Crossing a Ridge in Strong Föhn Conditions
You are ferrying an unpressurised, normally aspirated light twin from a valley aerodrome at 1,900 ft to the north, across a 9,000 ft ridge. The forecast ridge-level wind is 200°/45 kt, nearly perpendicular to the ridge, with 500 hPa wind 230°/60 kt. At 7,000 ft the temperature is −5 °C and the dew point −12 °C. The valley aerodrome reports QNH 1005 hPa and +12 °C. Lenticular altocumulus is visible over the ridge and a rotor cloud is reported on the lee side. The aircraft has no weather radar and no known-icing approval. The minimum safe altitude is 10,700 ft; the planned crossing is at 11,500 ft. Fuel allows 20 minutes of diversion. An alternative crossing 30 nm south has a 7,500 ft ridge and a wind of 190°/25 kt. The crew must choose a route and altitude before entering the valley.
1. Describe the expected flow, cloud and turbulence on the windward and lee sides, and explain the significance of the rotor cloud.
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The flow is nearly perpendicular to the ridge at 45 kt, with 60 kt at 500 hPa, so the ridge is a major obstacle. On the windward side, forced ascent produces orographic cloud and, at −5 °C, a risk of rime icing; the temperature/dew-point spread of 7 °C at 7,000 ft shows the air is not saturated at that level, so cloud base will be higher. Over and lee of the ridge, the stable layer supports a standing wave: lenticular altocumulus marks the wave crests, and the rotor cloud marks a rolled circulation beneath the crest on the lee side. The rotor is the warning sign for severe low-level turbulence, strong up- and downdrafts, and possible mountain-wave turbulence extending well above the ridge. The lee side may be clear because of subsidence, but clear air does not mean smooth air. Treat the rotor zone as no-fly for this aircraft and expect downdrafts capable of exceeding the aircraft's climb rate.
2. What are the specific icing, performance and altimetry concerns at the planned 11,500 ft crossing?
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At 11,500 ft, extrapolating the lapse rate from 7,000 ft gives roughly −14 °C, so any cloud at that level contains supercooled water and ice accretion is possible. The aircraft has no known-icing approval, so prolonged cloud penetration is not acceptable. Performance is also degraded: QNH 1005 hPa is below standard, so the pressure altitude at an indicated 11,500 ft is about 11,740 ft, and the high temperature in the valley gives a density altitude well above the indicated altitude. A normally aspirated twin will have a reduced climb rate and a lower ceiling margin, exactly when wave downdrafts may reach 1,000–2,000 ft/min. The QNH is 8 hPa below standard, so it is essential to set QNH 1005 hPa and not 1013 hPa; the latter would make the aircraft indicate about 240 ft higher than its true pressure altitude and erode terrain clearance.
3. Choose a route and altitude, and justify the decision using fuel, terrain clearance and aircraft limitations.
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Choose the southern crossing. The ridge is 7,500 ft and the wind only 25 kt, so the wave amplitude, rotor threat and downdraft strength are all substantially lower. Cross at 9,500–10,500 ft, giving 2,000–3,000 ft terrain clearance while remaining below the altitude at which the unpressurised aircraft's climb margin becomes critical. The 30 nm detour at about 150 kt takes roughly 12 minutes, which fits within the 20-minute diversion fuel allowance. The northern crossing at 11,500 ft gives 2,500 ft clearance over the ridge, but it places the aircraft in or near the rotor and wave, at high density altitude, with no known-icing approval and no weather radar. That combination is unacceptable when a lower, calmer crossing is available. Approaching the ridge at an angle is not a substitute for avoiding the rotor; the southern route avoids the strongest flow altogether.