Queenstown In-Flight: Severe Mountain Wave SIGMET
You are IFR at FL170 from Christchurch (NZCH) to Queenstown (NZQN) in a pressurised twin turboprop. Sixty nautical miles northeast of NZQN you receive a SIGMET for severe turbulence and severe mountain waves over the Southern Alps below FL250, and an AIRMET for moderate icing between FL120 and FL180. NZQN ATIS: wind 320/22G40 kt, visibility 10 km, few cloud 3000 ft, temperature 3°C, dewpoint 1°C, QNH 1004 hPa. The aircraft is known-icing certified and has serviceable weather radar. You have 45 minutes fuel to Dunedin (NZDN) and 35 minutes to Invercargill (NZNV), plus reserve. The Queenstown RNAV approach has a missed approach that climbs into terrain, and the airport is surrounded by high ground. The crew has not recently flown a mountain-wave recovery.
1. Interpret the SIGMET and AIRMET: which is operationally limiting and why?
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The SIGMET is the operationally limiting product. A SIGMET for severe turbulence and severe mountain waves warns of a hazard that can affect all aircraft types, including transport-category aircraft, and it is issued for conditions that are potentially dangerous. Mountain-wave severe turbulence can produce violent up- and downdrafts, large airspeed and altitude fluctuations, and terrain-related altimeter errors near high ground. The AIRMET for moderate icing between FL120 and FL180 is advisory and is within the capability of a known-icing-certified aircraft, although prolonged exposure would still need management. The AIRMET does not cancel or downgrade the SIGMET. At FL170 the flight is inside the vertical extent of both products, so the SIGMET governs the decision; the AIRMET is a secondary consideration for route and performance.
2. Assess whether the aircraft and crew can safely continue to NZQN. What specific hazards matter most?
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The aircraft is known-icing certified, so moderate icing is manageable, and the weather radar is serviceable. The critical hazards are the severe mountain-wave turbulence and the terrain. Queenstown is surrounded by high ground, and the RNAV missed approach climbs into terrain. With wind 320/22G40 kt and a severe mountain-wave SIGMET, a go-around or missed approach could be flown in severe turbulence and downdrafts near high ground. The crew has not recently practised a mountain-wave recovery, which raises the workload and error risk. Continuing to NZQN would rely on the approach being stable and the missed approach not being needed; that is not a safe assumption in a severe mountain-wave environment. The balance therefore favours avoiding the Queenstown terminal area unless the SIGMET is cancelled and conditions improve markedly.
3. State your decision and the diversion logic, including fuel and alternate choice.
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I would discontinue the NZQN approach and divert. The SIGMET severe mountain wave, the terrain-critical missed approach and the gusty surface wind make a go-around potentially more hazardous than the diversion. I would request the latest TAF and METAR for NZDN and NZNV, then choose the alternate that has a suitable forecast and the greater fuel margin. On the figures given, NZNV is 35 minutes away and lies south of the main wave area, so it is the preferred candidate; NZDN at 45 minutes is the fallback if NZNV weather is unsuitable. I would advise ATC, request a route clear of the wave and icing layer, avoid unnecessary altitude changes, and monitor fuel. Attempting one approach with a hard diversion trigger would only be reasonable if the SIGMET were cancelled and the crew had recent mountain-wave training.