RNP Approach with a RAIM Outage and Cold Temperature
You are planning an RNP APCH to runway 17 at a regional airport. The chart gives LNAV/VNAV DA 520 ft and LNAV MDA 620 ft; visibility 1,500 m. The aircraft is approved for baro-VNAV, but the chart states baro-VNAV is not authorized below -15°C. The airport temperature at ETA is -22°C. A NOTAM says GNSS RAIM outages are expected from 1200 to 1400 UTC. Your ETA at the initial approach fix is 1320. The destination has no ILS and no radar. The alternate has an ILS and is 25 minutes away. Fuel is sufficient for one approach at destination, a diversion, and final reserve. The first officer suggests using LNAV minima to avoid the temperature limitation. The captain suggests continuing because the RAIM outage is only 'expected'.
1. At 1320 UTC, can the crew conduct the LNAV/VNAV approach? Can they conduct the LNAV approach? Explain.
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LNAV/VNAV: no. The chart prohibits baro-VNAV below -15°C and the airport is -22°C, so the vertical path is not authorized. LNAV: also no, because LNAV still depends on GNSS and requires sufficient RAIM. The NOTAM predicts a RAIM outage from 1200 to 1400, which covers the 1320 ETA. The word 'expected' does not make the approach available; unless a current RAIM prediction or ATC confirms availability, the crew must treat the outage as active. The FO's suggestion to use LNAV solves the temperature limitation but not the RAIM problem. Without SBAS or another approved navigation source, no RNP APCH can be conducted at that time.
2. What are the realistic options, and which is the best decision given fuel for one approach plus diversion?
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Options are: (1) delay arrival until after 1400 by holding or reducing speed, but this consumes fuel and must preserve final reserve; (2) divert to the ILS alternate; (3) request a different approach, but the destination has no ILS and no radar. With ETA 1320 and fuel only for one approach plus diversion, a 40-minute hold is not realistic. The best decision is to divert to the ILS alternate while fuel is sufficient, or to hold only if the crew can arrive after 1400 with the required reserves. The captain should not launch an approach hoping RAIM returns; a loss of integrity on final would force a missed approach and may leave the aircraft without enough fuel for the alternate.
3. If RAIM is lost at the final approach fix during an LNAV approach, what should the crew do?
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If RAIM is lost at the FAF, the navigation integrity is no longer assured, so the crew must not continue the approach. They should initiate the published missed approach immediately, climb to the missed-approach altitude, and advise ATC. The missed approach must be flown using the available navigation, and the crew should request vectors or an alternate clearance if the missed approach also depends on GNSS. They should then divert, because another GNSS approach cannot be trusted without RAIM. The event should be reported. Continuing visually is only acceptable if the required visual reference is already established and the aircraft can be landed safely; otherwise the go-around is mandatory.