Nor'wester over the Main Divide: Queenstown to Milford Sound
You are planning a VFR crossing in a PA-28-161 from Queenstown (NZQN) to Milford Sound (NZMF) on a bright nor'wester day in spring. The graphical area forecast gives 320/40 kt at 10,000 ft, freezing level 8,000 ft, scattered lenticular cloud, and severe turbulence below 12,000 ft in the lee of the Main Divide. The divide in the Dart-Hollyford sector runs roughly 045/225. Your planned track crosses the divide at 9,500 ft near the Homer Saddle, giving about 3,000 ft above the highest ground. The aircraft climbs at 700 fpm at that altitude and is not turbocharged. Milford Sound is reporting 8 km visibility, low cloud at 2,000 ft and intermittent heavy rain; Queenstown is clear. You have 2 h 20 min of fuel and one nervous passenger.
1. Explain what the forecast wind implies for the crossing, and identify where the greatest hazard sits on this route.
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The divide here runs about 045/225, so a 320 wind is within a few degrees of perpendicular to the ridge line. That is the classic mountain-wave geometry: air is forced up the windward slope, oscillates downwind, and produces lenticular cloud, rotor turbulence and strong lee downdraughts. Two supporting conditions are implied: a stable layer (here, the frontal/inversion structure and the 8,000 ft freezing level) and ridge-top winds above roughly 25 kt. At 40 kt at 10,000 ft the threshold is comfortably exceeded, and the forecast of severe turbulence in the lee confirms it.
The lee side is the Queenstown side, so the departure and the first part of the crossing carry the wave and rotor hazard: strong updraughts on the upwind slope, violent downdraughts and rotor turbulence 5-10 nm downwind of the ridge, with 1,000-2,000 ft of height loss possible if a downdraught is entered. Once across, the windward side brings the other problem: orographic cloud, heavy rain and low visibility at Milford. So the flight has a wave problem at the start and a weather problem at the end, with Milford likely to be the limiting factor rather than the crossing itself.
2. If you enter a rotor downdraught of 1,200 fpm, what happens with your 700 fpm climb capability, and what is the correct escape manoeuvre?
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Net performance is 700 minus 1,200, i.e. 500 fpm of descent. With 3,000 ft above the high ground, that gives about 6 minutes before terrain becomes critical, and in a rotor the sink is usually not steady - it is worse close to the slope and can double. Rate of climb is irrelevant while the downdraught exceeds it, so the answer is lateral, not vertical.
Correct actions: lower the nose to best rate or best glide speed as appropriate, accept the height loss, and turn 90 degrees away from the ridge towards the lee side and lower ground - never turn towards rising terrain or attempt to out-climb a downdraught by pulling up. Keep the turn rate moderate; steep turns increase load factor and stall speed in turbulence. Set a hard floor: if you are not clear of the wave and back above 2,000 ft above ridge height by that floor, turn back. In wave conditions, plan at least 2,000 ft above ridge height rather than the 1,000 ft used for ordinary terrain clearance, and brief a turnback point before departing.
3. Given the information, what is your decision, and what would change it?
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Cancel or delay. Three factors combine: severe lee turbulence forecast below 12,000 ft on a route where you can only get 3,000 ft above the ridge, a light single with limited excess power climbing at 700 fpm, and a destination already at 2,000 ft cloud base in heavy rain on the windward side. Any one is manageable; together they remove the margins, and the passenger pressure is not a factor to weigh.
What would change the decision: the wind dropping below about 20-25 kt at ridge level and the severe turbulence forecast being withdrawn; a cloud base at Milford that allows a visual valley entry with a firm turnback; or a genuine change to fly the route later in the day only if the wind is forecast to ease - diurnal heating usually strengthens mechanical turbulence, so an afternoon departure is not an improvement unless the gradient wind drops.
If you do go, brief the wave escape, keep 2,000 ft above ridge height, and nominate a hard turnback point at the last valley before the saddle. Otherwise, the coast or a low-level valley flight with continuous visual reference to the valley floor is the only sensible alternative, and it is not available in this wind.