FAA Airline Transport Pilot (ATM)Navigation, Air Traffic Control and ProceduresMedium
A pilot conducting a VOR operational check under 14 CFR 91.171 cross-checks two independent VOR receivers against each other in flight because no VOT or designated ground checkpoint is available. What is the maximum allowable difference between the two indicated bearings for the check to be satisfactory?
- A8 degrees
- B2 degrees
- C6 degrees
- D4 degrees
Show answer & explanationAnswer & explanation
Correct answer: D. 4 degrees
Per 91.171, when checking dual VOR receivers against each other, the maximum permissible difference between the two indicated bearings to the same station is 4 degrees. VOT and ground checkpoints allow ±4°, while airborne checkpoints allow ±6°, but the dual-receiver cross-check tolerance is 4 degrees.
Why the other options are wrong
- A. 8 degrees exceeds any allowable VOR check tolerance.
- B. 2 degrees is tighter than the regulatory tolerance for this check.
- C. 6 degrees is the tolerance for an airborne checkpoint, not the dual-receiver check.
VOR Operational Check (91.171)
A required periodic accuracy check of VOR equipment before flight under IFR, with different tolerances depending on the check method used.
- VOT check tolerance: ±4 degrees
- Ground checkpoint tolerance: ±4 degrees
- Airborne checkpoint tolerance: ±6 degrees
- Dual VOR receiver cross-check: maximum 4-degree difference between receivers
Memory trick: 'VOT and Ground = 4, Air = 6, Dual = 4' — remember 4-6-4