GPS Approach with a RAIM Prediction Problem
You are planning an RNAV (GPS) approach to Runway 14 at a remote airport. The approach requires WAAS or, if WAAS is unavailable, RAIM with a prediction of at least five minutes before the final approach fix. Your aircraft has a WAAS-capable GPS but the receiver's fault detection and exclusion (FDE) function is inoperative, which is permitted for en route and terminal operations but not for the approach. You checked the RAIM prediction for your estimated time of arrival and it shows a 12-minute outage beginning 8 minutes after your ETA. Weather is 3,000 ft broken, 10 SM visibility. An alternate VOR approach exists 25 NM away with a 1,200 ft MDA.
1. Can you legally and safely fly the RNAV approach as planned?
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No. With FDE inoperative, the receiver cannot provide the fault detection required for the approach, and the RAIM prediction shows an outage that overlaps your arrival window. Even though the outage begins 8 minutes after ETA, you cannot guarantee you will be established and past the final approach fix before it starts, and an outage during the approach would leave you without integrity monitoring.
The legal and safe answer is to plan a different approach. The VOR approach 25 NM away is a viable alternative, and the weather (3,000 ft broken, 10 SM) is well above its 1,200 ft MDA. Alternatively, delay your arrival until after the outage ends, but that requires holding fuel and a re-prediction. Do not rely on the 8-minute buffer.
2. What would you need to verify before using the VOR approach as an alternate?
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Verify the VOR approach is operational and that you have a current chart. Check the VOR receiver and the identifier, and confirm the approach's minimums against the weather: 1,200 ft MDA with 3,000 ft broken and 10 SM is comfortable. Confirm the missed approach procedure and terrain clearance, since the airport is remote and the VOR may be the only navaid.
Also check that the VOR is not subject to a published restriction, that your aircraft's equipment is approved for the approach, and that you have enough fuel to fly 25 NM off route and still hold reserve. If the VOR is unserviceable, you would need a third option or to delay until the RAIM outage clears.
3. How would you re-plan if you chose to wait out the outage?
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Re-run the RAIM prediction for a later ETA that falls after the 12-minute outage ends, allowing at least five minutes of margin before the final approach fix. That means arriving at the FAF no earlier than the outage end plus five minutes. Calculate the additional fuel for holding or a slower cruise, and confirm you will still land with reserve.
Advise ATC of your revised ETA and request holding as needed. If the outage is long or the fuel cost is too high, the VOR approach remains the better option. The key discipline is to base the decision on a fresh prediction, not on the original one.