Diversion Fuel and the Committed Descent
You are captain of a twin turboprop on a scheduled night IFR sector. At top of descent, 95 NM from destination, ATC advises that the destination is now below minima with freezing drizzle and the alternate you filed (40 NM away, forecast temporarily 600 ft ceiling) is also trending down. Fuel state: 1,850 kg total remaining. Planned fuel at destination was 900 kg, of which the applicable reserve is 500 kg and final reserve 300 kg. Current burn is 480 kg/h at FL200; descent and approach to the alternate will consume 620 kg. A closer airport 25 NM off track has an ILS to 200 ft and a runway long enough, but you have never operated there and it is not on your company route list. The first officer suggests pressing to the filed alternate.
1. Compute the fuel margin if you proceed to the filed alternate and evaluate whether the first officer's suggestion is defensible.
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Fuel available for the diversion and approach: 1,850 kg.
Required to reach the filed alternate and land: descent/approach from present position 620 kg, plus you must still land with final reserve 300 kg. Minimum needed = 920 kg.
Margin = 1,850 − 920 = 930 kg, roughly 1 h 55 min at 480 kg/h. So on paper the filed alternate is fuel-feasible with a large buffer.
The problem is not arithmetic, it is the weather trend. The alternate forecast is temporarily 600 ft - a TEMPO condition is not a planning value and the trend is downward with freezing drizzle, which also implies airframe icing and possible runway contamination. A 930 kg margin looks generous but a missed approach, a hold, and a second approach at an airport already degrading can eat most of it. The suggestion is not automatically wrong, but it must be justified against the actual (not forecast) weather at the alternate at ETA, and against your ability to divert again if the alternate goes below minima. If the trend is confirmed worsening, the answer is no.
2. Assess the 25 NM airport option against the requirement to land with final reserve, and state what you must verify before committing.
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Distance is 25 NM off track, so the fuel to reach it and fly one approach is far less than 620 kg - call it on the order of 250–300 kg including a missed approach. Landing fuel would be roughly 1,850 − 300 = 1,550 kg, comfortably above the 300 kg final reserve. Fuel is not the constraint.
The constraint is operational suitability, which you must verify before committing:
- Is the airport available to your operation at this hour (opening hours, customs/security not required, RFF category adequate for your aircraft type)?
- Is the weather actually above the ILS minima and is the runway contaminated? Freezing drizzle affects it too.
- Do you have current charts and the approach plate for that ILS, and terrain/obstacle clearance on the arrival from your present position?
- ATC coordination to change destination and obtain the approach.
Unfamiliarity alone is not a legal bar, but it raises workload and error risk on a night approach in icing. If the airport is open, the weather is genuinely better, and you have the plates, it is the safer choice; if any of those is unverified, do not commit - declare and hold.
3. If you decide to divert to the closer airport, what should you communicate to ATC and what should you do about the passenger briefing?
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To ATC: declare the diversion clearly and early - "[callsign], diverting to [airport], request ILS [runway], we are [position], [altitude], request priority and vectors." If fuel becomes a concern at any point, use the standard urgency phrase PAN PAN (or MAYDAY if fuel is critical). Give them your fuel endurance in minutes so they can sequence you.
To the passengers: a short, calm PA explaining that weather at the destination has deteriorated and you are landing at a nearby airport, with an estimate of the delay. Do not speculate on cause or blame. If the landing will be at an airport without handling, tell them what happens next (remain seated, ground staff will advise). Cabin crew should be briefed before the approach so they can secure the cabin and prepare for a possible go-around.