VFR cross-country ahead of a forecast thunderstorm line
A VFR cross-country of 180 nm is planned at 5,500 ft, TAS 115 kt, departing 1430Z, ETA 1630Z. Sunset is 1745Z. Fuel on board 150 L, consumption 32 L/h, reserve 45 min. The destination TAF: 25012KT 9999 SCT025 BKN040 TEMPO 1500 +TSRA BKN008CB PROB30 0500 +TSRA. The only alternate is 38 nm away, TAF 27010KT CAVOK, but PPR closes at 1800Z and the pilot has no night rating. A cold front lies 40 nm west of track, moving east at 15 kt; forecast cloud tops 12,000 ft; en-route MSA is 6,500 ft. The destination has no fuel and no instrument approach; the alternate has fuel. The aircraft is not approved for flight in icing; freezing level is 9,000 ft.
1. Interpret the destination TAF and identify the main threat and its timing.
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The TAF is basically VFR: 12 kt, 10 km, scattered 2,500 ft, broken 4,000 ft. The threat is the TEMPO line: visibility 1,500 m in heavy thunderstorm rain and broken cumulonimbus at 800 ft, with a 30% probability of 500 m. That is below VFR minima and implies embedded CB, strong downdraughts, hail and wind shear.
The front 40 nm west, moving east at 15 kt, reaches the track in about 2.7 h - roughly 1710Z, close to the 1630Z ETA, so the TEMPO is not a remote possibility. Cloud tops at 12,000 ft rule out climbing over the cells at 5,500 ft. The destination has no instrument approach, so there is no benign fallback once the visibility drops.
2. Is fuel the limiting factor, and what go/no-go and diversion plan follows?
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Fuel is not the limiting factor: 180 nm at 115 kt is 1.6 h, about 50 L; the alternate is 20 min, about 11 L; reserve 45 min is 24 L. Total about 85 L against 150 L.
The limiting factors are time and alternates. The only alternate closes PPR at 1800Z and sunset is 1745Z; with no night rating, the latest safe arrival is before 1745Z. Set a firm decision point: if the destination is not observed or forecast VFR with good margin by 1615Z, divert immediately. That gives 20 min to the alternate, landing about 1635Z, with 70 min before PPR closes.
Do not press on hoping the TEMPO misses. The alternate is not available after 1800Z, and the destination has no fuel or approach to rescue a late arrival.
3. If a thunderstorm is encountered despite the plan, what should the pilot do?
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If a cell is encountered, do not enter it. Turn at least 20 nm around the visible precipitation, preferably on the upwind side; do not try to top it at 5,500 ft with tops at 12,000 ft. If caught inadvertently, reduce to turbulence penetration speed, maintain attitude, avoid abrupt control changes, turn 180° out of the cell, and expect hail, lightning and severe up- and downdraughts.
Report to ATC and land if the aircraft or pilot is affected. In this scenario the practical escape is to divert to the alternate before the front arrives; a VFR aircraft has no business in a forecast CB with a 500 ft PROB30. Do not rely on the destination METAR staying VFR once the TEMPO arrives.