the recurring airworthiness proof that the RVSM altitude-keeping equipment is accurate. RVSM eligibility is not a one-time stamp — it is contingent on the aircraft staying continuously airworthy, and the §91.411 test is the recurring linchpin of that. Treat the altimeter test due date as an RVSM due date, not just a maintenance date.
RVSM authorization is one of several FAA authorizations (alongside things like RNP and special navigation approvals) that a charter operator must keep continuously current. For how surveillance inspectors examine these authorizations and their supporting records, see how to prepare for a Part 135 FAA surveillance audit, and for who actually holds the authority to operate, see what operational control means in Part 135.
Record Discipline — What an Inspector Actually Looks For
The recurring tests are only as good as the records that prove them. Here is what a complete, audit-ready record set for the §91.411 / §91.413 tests and RVSM authorization looks like — and the specific ways these records fail.
Complete Record Set
- §91.411 altimeter/static sign-off with the test date and the next-due (24-calendar-month) date
- §91.413 transponder sign-off with its own test date and next-due date
- Identity and rating of the signer (repair station certificate number and ratings, or manufacturer, or CAMP reference)
- The aircraft RVSM approval / data package in the permanent records
- The current RVSM LOA (Part 91) or OpSpec authorization (Part 135) — or, for the Appendix G Section 9 ADS-B path, evidence of qualifying ADS-B Out and performance monitoring
- Height-monitoring results, where applicable, kept current
How These Records Fail
- Treating one combined "pitot-static / transponder" entry as covering both, with only one date
- Letting the two tests drift to different months, then losing track of the earlier due date
- A static-system-only A&P sign-off used to claim the full §91.411 test
- An expired or superseded RVSM LOA that was never refiled
- Calculating the next-due as 730 days instead of to the end of the 24th calendar month
- RVSM data package present, but no current operator authorization on file (for Section 3 operators — Section 9 ADS-B operators instead need their monitoring evidence)
FileFlo as the RVSM & Inspection-Records Layer
FileFlo is a compliance document intelligence platform — a read-only proof layer that sits alongside your maintenance-tracking stack and completes it. It does not perform the §91.411 or §91.413 tests, issue RVSM authorizations, or run a maintenance-tracking, dispatch, or flight-operations system. What it does is make the records that prove these things instantly producible.
- Classifies uploaded test sign-offs against their governing rule (§91.411 altimeter/static, §91.413 transponder) and separates them so neither is mistaken for the other
- Tracks the 24-calendar-month next-due date for each test independently and surfaces expiration alerts at 90/60/30 days
- Indexes the RVSM data package and the LOA / OpSpec authorization so the full RVSM evidence chain is one click from an inspector
- Flags an expired or superseded RVSM authorization so it gets refiled before it grounds an operation
- Generates an inspector-format records binder on demand, organized by requirement and due date
FileFlo classifies 600+ compliance document types and manages records across Part 91, Part 135, and Part 145 operators in a single platform. Pricing: Starter $89/mo, Professional $299/mo. 5-day free trial, no credit card required. FileFlo keeps the documents that prove compliance audit-ready — it does not provide or run a safety management system (SMS), dispatch system, or flight operations system.
The Tests Apply Across Operators — RVSM Raises the Stakes
The §91.411 and §91.413 tests are general Part 91 operating rules that apply regardless of whether the aircraft ever flies RVSM — with one carve-out: under §91.401(b), §91.411 does not apply to aircraft maintained under a continuous airworthiness maintenance program (Part 121, or 10-or-more-seat Part 135 aircraft under §135.411(a)(2)), whose parallel requirements live in their CAMP. The §91.413 transponder test applies to any aircraft using a transponder in the airspace specified by §91.215. What changes by operator and mission is the accountability and the consequence of a lapse.
Who is responsible
Owner/operator
Owner tracks both test due dates and, if RVSM-authorized, holds the LOA. Most exposed to silent drift between the two 24-month dates.
Who is responsible
DOM via maintenance program / OpSpecs
Tests fold into the §135.411 maintenance program; RVSM authorization lives in the OpSpecs. A Principal Maintenance Inspector reviews both.
Who is responsible
Operator + maintenance org
The §91.411 test becomes RVSM-critical, not just IFR-critical. Height-monitoring and the authorization document are added to the evidence chain.
These recurring inspection records sit alongside the other recurring obligations that ground an aircraft when they lapse — see airworthiness directive compliance records and the Part 91 §409 annual inspection requirements for the other recurring intervals an inspector cross-checks against the logbooks. And because RVSM is one of several authorizations a modern Part 135 fleet must keep current, the same discipline applies to the FAA Part 135 SMS 2027 deadline.
Frequently Asked Questions
How often must the altimeter and static system be tested under 14 CFR §91.411?
Every 24 calendar months. 14 CFR §91.411(a) prohibits operating an airplane or helicopter in controlled airspace under IFR unless, within the preceding 24 calendar months, the static pressure system, each altimeter instrument, and the automatic pressure altitude reporting system have been tested and inspected and found to comply with the standards in appendices E and F of part 43. "24 calendar months" means the test is good through the last day of the 24th month after the month in which it was performed — for example, a test completed on March 12, 2026 is valid through March 31, 2028. It is not a rolling 730-day window from the exact date.
How often must the ATC transponder be tested under 14 CFR §91.413?
Every 24 calendar months. 14 CFR §91.413(a) states that no person may use an ATC transponder (as specified in §91.215(a), §121.345(c), or §135.143(c)) unless, within the preceding 24 calendar months, it has been tested and inspected and found to comply with appendix F of part 43. The transponder test interval and the altimeter/static test interval are both 24 calendar months, but they are separate tests with separate due dates — a §91.411 sign-off does not satisfy §91.413, and vice versa. Many operators schedule them together, but the records must show each test independently.
Who is authorized to perform the §91.411 altimeter and static system test?
Per 14 CFR §91.411(b), the altimeter and static system tests may be performed by: (1) the manufacturer of the airplane or helicopter on which the tests are performed; (2) a certificated repair station holding the appropriate rating — an instrument rating, Class I; a limited instrument rating appropriate to the appliance; a limited rating appropriate to the test; or an airframe rating appropriate to the aircraft; or (3) a certificated mechanic with an airframe rating — but only for the static pressure system tests and inspections. A certificated mechanic with an airframe rating cannot perform the altimeter or automatic altitude reporting portions on their own; those require an appropriately rated repair station or the manufacturer.
Who is authorized to perform the §91.413 transponder test?
Per 14 CFR §91.413(c), the ATC transponder test may be performed by: a certificated repair station properly equipped to perform the test and holding the appropriate radio ratings; a holder of a continuous airworthiness maintenance program as specified in part 121 or §135.411(a)(2); or the manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by that manufacturer. A general A&P mechanic with an airframe-and-powerplant certificate is not, by that certificate alone, authorized to perform the §91.413 transponder test — it requires the radio-rated repair station, the CAMP holder, or the manufacturer.
What records prove RVSM authorization, and where does the 24-month altimeter test fit in?
RVSM authorization is established by two things working together. First, under 14 CFR §91.180(a), the operator and the aircraft must comply with the minimum standards of appendix G to part 91 (independent altitude-measurement systems, altitude alerting, an automatic altitude control system, and an altitude-keeping performance within defined tolerances), and the operator must be authorized by the Administrator — commonly via a Letter of Authorization for part 91 or operations specifications for part 135 under Appendix G Section 3, or, since 2019, under Appendix G Section 9: an aircraft with qualifying ADS-B Out meeting §91.227 whose height-keeping performance is monitored in a form and manner acceptable to the Administrator is authorized to fly RVSM with no LOA or OpSpec document at all. Second, the aircraft must remain continuously airworthy for RVSM, which depends on the altimetry equipment passing the §91.411 24-calendar-month altimeter and static system test. The §91.411 test record is therefore part of the evidence chain that the RVSM-critical altitude-keeping equipment is current — a lapsed §91.411 test undermines both basic IFR airworthiness and the RVSM altitude-keeping case.
What altitudes is RVSM airspace, and why does altimetry accuracy matter so much?
Per appendix G to part 91, within RVSM airspace air traffic control separates aircraft by a minimum of 1,000 feet vertically between FL290 and FL410 inclusive. Outside RVSM, the standard vertical separation above FL290 is 2,000 feet. Halving the vertical buffer is only safe if every aircraft's altitude-keeping is demonstrably accurate, which is why appendix G imposes altimetry system error and altitude-keeping tolerances, and why the §91.411 altimeter and static system test — the 24-calendar-month verification that the static system and altimeters read true — is the backbone of the airworthiness side of RVSM. An aircraft with an out-of-tolerance altimeter that still flies RVSM is an immediate hazard, which is why height-monitoring programs and the test records both exist.
What happens at a ramp check if the §91.411 or §91.413 test is expired?
An FAA Aviation Safety Inspector can request the maintenance records establishing the most recent §91.411 and §91.413 tests at any time. If either test is outside its 24-calendar-month window, the aircraft is not legally eligible for the operation that requires it — IFR operations in controlled airspace for §91.411, and transponder use for §91.413 — until a current test is accomplished and recorded. For an RVSM operator, an expired §91.411 altimeter test is doubly serious: it grounds IFR operations generally and removes the airworthiness basis for RVSM altitude-keeping. The inspector cannot distinguish "the test was done but the record is missing" from "the test was never done," so a missing record is treated the same as an expired one.
Does FileFlo perform RVSM, altimeter, or transponder tests?
No. FileFlo is a compliance document intelligence platform — a read-only proof layer. It does not perform the §91.411 or §91.413 tests, issue RVSM authorizations, or run a maintenance-tracking, dispatch, or flight-operations system. What FileFlo does is classify and index the test sign-offs, the RVSM LOA or operations specifications, and the supporting altimetry data package, then track the 24-calendar-month next-due dates and surface expiration alerts so an expired altimeter or transponder test never surprises you at a ramp check or in a records audit. The tests themselves are performed by an appropriately rated repair station, a CAMP holder, or the manufacturer.
Never let an altimeter or transponder test expire under you
FileFlo classifies your §91.411 and §91.413 sign-offs, tracks each 24-calendar-month next-due date independently, keeps the RVSM data package and LOA/OpSpec authorization indexed, and generates a complete inspector-format records binder on demand. Starter plan $89/mo. Professional $299/mo. 5-day free trial — no credit card required.
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Also related: Part 91 Aircraft Records · §43.9 Record Entries · AD Compliance Records · Part 135 Surveillance Audit
Written by Chad Griffith, Founder & CEO of FileFlo — compliance document intelligence. Reviewed June 9, 2026. FileFlo is a compliance document intelligence platform, not legal counsel, an A&P, or a repair station. Verify all test intervals, who-may-sign rules, and RVSM authorization requirements with a certificated A&P or IA, your repair station, and your FSDO.