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.
In Plain English
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.
Regulation Text
14 CFR § 91.171§ 91.171 VOR equipment check for IFR operations.
(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)
Oral Exam Questions a DPE Might Ask
Q1Before flying IFR using VOR navigation, what equipment check is required?
Per FAR 91.171, the VOR must either be maintained under an approved procedure or have had an operational check within the preceding 30 days that was found within the allowable bearing error.
Q2What are the acceptable methods of performing a VOR check and their tolerances?
FAR 91.171 allows a VOT or repair-station test signal (±4°), a ground checkpoint (±4°), an airborne checkpoint (±6°), an airway/ground-point check more than 20 NM from the station (±6°), or a dual-VOR cross-check (±4°).
Q3What must be recorded after completing a VOR operational check?
FAR 91.171(d) requires entering the date, place, bearing error, and your signature in the aircraft log or other record; if a repair station test signal was used, the repair station must also certify the transmitted bearing and date.
Studying for a checkride?
Related Sections in Part 91