Christchurch to Hokitika: mountain wave over the Southern Alps
You are planning NZCH–NZHK (Hokitika) in a Cessna 182 (TAS 135 kt, burn 12 gal/hr) over the Southern Alps. The route via Arthur's Pass is 110 nm; the alternate via Lewis Pass is 135 nm. Forecast: 250/45 at 10,000 ft, freezing level 7,000 ft, scattered CB embedded, mountain wave reported, severe turbulence below 12,000 ft. The pass is 3,000 ft with terrain to 7,000 ft within 5 nm. You plan 9,500 ft. The aircraft is not equipped for known icing. Fuel is 72 gal, reserve 45 min. The Hokitika TAF is fine, but the Christchurch TAF has a front arriving 2 hours earlier than forecast. A friend reports smooth conditions at 6,500 ft west of the divide.
1. Assess the route and planned altitude. What are the main navigation hazards?
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The main hazard is not distance but terrain and vertical motion. A 45 kt wind at 10,000 ft across the Alps is strong enough for mountain wave, rotor turbulence and large downdrafts. Severe turbulence below 12,000 ft rules out comfortable or predictable tracking at 9,500 ft. The freezing level at 7,000 ft means climbing through the pass in cloud carries airframe icing risk, and the aircraft is not known-ice equipped. Embedded CBs add hail, lightning and violent up/downdrafts.
At 9,500 ft the planned terrain clearance over the 7,000 ft terrain is 2,500 ft, which meets a 2,000 ft company mountain margin, but clearance is not the only issue: a downdraft can erode it quickly. The pass is narrow, so a 45 kt crosswind produces significant drift and turbulence on entry and exit. Fuel is not limiting: 110 nm at, say, 95 kt GS is 1.16 hr, about 14 gal, with 9 gal reserve, so 72 gal is ample. The friend's smooth report west of the divide does not validate the pass; wave effects are localised and can change within minutes.
2. Would you route via Arthur's Pass, Lewis Pass, or delay/cancel? Give your reasoning.
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Do not treat this as a simple short-route versus long-route choice. Arthur's Pass is shorter but the terrain is higher and the wave/turbulence forecast applies directly. Lewis Pass is longer but may offer a lower, wider crossing; however, it is still in the same strong northwesterly and wave environment. With severe turbulence below 12,000 ft, embedded CBs and a freezing level at 7,000 ft, neither pass is a good VFR option at 9,500 ft in a non-icing C182.
The safer decision is to delay until the front passes and the wave subsides, or cancel. If the flight must proceed, wait for a verifiable improvement: observed wind at 10,000 ft below about 25 kt, no rotor/lenticular cloud, and a freezing level above the planned crossing altitude. Then use Lewis Pass with a 2,000 ft terrain margin and a firm turn-back point. Fuel is sufficient for either route, but fuel is not the limiting factor. The Christchurch front arriving early also argues for delay, because the return or alternate options will deteriorate.
3. If you do go, what in-flight cues would make you turn back?
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Set clear abort criteria before entering the pass. Turn back if you encounter sustained downdrafts that cannot be countered with normal climb power, if turbulence becomes severe enough to make heading or altitude control unreliable, or if the pass is closing in with cloud or rotor cloud. Also turn back if you pick up airframe ice, if the freezing level is lower than forecast, or if you cannot maintain the planned 2,000 ft terrain margin. A lenticular or rotor cloud close to the pass is a warning, not scenery to inspect.
Use the GPS and chart to stay within the pass; do not drift downwind into higher terrain. Keep an alternate escape heading and note the last safe turning point. If you turn back, advise ATC and revise the fuel plan; the return to Christchurch may be into a strengthening front, so consider landing before it arrives rather than trying to outrun it.