Diversion with a Deteriorating Fuel Margin
You are the pilot-in-command of a twin-engine turboprop on a scheduled 95-minute sector. At the top of descent into the destination, the aircraft has 1,450 kg of fuel on board. Planned fuel at this point was 1,650 kg. The destination weather is at the edge of your minima with a 300 ft cloud base and 1,200 m visibility; the alternate is 40 minutes away and requires 700 kg to reach, hold 15 minutes and land with reserves. The company minimum is 30 minutes final reserve plus 10 percent of trip fuel. Cruise fuel burn is 480 kg per hour; descent and approach burn 300 kg per hour. ATC advises holding for 20 minutes before any approach. A runway inspection closes the destination for 25 minutes after that hold. Your first officer suggests pressing on because the weather may improve.
1. Calculate whether you can legally and safely continue to the destination and, if not, what the correct course of action is.
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Step 1: Establish available fuel and burn rates. You have 1,450 kg. Cruise burn is 480 kg/h, descent/approach is 300 kg/h.
Step 2: Compute the hold. A 20-minute hold at cruise power: 480 × (20/60) = 160 kg. Remaining: 1,290 kg.
Step 3: Compute the closure delay. The runway is closed for 25 minutes after the hold, so that is another 25 minutes of holding: 480 × (25/60) = 200 kg. Remaining: 1,090 kg.
Step 4: Compute the approach and landing. Descent and approach at 300 kg/h for roughly 30 minutes: 150 kg. Fuel at landing: 940 kg.
Step 5: Compare with reserves. Final reserve is 30 minutes at cruise: 240 kg. The 10 percent trip fuel margin is a planning buffer, not a landing reserve, but the company minimum effectively requires 240 kg plus the 10 percent trip buffer. With 940 kg at landing you are above 240 kg, but you have consumed the 200 kg shortfall and most of the 10 percent buffer.
Step 6: Decision. The margin is now thin and the weather is at minima. The correct action is to divert to the alternate now, before entering the hold, because the alternate requires 700 kg and leaves a defensible reserve. Pressing on assumes unforecast improvement and a clean approach, which is not a sound basis for fuel planning.
2. What regulatory and operational factors should drive the decision to divert rather than hold?
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Regulatory factors. Fuel planning rules require enough fuel to reach the destination, fly a missed approach, proceed to the alternate, and land with final reserve intact. Once the actual fuel state falls below the planned figure, the commander must reassess against those requirements, not against the original plan. The 10 percent trip fuel buffer is a planning allowance; it is not available for tactical holding.
Operational factors. The destination is at minima with a 300 ft cloud base and 1,200 m visibility, so a missed approach is a realistic outcome, and a missed approach consumes fuel you have not budgeted for. The alternate is 40 minutes away and needs 700 kg, which is a known, fixed commitment. Holding for a possible improvement converts a known diversion cost into an unknown one.
Human factors. The first officer's suggestion to press on reflects plan continuation bias. The disciplined response is to set a decision point: if the weather is not at or above minima by the time you reach the hold, divert. Document the fuel state and the decision.
3. If you divert, what should you tell ATC and what should you monitor during the diversion?
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ATC communication. Advise ATC that you are diverting to the named alternate, state your position, heading and requested level, and give the reason briefly (fuel and weather at destination). Request priority if the fuel state is below your committed diversion fuel. Confirm the alternate's weather and runway in use before committing.
Monitoring during the diversion. Track fuel against the 700 kg diversion allowance at each waypoint. Monitor the alternate's weather trend and any holding at the alternate. Watch for a change in wind that alters the 40-minute transit time. If the fuel state falls below the diversion allowance plus final reserve, declare a minimum fuel situation or an emergency, as appropriate. Keep the first officer engaged in cross-checking the fuel log and the navigation.