14 CFR 91.171 — VOR Equipment Check for IFR (Explained)

What does 14 CFR 91.171 require?

14 CFR 91.171(a) prohibits operating a civil aircraft under IFR using the VOR system unless the VOR equipment either meets an approved manufacturer maintenance standard or has been operationally checked within the preceding 30 days and found within permissible bearing error. The rule applies to any IFR flight that relies on VOR — including routes where VOR is required by the departure procedure, en route structure, or approach. The pilot in command is responsible for confirming currency before flight.

The regulation exists because VOR receivers degrade over time. A receiver with a 7-degree error may appear to work normally but will put you on a course that diverges significantly from the centerline — dangerous in IMC and on approaches. The 30-day interval balances operational practicality against the risk of undetected receiver drift.

What are the four VOR check methods?

14 CFR 91.171(b) lists four acceptable methods for the required VOR operational check. Each has a different maximum permissible bearing error:

When a repair station test signal is used in place of the FAA VOT, the same ±4° tolerance applies. The repair station representative must record the bearing transmitted and the date of transmission in the aircraft records — a separate documentation requirement from the pilot's own logbook entry.

How do you use a VOT to check VOR accuracy?

A VOR Test Facility (VOT) is a ground-based signal transmitter that broadcasts a single radial on a published frequency. Unlike a regular VOR station that transmits 360 radials, the VOT transmits only one omnidirectional test signal. To conduct a VOT check, tune the published VOT frequency and center the CDI. The OBS should read 0° (360°) with a FROM indication, or 180° with a TO indication — any other centered-needle result reveals receiver error. The difference between what you read and the published value is your bearing error. To find VOT frequencies, consult the Chart Supplement (Airport/Facility Directory), which lists VOT stations by airport and frequency. The AIM also addresses VOT use in AIM 1-1-3 .

How do designated ground checkpoints work?

Designated ground checkpoints are specific surface locations — typically on a taxiway or ramp — where you can check the accuracy of your VOR by tuning a nearby VOR station and comparing your indicated bearing to the published value. The FAA publishes designated checkpoint locations in the Chart Supplement. Each entry lists the airport, checkpoint surface location, VOR station to use, published radial to center, and any altitude or distance notes. The tolerance is ±4° from the published radial per 14 CFR 91.171(b)(2) . Ground checkpoints are the most common choice because they are easy to incorporate into a preflight at airports that have them.

How do designated airborne checkpoints work?

When no ground checkpoint or VOT is available at the departure airport, 14 CFR 91.171(b)(3) allows an airborne checkpoint. These are published checkpoint fixes — typically over a prominent landmark, at a defined altitude, a specific distance from a VOR — where the published radial is known and you can compare your receiver's indication. The wider ±6° tolerance reflects the additional error sources in flight: altitude variations, terrain scattering, and bank angle effects on the antenna. Airborne checkpoint locations are also listed in the Chart Supplement.

What is the dual VOR check method?

Aircraft equipped with 2 independent VOR receivers (each with its own antenna) may satisfy the 30-day requirement under 14 CFR 91.171(c) by comparing the two units against the same VOR ground station. Tune both receivers to the same VOR, note the indicated bearing on each, and compute the difference. If the two receivers agree within 4°, the check is complete. No external checkpoint is needed. Note: this method only confirms that the two receivers agree with each other — it does not reveal whether both are systematically shifted. A receiver pair drifting together in the same direction could still both be out of tolerance against the actual radial.

What must a VOR check logbook entry contain?

14 CFR 91.171(d) mandates that the following information be recorded in the aircraft maintenance records or in a separate record kept in the aircraft:

1. 1
   Date of the check
2. 2
   Place where the check was performed (airport, checkpoint ID, or VOR station name)
3. 3
   Bearing error found (in degrees)
4. 4
   Signature of the person performing the check

When a repair station signal is used, the repair station representative must separately record the bearing transmitted and the date of transmission. The pilot's log entry and the repair station's record together constitute the complete documentation for that check. There is no prescribed log format — the four elements above may be entered in any maintenance logbook, a dedicated VOR check log card carried in the aircraft, or the aircraft's airframe logbook.

What do DPEs ask about 91.171 during the oral exam?

This regulation is a fixture of IFR oral exams. Examiners frequently probe both the rule's requirements and the candidate's practical understanding of when and how to apply each check method. Know the tolerances cold — confusing ±4° with ±6° is one of the most common oral errors on this topic.

• "What are your options for satisfying the VOR check requirement before this IFR flight?"
• "Your departure airport has no VOT and no designated checkpoint. What do you do?"
• "You have a two-VOR aircraft. Walk me through how the dual VOR check works and what its limitation is."
• "What goes in the logbook entry for a VOR check, and where do you keep that record?"
• "Your VOR check was done 31 days ago. Are you legal to fly IFR?"
• "If your GPS is WAAS-certified and your route is GPS-navigable, do you still need a VOR check?"

Practice Questions

1. You are departing on an IFR cross-country. The airport has a VOT on 108.0 MHz. You tune the VOT, center the CDI needle, and the OBS reads 004°. Is this check within tolerance, and what do you log?

2. The Chart Supplement shows no VOT and no designated ground checkpoint at your departure airport. You find an airborne checkpoint listed 15 nm north. What is the maximum bearing error allowed at that checkpoint, and where does the 14 CFR cite for this tolerance?

3. Your aircraft has 2 independent VOR systems. You tune both to the KORD VOR. VOR No. 1 indicates 090°; VOR No. 2 indicates 095°. Does this satisfy the 30-day check requirement? What is the limitation of this method?

4. A DPE asks you to describe every field required in a VOR check logbook entry. Name all four and cite the regulation that requires them.

5. Your VOR check was performed 28 days ago. The bearing error logged was 5° airborne checkpoint. Are you legal to fly an IFR route today that relies on VOR navigation?

Frequently Asked Questions

Q: How often must you check VOR accuracy for IFR flight?

Every 30 days. Under 14 CFR 91.171(a)(2) , the VOR must have been operationally checked within the preceding 30 days and found to be within the permissible bearing error for your check method.

Q: What is the tolerance for a VOT check?

±4 degrees. When using an FAA-operated VOR Test Facility (VOT), the indicated bearing must be within 4 degrees of the published radial. The same ±4-degree limit applies to a repair station test signal and to a designated ground checkpoint.

Q: What is the tolerance for an airborne VOR checkpoint?

±6 degrees. Airborne checkpoints introduce more variables — altitude, terrain reflection, aircraft banking — so 14 CFR 91.171(b)(3) allows a wider 6-degree bearing error tolerance compared to the ±4-degree ground standard.

Q: Can you use two VORs instead of an external checkpoint?

Yes, if your aircraft has two independent VOR systems. Under 14 CFR 91.171(c) , you compare the two units against the same VOR ground station. If they agree within 4 degrees, the check satisfies the 30-day requirement.

Q: What must a VOR check logbook entry include?

Date, place of check, bearing error found, and the signature of the person who made the check. When a repair station signal is used, the repair station also records the bearing transmitted and the date of transmission per 14 CFR 91.171(d) .

Q: Can you fly IFR without a VOR check if you have GPS?

Only if GPS is the sole navigation system and VOR is not required by the route or ATC. If any segment of the IFR flight requires VOR navigation, 91.171 applies. GPS under 14 CFR 91.205(d) satisfies the IFR equipment requirement only where approved.

Q: Where do you find designated ground and airborne VOR checkpoints?

The FAA publishes designated VOR checkpoints in the Chart Supplement (formerly Airport/Facility Directory). Each listing includes the checkpoint location, the VOR to tune, and the published radial and distance.

Q: What happens if you fly IFR and your VOR check has lapsed?

You are not legal to use VOR navigation under IFR. Flying with an out-of-date VOR check violates 14 CFR 91.171(a) , which prohibits operating under IFR using the VOR system unless the check is current. The PIC is responsible.

Sources

• 14 CFR 91.171 — VOR Equipment Check for IFR Operations (Cornell LII)
• 14 CFR 91.171 — VOR Equipment Check for IFR Operations (eCFR)
• 14 CFR 91.205 — Powered Civil Aircraft: Instrument and Equipment Requirements
• AIM 1-1-3 — VOR Receiver Check (Aeronautical Information Manual)
• FAA Instrument Rating ACS (FAA-S-ACS-8C)

This article was researched from FAA primary sources (14 CFR Part 91 via Cornell LII / eCFR, Aeronautical Information Manual, Instrument Rating ACS) and citing current regulatory text — drafted by MockDPE. Last updated: May 2026. If you spot an inaccuracy, email corrections@mockdpe.org.