In-flight fuel monitoring with deteriorating destination weather
A twin-jet is 2 hours into a 4.5-hour flight. Planned fuel at destination was 3,600 kg. Actual fuel flow is 2,700 kg/h, 150 kg/h above plan, and the headwind is 35 kt stronger than forecast. The aircraft is at FL340 with TAS 470 kt and current ground speed 410 kt. Remaining distance to destination is 1,050 nm. Fuel remaining is 8,900 kg. Destination weather is now TEMPO TSRA, visibility 2,000 m, BKN 800 ft; the alternate is 110 nm beyond destination with CAVOK. Company reserves are 30 minutes final reserve at 1,200 kg/h, 5% contingency on trip fuel, and 200 kg approach fuel. A 15-minute hold at destination would burn 675 kg. The crew must decide whether to continue, hold, or divert, and when to declare minimum fuel.
1. Calculate the revised fuel at destination and the margin over final reserve, alternate and contingency.
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Ground speed is 410 kt, so time to destination = 1,050 / 410 = 2.56 h. Trip fuel = 2.56 × 2,700 = 6,914 kg. Fuel at destination after approach = 8,900 − 6,914 − 200 = 1,786 kg.
Required at destination: final reserve 30 min × 1,200 = 600 kg; alternate 110 nm / 410 kt = 0.268 h × 2,700 = 724 kg; contingency 5% × 6,914 = 346 kg. Total = 1,670 kg. Margin = 1,786 − 1,670 = 116 kg, about 2.6 minutes of fuel. That is a very small margin; the flight is legal on paper but has almost no room for a hold or a second approach.
2. Can a 15-minute hold be accepted? Show the fuel figures and state the action.
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A 15-minute hold burns 675 kg, leaving 1,786 − 675 = 1,111 kg at destination. That is 559 kg below the 1,670 kg needed for final reserve, alternate and contingency, so the hold is not acceptable unless the weather clears immediately and a landing is assured. Diverting now: distance to alternate = 1,050 + 110 = 1,160 nm; time = 2.83 h; fuel = 7,638 kg; approach 200 kg; fuel at alternate = 8,900 − 7,638 − 200 = 1,062 kg. Final reserve is 600 kg, so the margin is 462 kg. The safer decision is to divert now, or to continue only if destination weather is improving and no hold is expected. If fuel at the alternate is expected to fall below 600 kg, declare MAYDAY FUEL.
3. Explain the difference between MINIMUM FUEL and MAYDAY FUEL, and which applies here.
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“MINIMUM FUEL” is an advisory call to ATC when the aircraft is committed to land at a specific aerodrome and any change to the landing sequence would result in landing below the final reserve; it is not an emergency. “MAYDAY FUEL” is declared when the calculated fuel on landing is below the final reserve and the aircraft may need priority. In this case, after diverting, the expected fuel at the alternate is 1,062 kg, above the 600 kg final reserve, so neither call is yet required. If a further hold or a second approach would take the figure below 600 kg, declare MAYDAY FUEL. Do not wait until the fuel is actually below reserve to tell ATC.