Tropical disturbance and a night departure to Rodrigues
You are scheduled to depart FIMP at 0600 local for Rodrigues (FIMR), 340 NM east-northeast, in a twin-engine piston aircraft. A tropical disturbance about 300 NM northwest of Mauritius is moving southeast at 10 kt. The 0500 FIMP METAR gives wind 340/22 gust 35, visibility 4 km in heavy rain showers, few cumulonimbus at 800 ft, scattered at 1,500 ft, broken at 3,000 ft, QNH 1002 and falling. The TAF temporary 0500–0900 gives visibility 2 km, thunderstorms with rain, broken at 800 ft, wind gust 45 kt. Rodrigues reports wind 020/15, visibility 10 km, scattered at 3,000 ft. Route minimum safe altitude is 4,000 ft; the aircraft has no weather radar. It is June, so departure is in darkness. Fuel endurance is 4.5 hours; trip time 2.2 hours; alternate is FIMP.
1. Identify the hazards and explain how the weather pattern affects your go/no-go decision.
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Hazards are active cumulonimbus and thunderstorms with heavy rain, gusts of 35–45 kt, low cloud at 800 ft, visibility of 2–4 km, a falling QNH, and possible wind shear, microburst, hail, lightning and severe turbulence. Departure is at night with no weather radar, so visual CB avoidance is not possible. The route is over water and the alternate is the same deteriorating aerodrome. The disturbance being 300 NM away does not by itself mean FIMP will be hit, but the observed and forecast weather already exceeds safe VFR criteria. The go/no-go decision should be NO GO until the squall line passes and FIMP improves to a stable VFR condition with adequate cloud base and visibility and a usable alternate.
2. If the operator pressures you to depart, what specific conditions would you require before accepting?
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I would require a current METAR and TAF showing improvement: visibility at least 8 km, cloud base at least 1,500 ft, and no thunderstorms within a defined radius of the route. I would also require daylight or operable weather radar or storm scope, a route that avoids cumulonimbus by at least 20 NM with terrain clearance, a real alternate with acceptable weather and fuel to reach it, and gust spread within aircraft and company limits. Those conditions are not met at 0600. I would refuse the departure and explain the specific hazards, not just say the weather is bad.
3. If you were already airborne and a squall line lay ahead, what actions would you take?
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I would not penetrate the squall line. I would turn 180° or divert around the cells using at least 20 NM avoidance, slow to turbulence penetration speed, keep the wings level, maintain attitude by instruments, and not chase altitude. If I could not avoid the cells, I would land at the nearest suitable aerodrome. I would inform ATC and declare an emergency if fuel or weather became critical. Passenger or operator pressure would not change the hazard.