VOR check every 30 days, and the error limits.

What the rule says

14 CFR 91.171(a): no person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment "(1) Is maintained, checked, and inspected under an approved procedure; or (2) Has been operationally checked within the preceding 30 days, and was found to be within the limits of the permissible indicated bearing error." This page covers route (2).

Counting the 30 days

The rule says "within the preceding 30 days" and does not say how the days are counted. The checker shows the 30th day after the check date; it is a reading of the text, not an FAA interpretation.

Check methods and permitted error

• Test signal at the departure airport, FAA-operated or approved, or from an appropriately rated repair station: plus or minus 4 degrees (91.171(b)(1)).
• A designated VOR checkpoint on the airport surface: plus or minus 4 degrees (b)(2).
• If neither is available, an airborne checkpoint designated by the Administrator: plus or minus 6 degrees (b)(3).
• If none is available, in flight over a prominent ground point on a VOR radial along the centerline of an established VOR airway, preferably more than 20 nautical miles from the station, at a reasonably low altitude: maximum variation 6 degrees (b)(4).
• Dual VOR systems, tuned to the same ground facility: the two indicated bearings may differ by at most 4 degrees (c).

The required record

91.171(d): the person making the check "shall enter the date, place, bearing error, and sign the aircraft log or other record." If a repair-station test signal is used, the repair station must also enter the bearing transmitted and the date.

Worked example

Check on October 10, 2026, found within 4 degrees at a ground checkpoint and logged. Counting 30 days gives November 9, 2026 as the last day. An IFR flight on November 12, 2026 using VOR needs a new check or use of route (1).

FAQ

How often must a VOR be checked for IFR?

Within the preceding 30 days under 91.171(a)(2), unless the equipment is maintained and checked under an approved procedure.

What bearing error is allowed?

Plus or minus 4 degrees at a test signal or ground checkpoint, and 6 degrees for an airborne checkpoint or in-flight check.

Can I check one VOR against the other?

Yes, if dual independent VOR systems are installed, tuned to the same station, with at most 4 degrees between them.

What must go in the logbook?

The date, place, bearing error and a signature, plus the repair station entry if its test signal was used.

Is a VOR check needed for VFR flight?

Not under this rule. It applies to IFR operations using the VOR system.

Sources

Retrieved 2026-11-02. Regulation text read through the eCFR API (current as of 2026-10-29); AIM text read from the FAA publication URL below. Rules change; check the current text.

• 14 CFR 91.171, VOR equipment check for IFR operations: the 30-day check, error limits, check methods and record entry.