Diversion fuel decision over the South China Sea
You are commanding a Cessna 208B on a scheduled VFR flight from Brunei International (WBSB) to Mulu (WBMU) with 8 passengers and 60 kg of cargo. Departure mass is 3 560 kg, below the 3 629 kg MTOW. At 40 NM from WBSB, cruising at 9 500 ft, the pilot of a preceding aircraft reports Mulu is now below VMC minima with towering cumulus over the airfield, and the TAF suggests the weather will not clear for 2 hours. Your total usable fuel at start-up was 900 lb; you have burned 180 lb so far. You are now 30 minutes from Mulu. The nearest suitable alternates are Marudi (WBGM), 45 minutes away, and Miri (WBGR), 55 minutes away. You hold a 30-minute reserve.
1. Calculate your fuel state on arrival at Mulu, at Marudi and at Miri, and determine which alternate you can legally and safely reach.
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Fuel remaining at the decision point = 900 − 180 = 720 lb. Burn rate over the first segment = 180 lb in 40 NM. At the planned TAS and the reported winds, assume the aircraft burns approximately 270 lb/h (4.5 lb/min).
- Mulu in 30 min: 720 − 135 = 585 lb.
- Marudi (30 min to Mulu + 45 min to Marudi = 75 min): 720 − 338 = 382 lb.
- Miri (30 + 55 = 85 min): 720 − 383 = 337 lb.
Your reserve is 30 min at 4.5 lb/min = 135 lb. At Marudi you land with 382 lb, i.e. 247 lb above reserve - acceptable. At Miri you land with 337 lb, i.e. 202 lb above reserve - also acceptable, but the margin is thinner. Since Mulu is below VMC minima, you cannot plan to land there under VFR; the correct decision is to divert to Marudi now, which preserves the greatest fuel margin and keeps Miri as a further option if Marudi deteriorates.
2. What human factors and regulatory considerations should influence whether you continue towards Mulu for 10 more minutes to reassess?
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Continuing toward known poor weather introduces plan-continuation bias and get-there-itis, especially with passengers and a schedule. Under VFR you may not enter IMC, and the reported towering cumulus indicates embedded convective activity and possible severe turbulence, hail and icing above the freezing level. The 10-minute reassessment provides no new information: the TAF already predicts 2 hours of unsuitable weather. Regulatory considerations: you must remain clear of cloud and in sight of ground, maintain VMC minima, and land with the prescribed reserve intact. The safest decision is an early diversion to Marudi while fuel and options remain broad.