FAR 91.171 — VOR Equipment Check
FAR 91.171 explains the 30-day VOR check required for IFR flight, allowable bearing errors, and required logbook entries. Study guide for pilot students.
FAR 91.171 says you can't fly IFR using VOR navigation unless the VOR equipment is either maintained under an approved procedure or has been operationally checked within the preceding 30 days and found within tolerance.
You can satisfy the 30-day check using any of these methods:
- VOT (FAA test signal) at the departure airport — max error ±4°.
- Ground checkpoint designated on the airport surface — max error ±4°.
- Airborne checkpoint designated by the FAA — max error ±6°.
- Airway centerline check: pick a VOR radial along an established airway, fly over a prominent ground point (preferably more than 20 NM from the station) at low altitude, and compare the indicated bearing to the published radial — max variation 6°.
- Dual VOR cross-check: tune both receivers to the same station; bearings must agree within 4°.
After the check, log the date, place, bearing error, and your signature in the aircraft log or other record. This rule matters because IFR navigation accuracy is safety-critical — flying IFR with an uncertified VOR is both illegal and dangerous.
Five FAA-written-style questions. Instant explanations, every answer cites its source — no account needed.
You are flying VFR in Class E airspace at 8,500 feet MSL during the day. What are your minimum visibility and cloud clearance requirements?
Q1Before flying IFR using VOR navigation, what equipment check is required?
Q2What are the acceptable methods of performing a VOR check and their tolerances?
Q3What must be recorded after completing a VOR operational check?
VOR check tolerances by method
A VOR operational check is good for 30 days, and the maximum permissible bearing error is ±4° for a VOT or a designated surface checkpoint, ±6° for a designated airborne checkpoint, 6° against the published radial on the airway-centerline method, and 4° between the two indications of a dual-VOR cross-check (14 CFR § 91.171, verified 2026-08-13). The two numbers written without a plus-or-minus are not a typo in the CFR: those methods measure a variation between two readings, not an error against a known reference.
Every § 91.171 check method and its exact tolerance
| Citation | Check method | Where | Tolerance | How the CFR words it |
|---|---|---|---|---|
| § 91.171(b)(1) | FAA-operated or approved test signal (VOT), or a test signal radiated by a certificated radio repair station | At the airport of intended departure | ±4° | maximum permissible indicated bearing error |
| § 91.171(b)(2) | Point on the airport surface designated as a VOR system checkpoint by the Administrator | At the airport of intended departure, on the surface | ±4° | maximum permissible bearing error |
| § 91.171(b)(3) | Airborne checkpoint designated by the Administrator | Airborne, over the designated checkpoint | ±6° | maximum permissible bearing error |
| § 91.171(b)(4) | Airway centerline radial, flown directly over a prominent ground point preferably more than 20 NM from the VOR facility | Airborne, at a reasonably low altitude | 6° | maximum permissible variation between the published radial and the indicated bearing |
| § 91.171(c) | Dual-VOR cross-check — both units, independent except for the antenna, tuned to the same VOR ground facility | Ground or airborne | 4° | maximum permissible variation between the two indicated bearings |
What a legal bearing error costs in cross-track miles
The tolerance is angular, so the lateral error it permits grows with distance from the station — distance × tan(error).
| Bearing error | At 10 NM | At 20 NM | At 30 NM |
|---|---|---|---|
| 4° | 0.7 NM | 1.4 NM | 2.1 NM |
| 6° | 1.1 NM | 2.1 NM | 3.2 NM |
Which method you are allowed to use
- FirstUse a VOT or repair-station test signal, or a designated surface checkpoint, at the departure airport — § 91.171(b)(1) and (b)(2), ±4°.
- If neither is availableUse a designated airborne checkpoint — § 91.171(b)(3) opens only "if neither a test signal nor a designated checkpoint on the surface is available", ±6°.
- If no check signal or point is availableFly the airway-centerline method over a prominent ground point — § 91.171(b)(4), 6° against the published radial.
- Any time dual VOR is installedCheck one system against the other in place of the § 91.171(b) procedures entirely — § 91.171(c), 4° between the two indications.
- • Date of the check
- • Place of the check
- • Bearing error
- • Signature of the person making the check
Use a repair-station radiated signal under § 91.171(b)(1) and there is a second entry: the repair station certificate holder or its representative certifies the bearing transmitted for the check and the date of transmission.
The 30-day clock in § 91.171(a)(2) is not the only way to comply. Section 91.171(a)(1) is a full alternative: VOR equipment maintained, checked, and inspected under an approved procedure needs no 30-day operational check at all. Examiners ask for the tolerance and the log items; the (a)(1) escape is what separates a memorized answer from a read one.
VOR equipment check FAQ
What is the maximum permissible variation between two VOR bearing indications on a dual-VOR check?
Four degrees. Under 14 CFR § 91.171(c), when dual VOR systems (independent of each other except for the antenna) are installed, you may check one system against the other instead of the § 91.171(b) procedures. Both systems are tuned to the same VOR ground facility and the indicated bearings are noted — the maximum permissible variation between the two indicated bearings is 4 degrees. The CFR writes this as a flat 4 degrees, not "plus or minus 4 degrees", because it measures the spread between two receivers rather than error against a known reference.
What is the maximum bearing error allowed on a VOT check?
Plus or minus 4 degrees. Section 91.171(b)(1) covers both an FAA-operated or approved test signal (a VOT) and a test signal radiated by a certificated and appropriately rated radio repair station, and sets the maximum permissible indicated bearing error at plus or minus 4 degrees. A designated VOR system checkpoint on the airport surface carries the same plus or minus 4 degree limit under § 91.171(b)(2).
Which data must be recorded in the aircraft log by a pilot making a VOR operational check?
Under 14 CFR § 91.171(d), each person making the VOR operational check must enter the date, the place, and the bearing error, and must sign the aircraft log or other record — four items. If a test signal radiated by a repair station under § 91.171(b)(1) was used, the repair station certificate holder or its representative must additionally make an entry certifying the bearing transmitted for the check and the date of transmission.
How often is a VOR check required for IFR flight?
Within the preceding 30 days. Section 91.171(a)(2) prohibits operating a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment has been operationally checked within the preceding 30 days and found within the tolerances of § 91.171(b) or (c). There is an alternative: § 91.171(a)(1) satisfies the rule instead if the VOR equipment is maintained, checked, and inspected under an approved procedure, in which case the 30-day operational check does not apply.
What is the tolerance for an airborne VOR checkpoint?
Plus or minus 6 degrees for a designated airborne checkpoint under § 91.171(b)(3), which is available only if neither a test signal nor a designated checkpoint on the surface is available. The separate airway-centerline method in § 91.171(b)(4) — flying directly over a prominent ground point preferably more than 20 nautical miles from the VOR facility — allows a maximum permissible variation of 6 degrees between the published radial and the indicated bearing.
Can a pilot perform the VOR check, or does it require a repair station?
A pilot may perform it. Section 91.171(d) refers to "each person making the VOR operational check", and every method in § 91.171(b)(2) through (b)(4) and § 91.171(c) is flown or observed by the pilot. A repair station is involved only in the § 91.171(b)(1) case where the test signal is radiated by a certificated and appropriately rated radio repair station — and there the repair station makes its own additional log entry certifying the bearing transmitted and the date of transmission.