RAIM outage and a failed glideslope
You are planning an RNP APCH (LNAV minima) to runway 27 at an aerodrome where the only other instrument approach is an NDB procedure to runway 09. The aircraft has one GNSS receiver, no baro-aiding and no SBAS. The NOTAM service reports ILS RWY 27 GP U/S. Your ETA over the initial approach fix is 14:25 UTC. The RAIM prediction for a 15-minute window either side of the ETA shows a loss of fault detection from 14:20 to 14:38. The LNAV minima require RNP 0.3 and a 5-satellite geometry. Your alternate is 40 minutes away with an NDB approach; the aircraft's ADF is serviceable but the alternate's NDB is NOTAM'd unserviceable 13:00–17:00. Fuel on board at the IAF will be 2 800 kg, of which the final reserve is 1 100 kg and the alternate fuel is 900 kg. Holding burn is 2 000 kg/h.
1. Can the LNAV approach be flown at the planned ETA? Justify and list the alternatives.
Read the worked answer
No. The RAIM outage runs 14:20–14:38 and the ETA at the IAF is 14:25. With a single receiver, no SBAS and no baro-aiding, integrity cannot be assured inside that window, so the required 0.3 nm containment is unproven and the approach must not be commenced - a clean indication is not the same as a guaranteed one.
Alternatives:
- Delay past 14:38. A 13-minute hold costs 13/60 × 2 000 = 433 kg, leaving 2 367 kg. Against 900 kg alternate plus 1 100 kg final reserve that still leaves 367 kg - legal but thin, so one approach only.
- Fly the NDB to runway 09. The NOTAM affects the runway 27 glideslope only, so this is unaffected.
- Check the charts for a localiser-only (LLZ) procedure to runway 27 with its own minima. A CAT I ILS with the glideslope unserviceable does not automatically become a usable localiser approach.
Re-run the RAIM prediction for the new time before committing to the hold.
2. Assess the alternate. What is wrong with the plan as it stands?
Read the worked answer
The alternate's NDB is NOTAM'd unserviceable 13:00–17:00, which covers the entire arrival window. An aerodrome can only be used as an instrument alternate if a usable instrument approach is available at the expected time of arrival. A serviceable ADF in the aircraft does not compensate for a failed ground station.
Therefore:
- the nominated alternate is not valid and the flight cannot be dispatched as planned;
- either nominate a different alternate with a serviceable approach - which changes the alternate fuel and probably the take-off mass - or carry enough extra fuel to hold at the destination until the RAIM outage ends and still reach a distant alternate if the approach fails.
Whichever route is chosen, the fuel plan and the load sheet must be redone before departure. Discovering on the missed approach that the alternate has no approach is not a recovery plan; it is the accident sequence.
3. Assuming a valid alternate is found, what is the best course of action in flight?
Read the worked answer
Hold 13 minutes before the IAF to clear the RAIM outage, then fly the LNAV approach to runway 27 once only.
Fuel after the hold is 2 367 kg against 2 000 kg committed, so there is no room for a second approach. If the approach is not stable by 1 000 ft, or a go-around is flown, divert immediately.
Before entering the hold, confirm:
- the RAIM prediction is still valid for the revised window - predictions change with constellation status;
- the current METAR, not merely the TAF, puts the destination above the LNAV minima;
- the new alternate is still reachable with the fuel remaining at the revised ETA.
If any of the three fails, divert without holding. Also confirm the missed-approach procedure from runway 27 does not conflict with terrain or with the NDB traffic to runway 09.