Thunderstorm and Icing on a Mountain Route
You are planning a VFR flight in a single-engine piston aircraft from a valley airport (elevation 1,200 ft) to a mountain airport (elevation 4,800 ft), 180 nm direct. The route crosses a ridge at 8,500 ft. Forecast: freezing level 6,000 ft, scattered cumulus with tops at 12,000 ft, isolated thunderstorms after 15:00 local, visibility 15 sm, wind 240° at 25 knots at 9,000 ft. Your aircraft has a service ceiling of 14,000 ft, no known icing certification, and no oxygen. Departure time is 13:00 local. Fuel endurance is 4.5 hours; the flight should take 1.8 hours.
1. Can you complete this flight VFR under the forecast? Identify the limiting factors.
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Several factors combine to make this flight marginal or unsafe as planned. First, the freezing level at 6,000 ft means that any cloud or precipitation above 6,000 ft contains supercooled water. Your aircraft is not known-icing certified, so you cannot legally or safely enter cloud above the freezing level. The ridge at 8,500 ft is above the freezing level, and scattered cumulus tops at 12,000 ft mean you may encounter icing if you climb into cloud.
Second, isolated thunderstorms after 15:00 local. Your planned arrival is around 14:48, which is close to the convective window. Thunderstorms bring severe turbulence, hail, and rapid icing. Even isolated cells require a wide berth.
Third, no oxygen and a service ceiling of 14,000 ft. To clear the ridge at 8,500 ft with terrain clearance, you would fly at 9,500 to 10,500 ft, which is within the aircraft's capability but leaves little margin for downdrafts or mountain wave.
Fourth, the wind at 9,000 ft is 25 knots from 240°, which can produce mechanical turbulence and mountain wave on the lee side of the ridge.
The flight is not clearly safe. A better plan is to depart earlier, stay below the freezing level where possible, route around the ridge through a lower pass, and avoid the convective window.
2. What route and altitude strategy would you use, and what alternates would you plan?
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Route through a lower pass if one exists, keeping the aircraft below the freezing level (6,000 ft) as much as possible. If the pass requires climbing above 6,000 ft, ensure you remain in visual meteorological conditions with no cloud above the freezing level in your path. Fly the ridge crossing at 9,500 ft only if the sky is clear and you have a safe escape route.
Altitude strategy: stay low on the windward side, cross the ridge at the highest safe altitude with terrain clearance, and descend on the lee side. Anticipate downdrafts and turbulence; keep speed up and avoid abrupt power changes.
Alternates: identify a valley airport on the near side of the ridge with fuel and a runway long enough for your aircraft, and a second alternate on the far side. Check that both have weather reporting and are within your fuel endurance. Plan to arrive before 15:00 to avoid the convective window, and have a diversion plan if thunderstorms develop earlier than forecast.
Fuel: 4.5 hours endurance against 1.8 hours planned gives ample reserve, but if you divert or hold, recalculate. Do not press on into convective build-ups.