VOR equipment check for IFR14 CFR 91.171 VOR equipment check for IFR operations
To navigate by VOR under IFR, the aircraft's VOR equipment must either be maintained, checked and inspected under an approved procedure or have passed an operational check within the preceding 30 days. The allowed error is plus or minus 4 degrees for a test signal or surface checkpoint, plus or minus 6 degrees for an airborne checkpoint, 6 degrees for the in-flight airway check and 4 degrees between two receivers in a dual check.
By the ATIS Alerts team · Updated October 15, 2026 · Regulation text current as of October 14, 2026. A study aid based on the official regulation text, not legal advice or an FAA interpretation.
At a glance
- VOR equipment used under IFR must be maintained, checked and inspected under an approved procedure, or operationally checked within the preceding 30 days (a).
- Test signal or designated surface checkpoint at the airport of intended departure: plus or minus 4 degrees (b)(1) and (b)(2).
- Designated airborne checkpoint, if neither of those is available: plus or minus 6 degrees (b)(3).
- In flight over a prominent ground point on a radial along an established VOR airway, if no check signal or point is available: 6 degrees maximum (b)(4).
- Dual VOR systems tuned to the same ground facility: 4 degrees maximum between the two indicated bearings (c).
- Enter the date, place and bearing error and sign the aircraft log or other record (d).
In plain English
Section 91.171 applies when you operate a civil aircraft under IFR using the VOR system of radio navigation. Paragraph (a) gives two ways to meet it: the VOR equipment is maintained, checked and inspected under an approved procedure, or it has been operationally checked within the preceding 30 days and found within the permissible bearing error in (b) or (c).
Paragraph (b) lists the checks in a set order, and the airborne and in-flight options apply only when the earlier ones are not available. At the airport of intended departure, use an FAA-operated or approved test signal (or one radiated by a certificated and appropriately rated radio repair station), or a point on the airport surface designated as a VOR system checkpoint. Either allows a maximum error of plus or minus 4 degrees. If neither is available, use a designated airborne checkpoint, which allows plus or minus 6 degrees.
If no check signal or point is available at all, (b)(4) describes an in-flight check. Select a VOR radial along the centerline of an established VOR airway, pick a prominent ground point on that radial (preferably more than 20 nautical miles from the VOR ground facility), and fly directly over it at a reasonably low altitude. The bearing indicated over the point may differ from the published radial by no more than 6 degrees.
Paragraph (c) offers an alternative for aircraft with dual VOR systems that are independent of each other except for the antenna. Tune both to the same VOR ground facility and compare the indicated bearings to that station; they may differ by no more than 4 degrees. Whichever check you use, paragraph (d) requires the person making it to enter the date, place and bearing error and sign the aircraft log or other record.
Common mistakes
- Mixing up the tolerances: 4 degrees for a test signal, surface checkpoint or dual check; 6 degrees for an airborne checkpoint or the in-flight airway check.
- Leaving out part of the record. Paragraph (d) calls for the date, place, bearing error and a signature.
- Doing a dual check with systems that share more than the antenna. The (c) check requires units independent of each other except for the antenna.
The official text
14 CFR 91.171 as published on the eCFR, current as of October 14, 2026.
(a) No person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment of that aircraft—
(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 set forth in paragraph (b) or (c) of this section.
(b) Except as provided in paragraph (c) of this section, each person conducting a VOR check under paragraph (a)(2) of this section shall—
(1) Use, at the airport of intended departure, an FAA-operated or approved test signal or a test signal radiated by a certificated and appropriately rated radio repair station or, outside the United States, a test signal operated or approved by an appropriate authority to check the VOR equipment (the maximum permissible indicated bearing error is plus or minus 4 degrees); or
(2) Use, at the airport of intended departure, a point on the airport surface designated as a VOR system checkpoint by the Administrator, or, outside the United States, by an appropriate authority (the maximum permissible bearing error is plus or minus 4 degrees);
(3) If neither a test signal nor a designated checkpoint on the surface is available, use an airborne checkpoint designated by the Administrator or, outside the United States, by an appropriate authority (the maximum permissible bearing error is plus or minus 6 degrees); or
(4) If no check signal or point is available, while in flight—
(i) Select a VOR radial that lies along the centerline of an established VOR airway;
(ii) Select a prominent ground point along the selected radial preferably more than 20 nautical miles from the VOR ground facility and maneuver the aircraft directly over the point at a reasonably low altitude; and
(iii) Note the VOR bearing indicated by the receiver when over the ground point (the maximum permissible variation between the published radial and the indicated bearing is 6 degrees).
(c) If dual system VOR (units independent of each other except for the antenna) is installed in the aircraft, the person checking the equipment may check one system against the other in place of the check procedures specified in paragraph (b) of this section. Both systems shall be tuned to the same VOR ground facility and note the indicated bearings to that station. The maximum permissible variation between the two indicated bearings is 4 degrees.
(d) Each person making the VOR operational check, as specified in paragraph (b) or (c) of this section, shall enter the date, place, bearing error, and sign the aircraft log or other record. In addition, if a test signal radiated by a repair station, as specified in paragraph (b)(1) of this section, is used, an entry must be made in the aircraft log or other record by the repair station certificate holder or the certificate holder's representative certifying to the bearing transmitted by the repair station for the check and the date of transmission.
(Approved by the Office of Management and Budget under control number 2120-0005)
Test yourself
Practice questions from our free FAA written test prep that cite 14 CFR 91.171.
Private Pilot
When checking VOR accuracy using a VOT, the CDI should center within
- A. ±4° with the appropriate TO/FROM indication.
- B. ±6° with a FROM indication only.
- C. ±10° with any indication.
- D. exactly 0° with no tolerance.
Show answer
A. ±4° with the appropriate TO/FROM indication. Over a VOT, selecting 0° should give a centered CDI with FROM (or 180° with TO), and the allowable error is ±4°. Ground and airborne checkpoints have their own tolerances (±4° ground, ±6° airborne).
More Private Pilot practice →Instrument Rating
Before conducting an IFR flight using VOR for navigation, the VOR equipment must have been checked within the preceding:
- A. 30 days.
- B. 24 calendar months.
- C. 12 calendar months.
- D. 60 days.
Show answer
A. 30 days. 14 CFR 91.171 requires a VOR accuracy check within the preceding 30 days for IFR operations, with the date, place, bearing error, and pilot signature recorded.
More Instrument Rating practice →Commercial Pilot
A VOT facility is used to:
- A. check VOR receiver accuracy, with the CDI centered reading 0° FROM or 180° TO, ±4°.
- B. test the transponder's Mode C encoding.
- C. provide a DME range check.
- D. verify the accuracy of the altimeter setting.
Show answer
A. check VOR receiver accuracy, with the CDI centered reading 0° FROM or 180° TO, ±4°. A VOT transmits a signal representing the 360 radial from every direction. Remember it as "Cessna 182": Centered, 180 TO, or 0 FROM. Tolerance is ±4°, and the check must be logged with date, place, bearing error, and signature for IFR.
More Commercial Pilot practice →Instrument Ground Instructor
To use VOR navigation under IFR, the receiver must have been checked within the preceding 30 days. The maximum permissible bearing errors are:
- A. plus or minus 4 degrees on a VOT or ground checkpoint, plus or minus 6 degrees airborne, and 4 degrees between dual VORs.
- B. plus or minus 6 degrees for every kind of check.
- C. plus or minus 4 degrees for every kind of check, including airborne.
- D. plus or minus 8 degrees airborne and 4 degrees on the ground.
Show answer
A. plus or minus 4 degrees on a VOT or ground checkpoint, plus or minus 6 degrees airborne, and 4 degrees between dual VORs. Ground and VOT checks allow 4 degrees, airborne checks 6, and a dual-VOR cross-check allows 4 degrees between the indicated bearings. The check must be logged: date, place, bearing error, and signature.
More Instrument Ground Instructor practice →
Questions pilots ask
How often does the VOR need to be checked for IFR?
The VOR equipment must have been operationally checked within the preceding 30 days, unless it is maintained, checked and inspected under an approved procedure (91.171(a)).
What is the allowed error for a VOR check?
Plus or minus 4 degrees using a test signal or a designated surface checkpoint at the departure airport (91.171(b)(1) and (b)(2)), plus or minus 6 degrees using a designated airborne checkpoint (b)(3), 6 degrees for the in-flight check over a ground point on an airway radial (b)(4), and 4 degrees between two receivers in a dual check (c).
What has to be written down after a VOR check?
The date, place and bearing error, signed by the person who made the check, in the aircraft log or other record (91.171(d)). If a repair station test signal was used, the repair station certificate holder or its representative must also make an entry certifying the bearing transmitted and the date of transmission.