Regulation Text
(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)Research Notes
Section 91.171 requires VOR equipment used for IFR operations to be checked for accuracy within the preceding 30 days. The check confirms the VOR receiver is operating within tolerance.
The 30-day check options:
- VOT (VOR Test Facility) check, paragraph b(1): Tune to a published VOT frequency. With OBS set to 0°, the CDI should center with a FROM indication; with OBS set to 180°, the CDI should center with a TO indication. Tolerance: ±4°. Available at major airports.
- Ground checkpoint, paragraph b(2): Use a published designated ground checkpoint shown in the AFD/Chart Supplement. Tolerance: ±4°.
- Airborne checkpoint, paragraph b(3): Use a published airborne checkpoint. Tolerance: ±6°.
- Airborne dual VOR check, paragraph b(4): If aircraft has two VORs, both tuned to same station from the same location. The bearings must agree within 4° of each other.
- Airborne VOR over prominent landmark, paragraph b(5): Fly over an identifiable landmark at least 20 NM from the VOR. Tolerance: ±6°.
Documentation: The pilot must record the place, date, bearing error, and signature in the aircraft logbook or another permanent record. ('Date - Place - Bearing - Signature' is the standard memory device.)
30-day calendar window: The check must be within 30 days preceding the IFR flight. Don't confuse this with the 6-month VOR check for some other applications — the standard VOR equipment check is 30 days for IFR.
RNAV/GPS substitution: Many modern IFR-capable aircraft use RNAV/GPS for primary navigation and no longer need to satisfy § 91.171 — the VOR is supplementary. However, the equipment installed and used for navigation under IFR must be operationally checked per the AFM/POH and FAA guidance.
Reference: AIM 1-1 on Navigation Aids; FAA-H-8083-16 Chapter 3.
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