FAR Ninety-One Point Four Eleven, the Altimeter Inspection Every IFR Pilot Needs, and the Transponder Rule That Also Applies on VFR Days in Controlled Airspace

FAR 91.411 and FAR 91.413 don’t get the same airtime as 91.3 or the VFR weather minimums, but violations are surprisingly common. They show up on checkrides, and they can ground an airplane during preflight if you haven’t looked closely at the maintenance records. Here’s exactly what each regulation requires and where pilots consistently miss them.

What Does FAR 91.411 Actually Require?

FAR 91.411 states that no person may operate an aircraft in controlled airspace under instrument flight rules unless, within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system has been tested and inspected and found to meet the requirements of Appendix E of Part 43.

That covers three components: the static system, the altimeter itself, and the encoding altimeter (the piece that reports altitude to ATC through the transponder). All three must pass the same inspection before an IFR flight in controlled airspace.

Does FAR 91.411 Apply to VFR Pilots?

The trigger language is “controlled airspace under instrument flight rules.” A VFR-only pilot who never files IFR does not need a current altimeter and static system inspection to be legal. That much is correct.

But instrument-rated pilots sometimes miss the implication: if your altimeter and static system inspection is overdue, your airplane is grounded for IFR flight. You cannot legally file and fly IFR without it. And if you’re buying a used airplane you intend to fly IFR, this is one of the first things to verify - not the last.

What Does the Altimeter and Static System Inspection Involve?

A certified avionics shop, or an appropriately rated Airframe and Powerplant mechanic with inspection authorization (IA), pressurizes the static system and checks for leaks. They then verify the altimeter reads within tolerance across its entire range. If the aircraft has an encoding altimeter, the encoder is checked as part of the same inspection - it must pass separately.

From the pilot’s perspective, the process is straightforward: schedule it, the shop runs it, they write it up in the logbook. The complexity is on the technician’s side.

How Do You Calculate the 24-Calendar-Month Deadline?

The regulation specifies 24 calendar months - not 24 flight hours, not two years from a sticker date. Calendar months.

If the inspection was completed on November 15, 2023, it expires at the end of November 2025. The entire month of November counts. On December 1, it’s expired. This follows the same logic as the annual inspection, the medical certificate, and the flight review. And every year, pilots discover they miscounted or assumed “24 months” meant something slightly different than it does.

Where Do You Find the Inspection Records?

The official record is the aircraft maintenance logbook. The altimeter and static system inspection will have a signed logbook entry from an appropriately rated technician, including their certificate number.

Many shops also place a placard or sticker in the cockpit as a reminder. The sticker is not the official record. If the sticker says one date and the logbook says another, the logbook controls. Always go to the logbook.

Before flying any aircraft IFR - especially one you haven’t flown in a while - check the maintenance records before departure, not after landing.

What Does FAR 91.413 Require - and Why Does It Catch VFR Pilots Too?

FAR 91.413 states that no person may use an ATC transponder unless, within the preceding 24 calendar months, the transponder has been tested and inspected and found to comply with Appendix F of Part 43.

The critical word is “use.” The regulation doesn’t say “fly IFR.” It says use a transponder - any transponder, any flight.

If you’re flying VFR through Class Bravo airspace, you’re required to have an operating transponder under FAR 91.215. If that transponder hasn’t been inspected within the last 24 calendar months, you’re in violation of 91.413 - even though you’re VFR, even though you never touched the IFR system.

This catches pilots flying older aircraft that have been sitting. A current annual, a running engine, and working avionics don’t mean the transponder inspection is current. If the transponder is overdue, you cannot legally squawk into Class B, Class C, or Class E above 10,000 feet MSL without being in violation.

What Does a Transponder Inspection Involve?

The shop connects the transponder to test equipment that checks accuracy, reply frequency, pulse timing, and Mode C encoding. They verify that when you squawk 1200, ATC sees 1200. That when your aircraft is at 3,000 feet, the encoding altimeter is reporting 3,000 feet to ATC within the allowed tolerance. Mode S transponders receive additional tests for extended squitter function.

Like the altimeter inspection, this is a schedule-and-execute process from the pilot’s side. The shop runs the test and makes the logbook entry.

Are the Altimeter and Transponder Inspections the Same Thing?

No. They are separate items on the same 24-calendar-month interval, each requiring its own logbook entry.

You can have a current altimeter and static system inspection with an overdue transponder, or vice versa. When you’re doing a preflight records check on an unfamiliar aircraft, you need to locate both entries independently. Finding one does not confirm the other.

Do These Rules Apply to Glass Cockpit Aircraft?

Yes. The regulation refers to altimeter instruments and static systems, which includes the air data computers and altitude encoders in modern avionics suites. A Garmin G1000 has an air data computer that feeds altitude information to the transponder - that system must be inspected under the same 24-calendar-month requirement.

The glass panel doesn’t create an exemption. When scheduling the inspection, ask explicitly whether the shop is testing the full system, not just legacy gauges.

One additional note: the altitude encoder that feeds the transponder typically also serves as the altitude reference for an autopilot’s altitude-hold function. If that encoder is out of tolerance, the autopilot may hold an altitude that doesn’t match what ATC sees. The inspection catches this.

What Should You Check When Buying a Used Airplane?

Before closing on a used aircraft you plan to fly IFR, verify both inspections in the maintenance logbooks:

1. Altimeter and static system inspection - must be within 24 calendar months, logged by an appropriately rated technician with certificate number
2. Transponder inspection - same interval, separate logbook entry

If either is overdue, the airplane needs that inspection before the next applicable flight. Factor the scheduling and cost into your purchase decision.

What Do Instrument Checkride Examiners Ask About These Regulations?

The Instrument Rating Airman Certification Standards lists aircraft documents, logbook requirements, and airworthiness requirements as a task area. An examiner asking what must be current before departing IFR is testing exactly this.

Knowing the regulation is the first part. The follow-up question is where the records are found. The answer: the aircraft maintenance logbook, with a signed entry from an appropriately rated technician including their certificate number. Hesitating on the follow-up is a common mistake for students who memorized the list by acronym but didn’t work through the practical records check.

Key Takeaways

• FAR 91.411 requires a 24-calendar-month inspection of the static pressure system, altimeter, and automatic pressure altitude reporting system before any IFR flight in controlled airspace
• FAR 91.413 requires a 24-calendar-month transponder inspection before using a transponder in any airspace where one is required - this applies to VFR flights too
• Both inspections run on the calendar month interval, not flight hours; the entire final month counts, and expiration begins on the first day of the following month
• The official record for both is the aircraft maintenance logbook - a cockpit sticker is a reminder, not proof
• These are separate inspections requiring separate logbook entries; always verify both when doing a preflight records check on an unfamiliar aircraft