Split VOR Indications in IMC
VT-RNA, a twin-engine aircraft, is cleared on an IFR route using VOR XYZ on 114.60. Ten minutes after station passage, the HSI CDI swings fully left and the NAV warning flag flickers. The audio ident is present but distorted by a high-pitched whine. The No. 2 VOR, tuned to the same station, shows a stable course but its CDI is 12 degrees different from the No. 1. ATC asks: 'VT-RNA, report distance from XYZ.' The aircraft has a panel-mounted GPS, but its database is 40 days out of date and it is not approved for the approach. The pilot is in IMC, 25 NM from a hold fix, with no radar coverage below 6,000 ft. The pilot must decide which navigation information, if any, can be trusted.
1. How should the pilot interpret the No. 1 and No. 2 VOR indications?
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The No. 1 VOR is not usable for navigation. A full-scale CDI deflection with a flickering NAV flag is a failure indication, not a normal off-course indication. The distorted audio ident and high-pitched whine also suggest a receiver or signal-quality problem. Do not follow the No. 1 CDI.
The No. 2 VOR is more stable, but a 12-degree disagreement with No. 1 cannot simply be averaged. Since No. 1 is suspect, the No. 2 indication may be the better one, but it still needs independent confirmation. Check the No. 2 ident, the published radial, the heading, and any other available bearing source. If no independent confirmation is available, treat the No. 2 VOR as degraded rather than fully reliable. The out-of-date, unapproved GPS must not be used as the primary means for the approach.
2. What should the pilot tell ATC, and what should be requested?
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The pilot should tell ATC about the degraded navigation capability without overstating it:
'VT-RNA, No. 1 VOR unreliable, No. 2 VOR indications suspect. Request radar vectors to the hold fix and confirmation of our position.'
This informs the controller that the aircraft may not be able to fly a VOR-defined route accurately. Requesting vectors is appropriate because radar coverage exists above 7,000 ft and the aircraft is in IMC. The pilot should not simply report 'distance from XYZ' if the distance cannot be verified. If ATC asks for distance, state that the DME or VOR-derived distance is unreliable and give the best independent estimate only if it can be supported. Request a climb into radar coverage if terrain and clearance permit.
3. What navigation plan should the pilot follow if ATC cannot provide vectors?
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If vectors are not available, the pilot should continue on the last cleared route using the most reliable available information while keeping a safe terrain margin. Use the No. 2 VOR only after confirming its ident and cross-checking it against the aircraft heading, expected radial, and any other bearing source. If the No. 2 indication remains inconsistent, declare the navigation capability degraded and request an amended clearance, such as a climb to a surveillance-covered level or a route that does not depend on the failed VOR.
Do not use the expired GPS database for the approach. It may be used only as a situational-awareness aid if company or regulatory policy permits, not as the primary navigation source. If the aircraft cannot maintain the required navigation performance in IMC, the pilot should consider diverting to an airport with radar coverage or better navigation aids, and should inform ATC early rather than waiting until the hold fix.