RAIM outage and an unavailable fallback approach
An IFR flight is cleared for the GPS approach at destination, ETA 1415Z. The aircraft has a single GNS 430 (GPS) and a VOR/ILS receiver, but no DME. Destination weather is OVC005, visibility 2 km. The only other approach is a VOR approach with DME minimums; the VOR facility is serviceable. A NOTAM gives a GPS RAIM outage from 1400Z to 1430Z. The alternate is 30 minutes away with an ILS, weather OVC008, visibility 4 km, and fuel at destination would leave 1 h 10 min. ATC has said 'expect the GPS approach'. The aircraft's GPS is approved for IFR approach operations, but the pilot has not updated the navigation database for 40 days; the approach is in the database. There is no icing forecast.
1. Can the GPS approach be flown at ETA? Explain the RAIM issue and any second problem.
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No. RAIM is the receiver's ability to detect a faulty satellite signal. The NOTAM says RAIM is unavailable 1400-1430Z, and ETA is 1415Z. During that window the GPS approach cannot be relied on for integrity; the receiver may flag or, worse, provide misleading guidance.
The expired database is a second problem: if the navigation database is not current, the approach should not be flown even outside the outage. The approach is therefore not available at ETA. ATC's 'expect the GPS approach' is not a clearance to fly it when the required integrity is missing; the pilot must advise ATC.
2. Can the VOR approach be used as a fallback?
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No. The VOR approach has DME minimums and the aircraft has no DME. A GPS distance may not be substituted for DME unless the procedure and the installation are approved for it; the pilot cannot simply read the GPS mileage and call it DME. The VOR receiver being serviceable is irrelevant if the approach cannot be flown to its minima.
So the only usable approach at destination is the GPS approach, and it is unavailable. This is exactly the kind of case where the fallback must be identified before departure, not after the RAIM outage begins.
3. What is the correct diversion decision and communications plan?
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Divert to the ILS alternate. Fuel at destination is 1 h 10 min; the alternate is 30 min away, leaving 40 min. Holding 15 min for RAIM would leave about 25 min - less than the 30-min alternate plus any reserve - so waiting is not a safe option.
Advise ATC: 'Unable GPS approach due RAIM, request diversion to [alternate].' Do not declare an emergency unless fuel or another factor requires it; if the diversion plus reserve becomes tight, declare a PAN PAN. Brief the ILS and keep the VOR receiver as a backup for en-route navigation. Also record the database expiry so it is corrected before the next IFR flight.