GNSS Approach with a RAIM Alert and Deteriorating Weather
You are flying a regional jet on a GNSS (LNAV/VNAV) approach to runway 18 at an airport with a 2400 m runway. The weather is 600 m visibility, RVR 800 m, ceiling 300 ft, and the LNAV minima are 350 ft MDA and 1200 m visibility. The GNSS receiver shows a RAIM prediction of 5 satellites with a fault detection availability of 99.9% for the approach window, but during the approach the RAIM alert triggers at 8 NM final, indicating a loss of integrity. The tower reports the wind as 180/12, and the runway is dry. The missed approach requires a climb to 3000 ft and a turn to the right to a holding fix. Your alternate is 40 NM away with an ILS and weather 800 ft ceiling and 3000 m visibility.
1. What does the RAIM alert mean, and what are your options at 8 NM final?
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RAIM (Receiver Autonomous Integrity Monitoring) detects whether the GNSS position is consistent with the received satellites; a RAIM alert means the receiver cannot guarantee the integrity of the position to the level required for the approach, so the GNSS guidance is no longer usable for the approach. At 8 NM final you are still outside the final approach segment and have time to act. Options, in order: (1) abandon the GNSS approach and request an alternative - if the airport has an ILS or other ground-based approach, request it; (2) if no alternative, divert to the alternate 40 NM away, which has an ILS and weather above minima; (3) do not continue the GNSS approach hoping the alert clears, because integrity is not assured.
The correct action is to inform ATC immediately, request either a different approach type or a diversion, and fly the published missed approach if already inside the final approach segment. Since you are at 8 NM, you can likely be vectored for an ILS or cleared to the alternate without flying the missed approach.
2. Compare the weather against the LNAV minima and decide whether the approach would have been legal even without the RAIM alert.
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LNAV minima are 350 ft MDA and 1200 m visibility. The reported weather is a ceiling of 300 ft and visibility 600 m with RVR 800 m. The ceiling of 300 ft is below the 350 ft MDA, so the aircraft would not have been able to descend to the MDA and see the runway - the approach would not be legal to commence under the operator's rules if the ceiling is below the MDA. The visibility of 600 m (RVR 800 m) is also below the 1200 m required. So even without the RAIM alert, the approach should not have been attempted: the weather is below minima.
The judgement lesson: the decision to commence an approach must be made against the minima before final, and the reported weather must meet or exceed both the ceiling and visibility criteria. The RAIM alert is a second, independent reason to discontinue. The alternate at 800 ft ceiling and 3000 m visibility is comfortably above its ILS minima and is the correct destination.
3. If you are inside the final approach segment when the alert occurs at 2 NM, describe the missed approach procedure and the considerations for the turn.
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Inside the final approach segment, the missed approach must be flown immediately: apply go-around power, pitch to the missed approach climb attitude, and follow the published procedure - climb to 3000 ft and turn right to the holding fix. The turn direction matters: with a wind of 180/12 (a direct headwind on runway 18), the right turn takes you crosswind then downwind; keep the turn rate standard and do not exceed bank limits in the missed approach. Maintain the climb gradient published for the missed approach; if the aircraft cannot achieve it, advise ATC.
Considerations: obstacle clearance in the missed approach is based on the published procedure, so do not turn early or late. The RAIM alert means the GNSS guidance may be unreliable during the missed approach; if the missed approach procedure is GNSS-based, use the initial climb and turn based on the published track and your other navigation aids (VOR/ILS) if available. Communicate with ATC: "Going around, RAIM alert, request vectors for the ILS or diversion." Do not attempt to re-fly the GNSS approach until RAIM is predicted available.