Fuel, Reserves and a TEMPO: Is 360 kg Enough?
You are captain of a twin-engine aeroplane on a commercial IFR charter. Cruise fuel flow 120 kg/h total at TAS 150 kt. Planned trip 300 nm. Taxi fuel 10 kg. The applicable IFR fuel rule requires contingency fuel of 5% of trip fuel, or fuel to hold 5 minutes at 1,500 ft above the destination, whichever is greater; an alternate; and a 45-minute final reserve. The destination TAF carries a TEMPO of 1,500 m visibility and BKN 600 ft spanning your ETA, so an alternate is required. The alternate is 25 nm away, forecast CAVOK. Ramp fuel on board is 360 kg.
1. Calculate the minimum fuel required and determine whether the flight may depart.
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| Item | Calculation | Fuel |
|---|---|---|
| Taxi | given | 10 kg |
| Trip | 300 nm ÷ 150 kt = 2.0 h × 120 kg/h | 240 kg |
| Contingency | 5% × 240 = 12 kg; 5 min holding = 10 kg; greater | 12 kg |
| Alternate | 25 nm ÷ 150 kt = 10 min × 120 kg/h | 20 kg |
| Final reserve | 45 min × 120 kg/h | 90 kg |
| Total required | 372 kg |
Available 360 kg against 372 kg required: 12 kg short, about 6 minutes of flying. The flight may not depart.
The tempting argument - that the flight is only two hours and you would land with 126 kg (63 minutes) on board - is not the test. The final reserve is not fuel you may plan to use; it is the fuel that must remain when you land. The legality turns on a 12 kg figure, and 12 kg is the difference between a legal departure and an enforcement action.
2. Suppose the TAF is amended and the destination now meets the criteria that make an alternate unnecessary. Would 360 kg be sufficient? What does that tell you about the margin?
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Without an alternate the requirement becomes:
10 (taxi) + 240 (trip) + 12 (contingency) + 90 (final reserve) = 352 kg.
360 kg is sufficient, with 8 kg - about 4 minutes - of discretionary fuel.
What this tells you:
- The whole legality of the flight rests on a TEMPO group in a forecast. If the TEMPO becomes BECMG, or the alternate requirement is reinstated by an amendment en route, the flight becomes illegal in the air.
- An 8 kg margin is not a margin; it is a rounding error. The correct planning decision is to uplift more fuel, not to celebrate a technically legal figure.
- Where the destination forecast is borderline, treat the alternate as required for planning purposes even when the letter of the rule would let you drop it. The rule sets a floor, not a target.
3. In flight, 1.2 hours after start, the fuel indicates 206 kg. ATC advises the destination is now below alternate minima in fog; the alternate is 125 nm from your present position. What do you do?
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Check the fuel state first. Used = 1.2 h × 120 = 144 kg, plus 10 kg taxi, so 360 − 154 = 206 kg remains - the indication is consistent, not a gauge fault.
Option A - continue to the destination (100 nm, 40 min, 80 kg): land with 126 kg, i.e. 63 minutes. Required at landing if the destination has gone below minima: contingency on the remaining trip (about 4 kg) + alternate (20 kg) + final reserve (90 kg) = 114 kg. So 126 kg is 12 kg above the requirement - 6 minutes.
Option B - divert direct to the alternate now (125 nm, 50 min, 100 kg): land with 106 kg, i.e. 53 minutes, which is 16 kg (8 minutes) above the 45-minute final reserve.
Decision: divert now, and tell ATC. The two options cost roughly the same fuel only because the alternate happens to lie on the same track beyond the destination. What differs is optionality: continuing commits you to a destination that is currently below minima, leaves no fuel to hold for an improvement, and makes the decision irreversible at the worst moment - on approach, at low level, in fog. Diverting while you still have 8 minutes of margin above the final reserve keeps the decision reversible.
Communications and law: declare minimum fuel to ATC as soon as it is clear you will land with less than final reserve plus normal contingency, and be prepared to upgrade to a MAYDAY FUEL if the diversion lengthens. If the destination were forecast to clear within 30 minutes, holding would be a legitimate choice - but only with a written hold time computed from the fuel available above the reserve, not an open-ended hold.