Mountain-wave crossing at 9,500 ft
You are planning a VFR crossing of a north-south ridge whose highest crest is 7,800 ft. The planned crossing altitude is 9,500 ft; the MEA is 9,000 ft. Winds are 250° at 35 kt at 9,000 ft and 250° at 55 kt at 14,000 ft. The air is stable, and lenticular altocumulus is visible downwind of the ridge. The forecast warns of mountain wave turbulence, moderate to severe, below 16,000 ft, with downdrafts up to 1,500 ft/min on the lee side. The temperature at 9,500 ft is −3°C with a dewpoint of −8°C. Your normally aspirated aeroplane has a service ceiling of 12,500 ft and climbs at 300 ft/min at 9,500 ft. The route has a 5 nm wide pass 20 nm north of the planned crossing, but it adds 15 minutes and has a 9,200 ft MEA. There is no oxygen on board.
1. Identify the mountain-wave indicators in this scenario and explain the mechanism.
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The wind at 9,000 ft is 250° at 35 kt, which is strong and nearly perpendicular to a north-south ridge; 55 kt at 14,000 ft shows that the flow strengthens with height. The air is stable, and lenticular altocumulus downwind is the classic visible signature of standing wave crests. The mechanism is that stable air is forced upward over the ridge, then oscillates downwind; the lee side can contain rotors and strong downdrafts. The forecast of moderate to severe turbulence below 16,000 ft and downdrafts up to 1,500 ft/min is consistent with a well-developed wave. A 35 kt cross-ridge wind is above the usual 20 kt threshold for significant wave activity, so the warning should be treated as operationally relevant, not as a forecast for someone else.
2. Calculate the terrain-clearance problem at 9,500 ft and explain why normal climb performance does not solve it.
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At 9,500 ft over a 7,800 ft crest the clearance is only 1,700 ft. A 1,500 ft/min downdraft combined with a 300 ft/min climb gives a net descent of 1,200 ft/min. In one minute the aircraft would lose 1,200 ft, leaving about 500 ft; in another 25 seconds it would be at the terrain. A 5 nm crossing at, say, 100 kt ground speed takes about 3 minutes, so the aircraft cannot outclimb the downdraft. Climbing to 12,000 ft would give more margin, but the aeroplane must first gain 2,500 ft while possibly in the downdraft, and the service ceiling is 12,500 ft with no oxygen. The calculation shows that 9,500 ft is not a safe crossing altitude in this forecast.
3. What is your decision, and what alternatives would you brief?
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I would not cross at 9,500 ft. The practical alternatives are to delay until the cross-ridge wind weakens below about 20 kt, to route far around the ridge, or to use the pass only if the latest forecast shows weaker wave activity and I can cross with at least 3,000 ft terrain clearance. The pass adds 15 minutes and has a 9,200 ft MEA, so it does not automatically solve the problem; it must be flown at a higher altitude, with an escape route and a plan for a downdraft. Climbing to 12,000 ft in the wave is not a simple fix because the climb may be fought by the same downdraft, and oxygen is not available. My brief would be: no crossing today unless conditions change; divert or wait.