Fuel Planning Against MTOW: Contingency Is Not a Fuel Tank
You are planning a 2 h 05 min sector at a cruise flow of 600 kg/h. Taxi fuel 25 kg. The operator's approved fuel scheme requires 5% contingency on trip fuel. The destination forecast carries TEMPO TSRA for your arrival window, and the alternate is 20 minutes away at 600 kg/h. Final reserve is 30 minutes at 600 kg/h. Zero fuel mass is 4,300 kg; MTOW 6,200 kg, MLW 5,900 kg, MZFW 5,400 kg, fuel capacity 2,400 kg. Actual fuel on board is 1,900 kg. The commander wants an extra 30 minutes of holding fuel because of the thunderstorms and suggests the dispatcher "take it out of the contingency" so the aircraft stays under MTOW. The sector is fully booked and the payload is already firm.
1. Compute the minimum required fuel and the discretionary fuel, and state whether 1,900 kg is acceptable.
Read the worked answer
Trip fuel = 2.083 h × 600 = 1,250 kg.
- Taxi: 25 kg
- Trip: 1,250 kg
- Contingency 5%: 63 kg
- Alternate 20 min: 200 kg
- Final reserve 30 min: 300 kg
Minimum required = 1,838 kg. Fuel on board 1,900 kg gives 62 kg discretionary - about six minutes of holding.
The flight is legal as loaded, but the discretionary margin is negligible against a TEMPO TSRA forecast, and the commander's request for 300 kg of extra holding fuel is entirely reasonable. The question is where it comes from, and it cannot come from the contingency: 5% of trip fuel is only 63 kg, so even reducing the contingency to zero would not fund 300 kg.
2. Show why the extra 300 kg cannot be carried at the current payload, and quantify what must be offloaded.
Read the worked answer
Take-off mass at 1,900 kg fuel = ZFW 4,300 + 1,900 = 6,200 kg, exactly MTOW. There is no mass available for additional fuel.
To carry 1,900 + 300 = 2,200 kg of fuel at MTOW:
ZFW max = 6,200 − 2,200 = 4,000 kg → payload must be reduced by 4,300 − 4,000 = 300 kg.
Other checks: MZFW 4,000 kg is well inside 5,400 kg; landing mass at the destination = 6,200 − 1,250 = 4,950 kg, inside MLW 5,900 kg; the 2,200 kg is inside the 2,400 kg tank capacity, so tank volume is not limiting either. Only MTOW is.
So the choice is 300 kg of payload or 300 kg of fuel - and the decision belongs to the commander and dispatcher together, documented on the operational flight plan, not absorbed silently by trimming reserves.
3. Evaluate the "take it out of the contingency" proposal and list the alternatives.
Read the worked answer
Reducing the contingency from 5% to 3% saves 2% × 1,250 = 25 kg, or about 2.5 minutes of holding - it does not come close to 300 kg, and it is not permitted: the contingency is a planning minimum in the operator's approved fuel scheme and cannot be planned away to fund discretionary fuel. It can be used in flight when it becomes clear it will not be needed, but that is a different decision, taken later, with the fuel state known.
It is also self-defeating: the contingency exists precisely to absorb forecast error, and TEMPO TSRA is exactly that forecast error. Removing it would leave you with less protection for the hazard you are trying to protect against.
Alternatives:
- Offload 300 kg of payload and carry the extra fuel, accepting the commercial cost.
- Keep the payload and accept the 62 kg discretionary margin, with an early in-flight decision to divert if the destination weather deteriorates.
- Consider a technical stop or a closer alternate if that reduces the total fuel requirement more cheaply than offloading payload.
Any of these is defensible; taking it out of the contingency is not.