RAIM Loss on an RNAV Approach
You are cleared for an RNAV (GNSS) approach to Runway 27 at a regional airport. The aircraft is at 15 NM final, 3,000 ft, groundspeed 140 kt. The GNSS receiver predicts RAIM availability until 14:32; the current time is 14:27. The approach minima are 600 ft, and the missed approach requires a climb to 4,000 ft. The airport has a VOR/DME on the field, but the VOR is 4 NM off the extended centreline and has a published 3° bearing error. There is no published VOR approach to Runway 27. ATC has advised that the destination weather is at the approach limit, with no improvement expected before 14:40. You must decide whether to continue the approach and what to do if the RAIM flag appears at 900 ft.
1. Calculate whether RAIM loss is predicted before the missed approach point and at approximately what altitude.
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Time to the missed approach point = 15 NM / 140 kt = 0.107 h = 6.43 min. RAIM expires in 5 min, so predicted loss is 1.43 min before the MAPt, or 140 × (1.43/60) = 3.3 NM before it.
The descent from 3,000 ft at 15 NM to 600 ft at the MAPt gives an average slope of (3,000 − 600) / 15 = 160 ft/NM. At 3.3 NM before the MAPt the aircraft is about 600 + 3.3 × 160 = 1,130 ft.
So RAIM loss is predicted at about 1,100 ft, above the 600 ft minima and before the MAPt. The approach cannot be assured under the current timing; expect to discontinue or delay.
2. Can the VOR/DME be used to complete the approach if RAIM is lost? What action would you take at 900 ft if the RAIM flag appears?
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No. The VOR/DME is not on the extended centreline and has a published 3° bearing error; it is not a substitute for the lateral guidance required by the RNAV approach. A 3° error at 4 NM is about 0.2 NM, but obstacle clearance is based on the surveyed RNAV path, not on a nearby navaid. It may be used for situational awareness or for a separately published VOR procedure, but there is none here.
If the RAIM flag appears at 900 ft, go around immediately. Do not descend below 600 ft without valid lateral guidance. Fly the published missed approach, advise ATC of the RAIM loss, and either hold for RAIM recovery or divert. Because the weather is at the limit and no improvement is expected before 14:40, delaying for a short RAIM outage is not a sound plan.
3. What pre-approach decision would you make at 15 NM, and why?
Read the worked answer
At 15 NM the predicted RAIM loss occurs before the MAPt, so the approach should not be continued as cleared. The safer decision is to discontinue before the final approach segment, request vectors or holding, and reassess RAIM after the outage window. If RAIM is not expected to return before the fuel or weather limit, divert to an airport with a conventional approach. Continuing and hoping the flag appears after landing is not acceptable because the loss is predicted above minima; the crew would be committed to a missed approach at low altitude with the weather at the limit.