Holding Fuel, Minimum Fuel and MAYDAY FUEL
At 12:40 you enter the hold at the destination with 1,150 kg of usable fuel. Flow in the hold and in the diversion is 620 kg/h. From the holding fix, the approach and landing at the destination takes 8 minutes. The alternate is 25 minutes from the hold, including its approach. Final reserve is 30 minutes at 620 kg/h. Thunderstorms are over the field and the tower says the approach "may open up in 10 to 20 minutes". At 13:20, after 40 minutes in the hold, ATC offers either about 10 more minutes of holding followed by the approach, or an immediate diversion to the alternate. Your operator's standard operating procedures require the commander to declare MINIMUM FUEL or MAYDAY FUEL using the ICAO definitions, and to record the times and fuel figures at each declaration.
1. Compute how long you can hold before the planned reserve is eroded.
Read the worked answer
Fuel burn rate = 620 kg/h = 10.33 kg/min.
Required to complete normally from the holding fix:
- Approach and landing at destination, 8 min: 83 kg
- Alternate, 20 min... corrected: 25 min diversion: 258 kg
- Final reserve, 30 min: 310 kg
Total = 600 kg (using the 25-minute diversion of 258 kg).
Fuel available for holding = 1,150 − 600 = 550 kg = 53 minutes of holding.
So the plan can be sustained until about 13:33. Beyond that, either the alternate or the final reserve is being spent. Note that this is not a limit you can "see" from the fuel gauges alone - it has to be recomputed at each holding entry and when the diversion time or alternate weather changes.
2. At 13:20 (40 minutes in the hold) evaluate the two options and recommend one.
Read the worked answer
Fuel at 13:20 = 1,150 − (40 × 10.33) = 737 kg.
Option A - hold 10 more minutes then approach. Fuel after the hold = 737 − 103 = 634 kg; landing at the destination after the 8-minute approach = 634 − 83 = 551 kg, which is above the 310 kg final reserve, so that landing is legal. But if the approach fails and you go around, you are at about 551 kg with 258 kg needed to reach the alternate: 551 − 258 = 293 kg at the alternate, 17 kg below the 310 kg final reserve. That is a MAYDAY FUEL situation created by a decision taken on the ground at 737 kg with thunderstorms overhead.
Option B - divert now. 737 − 258 = 479 kg at the alternate, 169 kg above the final reserve.
Recommendation: divert now. With TS over the field, a go-around is a realistic outcome, and option A leaves insufficient fuel to protect the diversion. Option B keeps the final reserve intact and leaves a decision margin if the alternate weather also changes.
3. Define the MINIMUM FUEL and MAYDAY FUEL thresholds for this diversion and state what you would declare at 737 kg and below.
Read the worked answer
Committed to the alternate, you need 258 kg to reach it, and the planned final reserve at that point is 310 kg. The fuel on board at which you would land at the alternate with exactly the final reserve is:
258 + 310 = 568 kg.
- At 737 kg you are 169 kg above that figure: no declaration is required, but you should tell ATC you are diverting and give the fuel state so priority can be arranged.
- At 568 kg, still with the diversion to fly, you are committed to the alternate and any change to the plan - a longer track, a hold, a second approach - would put you below the planned final reserve: declare MINIMUM FUEL.
- Below 568 kg, the calculation at the nearest adequate aerodrome gives less than 30 minutes remaining, so declare MAYDAY FUEL and request priority. Note that in this case the nearest adequate aerodrome is the alternate itself, so the two thresholds coincide numerically - the declaration at 568 kg is MINIMUM FUEL, and it becomes MAYDAY FUEL once the fuel is actually below that value, for example because the diversion is taking longer than 25 minutes.
Record the time and fuel at each declaration, and make the call early rather than when the gauges are already near the reserve.