2.4 VOR Equipment Accuracy Checks (14 CFR 91.171)

VOR Equipment Accuracy Checks (14 CFR 91.171)

Very High Frequency Omnidirectional Range (VOR) ground stations and airborne receivers form the foundational terrestrial radio navigation infrastructure in the National Airspace System. Because signal drift, receiver decalibration, or antenna deterioration can introduce dangerous navigational errors during an instrument approach or en route navigation, federal aviation regulations require regular verification of receiver accuracy.

Under 14 CFR § 91.171, no person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment has been operational and checked for accuracy within the preceding 30 days.

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1. The 30-Day Operational Requirement

• Time Window: The check must have been performed within the preceding 30 days (note: this is 30 calendar days or 30 consecutive days from the date of the check, not the end of the month).
• Applicability: Required whenever VOR navigation will be utilized during any portion of an IFR flight (including primary en route navigation, intersection identification, or flying a VOR/LOC instrument approach).
• Who May Perform the Check: Any certified pilot (Private, Commercial, or ATP) holding an instrument rating or conducting an instrument flight, as well as an authorized repair technician or mechanic.

+-----------------------------------------------------------------------------+
|                  14 CFR 91.171 VOR ACCURACY CHECK METHODS                   |
|                                                                             |
|   [GROUND CHECKS: +/- 4° TOLERANCE]                                         |
|   1. VOT (FAA VOR Test Facility)      ---> 180° TO / 360° FROM (+/- 4°)    |
|   2. Certified Ground Checkpoint      ---> Published Radial on Surface (+/- 4°)|
|   3. Dual VOR System Cross-Check      ---> 4° Max Difference Between Units  |
|                                                                             |
|   [AIRBORNE CHECKS: +/- 6° TOLERANCE]                                       |
|   4. Certified Airborne Checkpoint    ---> Published Landmark/Altitude (+/- 6°)|
|   5. Prominent Airway Landmark Check  ---> Airway Centerline (+/- 6°)       |
+-----------------------------------------------------------------------------+

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2. Permissible Methods of VOR Accuracy Checks

Under 14 CFR § 91.171(b) and (c), the FAA permits five specific methods for verifying VOR receiver accuracy. These methods are prioritized based on availability and ground versus airborne conditions:

1. FAA VOR Test Facility (VOT) — Tolerance: ±4°

• What is a VOT? An automated, low-power FAA transmitter installed on designated airports that transmits a constant test signal on a published VHF frequency.
• Where to Find VOT Frequencies: Listed in the Chart Supplement (formerly Airport/Facility Directory - A/FD), instrument approach plates, and VOR frequency boxes on navigation charts.
• Signal Characteristic: Unlike a standard VOR that transmits 360 individual radials, a VOT radiates only the 360° radial in all directions.
• Tuning and Indications:
  1. Tune the VOR receiver to the published VOT frequency and identify the continuous series of dots/audio tone or morse code identifier ("V").
  2. Center the CDI needle:
     • With the Omni Bearing Selector (OBS) set to 360° (000°), the flag must indicate FROM.
     • With the OBS set to 180°, the flag must indicate TO.
• Memory Rule: "Cessna 182" $\rightarrow$ 180° TO, or 360° FROM.
• Allowable Tolerance: ±4° of error.
  • Acceptable OBS readings for a centered needle: 176° to 184° with a TO flag, or 356° to 004° with a FROM flag.

[!NOTE] Why Does a VOT Give 180° TO / 360° FROM? Because the VOT station transmits the 360° radial outward in all directions, if you dial 360° into the OBS, you are tracking FROM the station (outbound). If you dial 180° into the OBS, you are selecting a course TO the station (inbound). If the needle centers on 180° with a FROM indication, or 360° with a TO indication, the receiver is reverse-sensing or malfunctioning.

2. Certified Ground Checkpoint — Tolerance: ±4°

• Description: A designated painted point on an airport taxiway, apron, or run-up area where a line of sight to a nearby VOR ground station is established.
• Information Source: Chart Supplement lists the airport, specific taxiway location, VOR frequency, and the exact published radial.
• Procedure: Taxi the aircraft to the painted ground checkpoint dot, tune the VOR station, and dial the published radial into the OBS. Center the CDI.
• Indication & Tolerance: Must indicate the published radial within ±4° with a FROM indication.

3. Certified Airborne Checkpoint — Tolerance: ±6°

• Description: An established geographic fix or landmark located along an airway within line of sight of a VOR station.
• Information Source: Chart Supplement lists the specific visual landmark, VOR frequency, published radial, and the mandatory check altitude.
• Procedure: Fly directly over the designated landmark at the prescribed altitude, tune and identify the VOR station, and center the CDI on the published radial.
• Indication & Tolerance: The indicated bearing must be within ±6° of the published radial with a FROM indication.

4. Dual VOR System Cross-Check — Tolerance: 4° Maximum Difference

• Applicability: Aircraft equipped with two independent VOR receivers (receiver units and indicators, though they may share a common antenna).
• Procedure: Tune both VOR receivers (NAV 1 and NAV 2) to the same VOR ground station (can be performed either on the ground or in flight).
• Centering: Center both CDI needles or note the indicated radials.
• Tolerance: The maximum permissible variation between the two indicated bearings is 4° (i.e., $|\text{Bearing}_1 - \text{Bearing}_2| \le 4^\circ$).
• Example: NAV 1 indicates 092° radial, NAV 2 indicates 095° radial $\rightarrow$ Difference is 3° (Legal). If NAV 2 indicates 097° $\rightarrow$ Difference is 5° (Illegal).

5. Prominent Airborne Landmark along Airway — Tolerance: ±6°

• Contingency Method: Permitted only if no VOT, ground checkpoint, or certified airborne checkpoint is available.
• Procedure: Select a prominent landmark on the ground located along the centerline of an established VOR airway. Overfly the landmark at a reasonable altitude, note the VOR radial, and compare it against the charted airway centerline.
• Tolerance: Maximum permissible error is ±6°.

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3. VOR Tolerance & Method Summary Table

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4. Mandatory Logbook Entry Requirements: 14 CFR 91.171(d)

Conducting the VOR accuracy check is only half the legal requirement; the check is invalid unless properly recorded. Under 14 CFR § 91.171(d), every pilot who performs a VOR check must enter a record in the aircraft log or other permanent record.

The required logbook entry items are remembered by the mnemonic DPBS (or D-P-B-S):

+-----------------------------------------------------------------------------+
|                     14 CFR 91.171(d) - DPBS LOG ENTRY                       |
|                                                                             |
|   [D] DATE        ---> Date the check was conducted (e.g., 08/21/2026)      |
|   [P] PLACE       ---> Location of check (e.g., KDTO Ground, KDAL VOT)      |
|   [B] BEARING ERR ---> Amount and direction of error (e.g., +2° error, -1°) |
|   [S] SIGNATURE   ---> Signature of the pilot performing the check          |
+-----------------------------------------------------------------------------+

Sample Compliant VOR Log Entries

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Date        Place / Facility    Bearing Error    Signature & Cert
--------------------------------------------------------------------------------
08/21/2026  KADS VOT 111.4      +1° (#1) / 0° (#2)  John Doe (PPL 3948201)
08/21/2026  KDTO Ground Check   -2° Error        Jane Smith (CPL 4829103)
08/21/2026  In-Flight (Dual)    2° Diff (090/092) R. Chen (CPL 5928104)
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[!WARNING] Common FAA Knowledge Exam Pitfalls:

1. 30 Days vs. Calendar Month: The VOR check is strictly a 30-day limit, unlike the 24-calendar-month pitot-static rule or the 12-calendar-month annual rule. If checked on March 1st, it expires on March 31st (30 days), not the end of April.
2. Omission of DPBS Items: Failing to sign the log, omitting the location, or not recording the specific degree error violates 14 CFR 91.171(d) and invalidates the check for an IFR flight.