FAA Flight Instructor Airplane (FIA)Navigation and AirspaceMedium
During ground school, a CFI explains the operational check tolerances required by 14 CFR 91.171 for VOR equipment used under IFR. A pilot performs a dual VOR check using two independent receivers tuned to the same VOR station. Receiver A indicates a bearing of 182°, and Receiver B indicates 177°. What should the pilot conclude?
- AThe system is airworthy because the difference is within the 6° airborne checkpoint tolerance
- BThe system is not airworthy because the 5° difference exceeds the 4° maximum allowable difference between receivers
- CThe system is airworthy as long as both readings are within 10° of each other
- DThe system is airworthy because the difference is within the 4° VOT tolerance
Show answer & explanationAnswer & explanation
Correct answer: B. The system is not airworthy because the 5° difference exceeds the 4° maximum allowable difference between receivers
14 CFR 91.171 allows a maximum difference of 4° between two independent VOR receivers checked against each other. Here 182° − 177° = 5°, which exceeds the 4° tolerance, so the equipment does not meet the requirement for IFR flight.
Why the other options are wrong
- A. 6° tolerance applies to airborne checkpoints against a designated ground point, not dual-receiver checks.
- C. 10° is not a tolerance specified anywhere in 91.171.
- D. 4° VOT tolerance applies to a VOR test signal, not a dual-receiver comparison, and the math shown is for dual check.
VOR Equipment Check Tolerances
14 CFR 91.171 specifies allowable error tolerances for various VOR accuracy checks required for IFR flight.
- VOT check: ±4°
- Ground checkpoint: ±4°
- Airborne checkpoint: ±6°
- Dual VOR receiver check: maximum 4° difference between receivers
Memory trick: 'VOT and Ground are 4, Air is 6, Dual is 4 too'