Queenstown RNAV approach: cold temperature and a RAIM outage
You are planning an RNAV (GNSS) RWY 05 approach at Queenstown (NZQN) in a Cessna 172S with a Garmin GTN 650 (TSO-C146). Aerodrome elevation is 1,171 ft; the LNAV MDA is 1,720 ft. The procedure requires baro-VNAV and notes that baro-VNAV is not authorised below 0°C. Forecast temperature at the aerodrome is -4°C, QNH 1012 hPa. The TAF has BKN 3,000 ft and 10 km. Company SOP requires an alternate when the approach is used in IMC. You have filed Dunedin as alternate, 150 nm away. A NOTAM says GNSS RAIM outage for the area 1200–1230; your ETA is 1215. Your GPS predicts RAIM availability, but the prediction was made 3 hours earlier. Fuel is sufficient for one approach plus 45 min reserve and the alternate.
1. Can you plan the RNAV approach as the primary approach? Consider temperature, RAIM and the alternate.
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Not as a baro-VNAV approach. At -4°C the plate prohibits baro-VNAV below 0°C, so LNAV/VNAV minima are not available. You may still use LNAV minima, but the MDA is a barometric altitude and cold-temperature altimetry means the true height is lower than indicated. Apply the published cold-temperature correction to the LNAV MDA; do not simply use 1,720 ft. The correction is typically about 100 ft at this temperature, but use the chart/operator table.
The RAIM NOTAM is the bigger problem. It covers 1200–1230 and your ETA is 1215, inside the outage. A TSO-C146 receiver needs adequate RAIM for the approach; without it, the GPS approach is not available. The prediction was made 3 hours earlier and is stale, so it cannot override the NOTAM. Plan either to hold until after 1230, or to expect the alternate. Because the weather is IMC and company SOP requires an alternate, Dunedin must remain viable with fuel to reach it. The approach can be planned only as a late-arrival option, not as the primary plan at 1215.
2. What would you do if RAIM is lost at 10 nm final?
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Execute the published missed approach immediately. Do not continue just because the GPS still shows a position: RAIM loss means the receiver cannot assure integrity, so the displayed position may be outside the required accuracy. Climb on the published missed-approach track, advise ATC, and request diversion. If conventional radio aids are available and you are qualified, use them; otherwise proceed on the cleared route to the alternate. Do not attempt to improvise a GPS approach or continue visually unless the aerodrome is genuinely in sight and you can maintain terrain clearance under VFR. Record the time and reason for the missed approach. If the loss occurs before the final approach fix, the safest action is to hold or divert rather than start the approach.
3. How would you manage fuel and the alternate if you have to hold for the RAIM outage?
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The hold must be treated as extra fuel, not as part of the 45 min reserve. Calculate the hold fuel from actual burn and add it to the approach, missed-approach and diversion fuel. If the hold is 15 min, that is a known increment; if ATC cannot give a delay estimate, set a hard decision point: when remaining fuel reaches approach fuel plus reserve plus alternate fuel, leave for Dunedin. Do not allow the hold to consume the alternate fuel. Confirm Dunedin weather and the alternate's own approach capability, and ensure you have an alternate alternate if Dunedin is marginal. If the outage is likely to persist, divert early rather than holding to the limit; a diversion with reserve intact is preferable to arriving at Queenstown with minimum fuel and no GPS integrity.