FAR 91.413 Transponder Check: Owner’s Compliance Guide

Hands testing aircraft transponder with instruments

Under 14 CFR § 91.413, no person may operate an ATC transponder in airspace where 14 CFR § 91.215 requires one unless that transponder has been tested, inspected, and found to comply with Appendix F of Part 43 within the preceding 24 calendar months. A second trigger applies regardless of the calendar interval: any installation or maintenance that could introduce a data-correspondence error requires testing before the aircraft returns to service.

Your first action is straightforward: open your aircraft logbook, find the last transponder compliance entry, and check the date. If that date is more than 24 calendar months ago, or if you have had avionics work done since then that touched the encoder, altitude reporting system, or transponder itself, you need to schedule a test before your next flight in transponder-required airspace.

Check the logbook for the last 91.413 compliance entry and its date.
Identify any recent maintenance that may have introduced a data-correspondence error.
Confirm whether the test covered Appendix F in full (including Mode S checks if applicable).
Schedule a test with a certificated repair station if you are at or past the 24-month mark.

Pro Tip: Set a calendar reminder 60 days before your transponder test expires. That window gives you time to find a qualified repair station, coordinate with other scheduled maintenance, and avoid grounding the aircraft if the shop has a backlog.

Key Takeaways

Under 14 CFR § 91.413, every ATC transponder must be tested to Appendix F of Part 43 within the preceding 24 calendar months, and any maintenance that could introduce a data-correspondence error triggers an immediate re-test requirement regardless of the calendar interval.

Point	Details
24-calendar-month interval	Test expires at the end of the 24th calendar month after the month the last test was performed.
Post-maintenance trigger	Any installation or repair that could introduce a data-correspondence error requires a new Appendix F test before return to service.
Authorized providers	Only certificated repair stations with a radio rating, Class III, and appropriate limited ratings may perform the test for most GA owners.
Appendix F scope	Test must cover reply frequency, receiver MTL (−73 ±4 dBm ATCRBS; −74 ±3 dBm Mode S), RF output, Mode S address/format, and squitter.
Squawkfree for tracking	Squawkfree automates 24-month reminders and lets you attach Appendix F test reports directly to maintenance entries for audit-ready recordkeeping.

What § 91.413 actually requires and how it connects to 91.215 and Appendix F
Who is authorized to perform the 91.413 transponder test
What Appendix F actually tests and why each item matters
When to schedule your test: the 24-month cadence and additional triggers
How bench tests and portable radiated tests differ
How to log the test correctly and what records to keep
Step-by-step compliance checklist for owners and operators
Why proactive transponder compliance is worth the discipline
Squawkfree keeps your 91.413 compliance on track automatically
Sources

What § 91.413 actually requires and how it connects to 91.215 and Appendix F

The 24-calendar-month interval is not a rolling 730-day count. It runs from the first day of the month in which the last test was performed. If your last test was completed on March 15, 2024, the test expires at the end of March 2026, not on the exact anniversary date. That distinction matters when you are scheduling close to the deadline.

AOPA’s guidance on transponder inspections confirms this 24-calendar-month standard and notes that Appendix F defines the checklist the inspection must follow. The practical implication: a shop that tests your transponder but does not reference Appendix F compliance in the logbook entry has not completed the work in a way that satisfies the regulation.

The post-maintenance trigger is separate from the calendar interval. Appendix E to Part 43 governs integrated system tests after certain installations. If a shop replaces your encoder, swaps an altimeter, or installs new avionics that interface with the transponder, an Appendix E integrated system check may be required before the aircraft returns to service, in addition to the Appendix F transponder test. These are two distinct requirements, and completing one does not substitute for the other.

Section 91.215 is where you find the airspace rules that make the transponder mandatory. Controlled airspace, Class A through Class E where required, and certain other designated areas all require a functioning transponder. The 91.413 test requirement exists to confirm that the transponder you are relying on in those environments actually performs to the standard ATC expects. A transponder that has not been tested within 24 calendar months is not legal to use in that airspace, even if it appears to be working normally on the ground.

Your immediate action items: pull your logbook, verify the last test date, review any maintenance records from the past two years for work that may have touched the transponder or altitude reporting system, and compute your next due date using the calendar-month method above.

Who is authorized to perform the 91.413 transponder test

Not every avionics shop is authorized to conduct a 91.413 transponder inspection. The regulation specifies three categories of authorized providers:

Certificated repair stations with a radio rating, Class III, and the appropriate limited rating for the make and model of transponder being tested.
Holders of continuous airworthiness maintenance programs under Part 121 or certain Part 135 operations, where the program specifically covers transponder testing.
The aircraft manufacturer, in cases where the manufacturer performs the test under its own maintenance authority.

For most general aviation owners, the relevant option is the certificated repair station. Before you drop off the aircraft, confirm these specifics with the shop:

Does the station hold a radio rating with Class III authorization?
Does it hold the appropriate limited rating for your transponder make and model?
Can it perform the full Appendix F test sequence, including Mode S address and format checks if your aircraft has a Mode S transponder?
If you have had recent avionics work done, can it also perform the Appendix E integrated system check?
Will it provide a written test report that references the specific Appendix F test paragraphs?

Practical questions worth asking before you commit: What is the typical turnaround time? Will the technician use a bench test or portable radiated test equipment? What documentation will you receive at pickup? A shop that cannot answer these questions clearly is worth reconsidering.

What Appendix F actually tests and why each item matters

Appendix F to Part 43 covers test items (a) through (j), each targeting a specific performance parameter. Here is what each check verifies and why it matters to you operationally.

Reply frequency. The transponder must reply on 1090 MHz ±3 MHz. A drift outside that tolerance means ATC secondary surveillance radar may not decode your replies correctly.

Receiver sensitivity (MTL). The minimum triggering level tells you how weak an interrogation signal the transponder will still respond to. Per Appendix F and Gleim’s FAR/AIM summary, ATCRBS transponders are tested to an MTL of −73 ±4 dBm and Mode S transponders to −74 ±3 dBm. A transponder that fails this check may not reply to ATC interrogations at range, creating a surveillance gap neither you nor the controller will see in the cockpit.

RF peak output power. The transponder must transmit at sufficient power for ATC radar to receive your reply at the distances required for the airspace you operate in.

Suppression behavior. The transponder must suppress replies correctly when interrogated by TCAS or other aircraft equipment, preventing interference with collision avoidance systems.

Mode S address and format replies. For Mode S transponders, Appendix F requires verification that the unit replies only to its assigned 24-bit ICAO address, that it correctly handles uplink formats UF=4 and UF=5, and that altitude and identity data in Mode S replies match the corresponding ATCRBS Mode C and Mode 3/A outputs. A mismatch here can cause your aircraft to appear incorrectly in ATC surveillance systems.

Squitter behavior. Mode S transponders must transmit spontaneous squitter messages at the correct rate and format. This is the foundation of ADS-B Out surveillance.

Parameter	Standard / Threshold	Why It Matters
Reply frequency	1090 ±3 MHz	ATC radar decoding accuracy
ATCRBS receiver MTL	−73 ±4 dBm	Replies to weak interrogations at range
Mode S receiver MTL	−74 ±3 dBm	Mode S surveillance coverage
RF peak output power	Per Appendix F spec	ATC radar return strength
ATCRBS interrogation rate (portable)	~235 per second	Portable test equipment standard
Mode S interrogation rate (portable)	~50 per second	Portable test equipment standard

When portable test equipment is coupled to the aircraft antenna system rather than connected directly to the transponder, Appendix F allows a 3 dB additional loss allowance during receiver sensitivity measurements to compensate for antenna coupling errors. Ask your shop whether that allowance was applied and whether the reported MTL values reflect it.

Statistic callout: ATCRBS receiver sensitivity is tested to a value near −73 dBm with allowable variation; Mode S sensitivity is tested to a value near −74 dBm with allowable variation. A transponder that falls outside these thresholds may not reply reliably to ATC interrogations at operational distances.

When to schedule your test: the 24-month cadence and additional triggers

The calendar-month calculation is simple once you know the rule. If your last Appendix F test was completed in any day of April 2024, the test expires at the end of April 2026. You may not operate in transponder-required airspace on May 1, 2026 without a new test on record.

Beyond the regular cadence, these events require a test before returning to service:

Replacement of the transponder unit.
Replacement or repair of the altitude encoder.
Any maintenance on the static system that could affect altitude reporting accuracy.
Installation of new avionics that interface with the transponder or altitude reporting system.
Any repair or modification that could introduce a data-correspondence error between the transponder’s altitude output and the altimeter system.

For items 2 through 5, an Appendix E integrated system check is typically required in addition to the Appendix F transponder test. Confirm with your shop which tests apply before authorizing the work.

Practical scheduling advice: book the transponder test at the same time as your 91.411 altimeter and static system check, which runs on the same 24-calendar-month cycle. Combining both tests in one shop visit reduces ferry time, minimizes downtime, and often lowers the total labor cost compared to two separate appointments.

Pro Tip: Keep a single maintenance calendar that tracks both the 91.411 altimeter/static check and the 91.413 transponder test together. They share the same interval and the same authorized provider pool, so scheduling them together is almost always more efficient than managing them separately.

How bench tests and portable radiated tests differ

A bench test removes the transponder from the aircraft and connects it directly to calibrated test equipment in the shop. This method gives the technician a clean, interference-free signal path and the most accurate sensitivity and output power readings. It is the preferred method when the transponder is being replaced or when the shop needs to verify performance to the tightest tolerances.

A portable radiated test keeps the transponder installed in the aircraft and uses test equipment that interrogates through the aircraft’s antenna system. This approach is faster and avoids the labor of removing and reinstalling the unit, but the antenna coupling introduces signal loss. That is precisely why Appendix F permits the 3 dB allowance during receiver sensitivity measurements with portable equipment. The allowance accounts for coupling losses so the reported MTL value reflects the transponder’s actual performance rather than the test setup’s attenuation.

When you authorize a portable test, ask the technician to confirm the interrogation rates used: ATCRBS equipment should run at approximately 235 interrogations per second and Mode S equipment at approximately 50 per second, as specified in Appendix F. Rates outside those values can affect whether the transponder’s reply behavior is accurately characterized.

For your work order, request explicit coverage of these items regardless of which test method is used:

Mode S address verification (24-bit ICAO address confirmation).
UF=4 and UF=5 uplink format checks.
Squitter verification.
Receiver sensitivity measurement with the applied allowance noted.
RF output power measurement.
Reply frequency verification.

Ask the shop to provide a written test report that references the specific Appendix F paragraph numbers for each test performed. A report that simply says “transponder checked, OK” does not give you or an FAA inspector the detail needed to confirm Appendix F compliance.

How to log the test correctly and what records to keep

The maintenance entry following a 91.413 test must contain specific elements to satisfy FAA requirements. A vague or minimal entry creates unnecessary questions during ramp checks and can complicate the aircraft’s sale or transfer later.

A compliant logbook entry should include:

The date the test was performed.
A description of the work: “ATC transponder tested and inspected per 14 CFR § 91.413 and found to comply with Appendix F of Part 43.”
The name of the repair station and its certificate number.
The signature and certificate number of the authorized technician.
A reference to the test report number or document, if one was issued.

Beyond the logbook entry, keep these documents accessible:

The written Appendix F test report from the shop.
The bench or radiated test equipment identification (make, model, and calibration status if provided).
Mode S address and format test results.
Any Appendix E integrated system test report if one was required.

For aircraft recordkeeping requirements, the FAA expects you to be able to produce these records on request during a ramp check. Storing scanned copies in a digital maintenance system alongside the logbook entry means you can pull them up immediately rather than searching through paper files. A digital system also lets you attach the test report directly to the maintenance entry so the two are permanently linked.

Step-by-step compliance checklist for owners and operators

Follow these steps to close the 91.413 loop completely:

Locate the last transponder test entry in your aircraft logbook and note the date and the repair station that performed it.
Calculate your due date using the calendar-month method: the test expires at the end of the 24th calendar month after the month in which it was performed.
Review maintenance records from the past 24 months for any work on the transponder, encoder, static system, or interfacing avionics that may have triggered an immediate re-test requirement.
Confirm the scope of the last test — verify that the entry references Appendix F compliance and, for Mode S transponders, that Mode S address and format checks were included.
Contact a certificated repair station with a radio rating, Class III authorization, and the appropriate limited rating for your transponder make and model.
Schedule the test and coordinate it with your 91.411 altimeter/static check if both are due within the same maintenance window.
Confirm the work order scope before authorizing: request Mode S address verification, UF format checks, squitter verification, receiver sensitivity measurement, RF output power measurement, and a written report referencing Appendix F paragraphs.
Verify the logbook entry when you pick up the aircraft: confirm it includes the date, a reference to § 91.413 and Appendix F, the station’s certificate number, and the technician’s signature.
File the test report with your aircraft records and attach a scanned copy to your digital maintenance log.
Set your next reminder for 22 months out so you have a two-month scheduling buffer before the next expiration.

Common pitfalls to avoid: accepting a logbook entry that does not reference Appendix F by name; relying on a bench check alone when an Appendix E integrated system check is also required after an installation; and forgetting to verify the Mode S address after an encoder or avionics swap.

Pro Tip: When requesting a test from a repair station, send a short email with three items: your aircraft N-number and transponder make/model, the date of the last 91.413 test, and a note requesting a full Appendix F test report with Mode S checks if applicable. This gives the shop everything it needs to quote the job and prepare the right test equipment before your aircraft arrives.

Pro Tip: Combine your 91.413 transponder test with your IFR currency check or annual inspection whenever the timing allows. Fewer shop visits means less ferry time and lower total maintenance cost per year.

Why proactive transponder compliance is worth the discipline

Letting a transponder test lapse is not a minor paperwork issue. Operating in transponder-required airspace with an out-of-compliance unit is a regulatory violation under 14 CFR § 91.413, and it affects the aircraft’s airworthiness status. An FAA ramp check that surfaces an expired test can ground the aircraft on the spot and trigger an enforcement action. Beyond the regulatory exposure, a transponder that has not been tested in over two years may have drifted outside the Appendix F tolerances without any visible cockpit indication, creating a real surveillance gap between you and ATC.

Technician’s hand with maintenance documents near aircraft tail

The owners who handle this cleanly are the ones who treat the 91.413 test as a scheduled event rather than a reactive one. They book it alongside the altimeter/static check, they keep the test report attached to the logbook entry, and they set a reminder before the window closes. That discipline takes about 20 minutes of administrative work per cycle and eliminates the risk of a last-minute scramble or an inadvertent violation.

One thing worth noting: the post-maintenance trigger catches many owners off guard. An avionics shop that installs a new GPS or replaces an encoder may not automatically flag that a new Appendix F test is required. You, as the aircraft owner, are responsible for knowing when that trigger applies. Staying current on FAA AD compliance and maintenance triggers is part of owning an airworthy aircraft.

Squawkfree keeps your 91.413 compliance on track automatically

Tracking a 24-calendar-month interval across multiple aircraft, coordinating with repair stations, and keeping test reports attached to the right logbook entries is exactly the kind of administrative work that falls through the cracks in a paper-based system. Squawkfree gives GA owners and flight schools a purpose-built alternative.

Squawkfree

With Squawkfree, you set the 91.413 due date once and the platform sends automated reminders before the window closes. You can attach scanned Appendix F test reports directly to the maintenance entry, so the report and the logbook record are permanently linked and instantly accessible during a ramp check. Shared access means your mechanic, co-owner, or dispatcher sees the same compliance status you do, with no version-control issues.

Squawkfree does not replace a qualified repair station. It handles the tracking, documentation, and reminder work so you can focus on flying. Start your 60-day free trial at Squawkfree and see how much simpler compliance tracking can be.

Sources

These are the primary sources for everything covered in this article. Bookmark them for audits or deeper technical reading.

For a broader look at aircraft maintenance logs and how to structure compliant entries across all your periodic inspections, the Squawkfree blog covers FAA recordkeeping requirements in practical detail.

This article is general information, not a substitute for advice from a qualified lawyer. Consult a qualified legal professional about your own circumstances before acting on anything here.

FAR 91.413 Transponder Check: Owner's Compliance Guide