Block Fuel: 5% Verses 5 Minutes Verses 3% – Contingency Fuel
When we are doing our Block Fuel questions, we need to read the question carefully to determine what amount of contingency is required. We normally use 5% of trip fuel or 5 minutes of holding fuel @1500 ft overhead the destination aerodrome – whichever is the GREATER.
If we have an ERA (nominated for fuel purposes) we can drop the requirement to 3% or 5 mins – whichever is the GREATER.
If your trip time is short (under 2 hours), then there is a high probability that 5 minutes will be greater than 5%.
Let us have a look at a few questions:
1. For a jet aircraft, given:
- Taxi Fuel 500 kg
- Trip Time 7 hrs
- Average Trip Fuel Consumption 9000 kg/hr
- Destination Holding Fuel Flow 8500 kg/hr.
- Alternate Holding Fuel Flow 8000 kg/hr
- Alternate Fuel 10200 kg
What is the minimum Block Fuel required?
- Taxi Fuel 500 kg
- Trip Fuel (7 x 9000) 63000 kg
- 5% contingency (0.05 x 63000) 3150 kg (use)
- 5 mins (5 ÷ 60 x 8500) 708 kg
- Alternate Fuel 10200 kg
- Final Reserve (0.5 x 8000) 4000 kg
- Total 80850 kg
With this question we need the full 5% as it is a long flight where 3150 kg is greater than 708 kg.
2. For a jet aircraft, given:
- Taxi Fuel 600 kg
- Trip Fuel 7200 kg
- Alternate Fuel 4380 kg
- Destination Holding Fuel Flow 6500 kg/hr
- Alternate Holding Fuel Flow 6000 kg/hr
What is the minimum take-off fuel?
- Taxi Fuel N/R
- Trip Fuel 7200 kg
- Contingency (0.05 x 7200) 360 kg
- Contingency (5 ÷ 60 x 6500) 542 kg (use)
- Final Reserve 3000 kg
- Total 15122 kg
We must be careful with this question as they do not give us a trip time but by looking at trip fuel and holding fuel flows, we can deduce that the trip time is under 2 hours. The other thing is we were asked for, is minimum take-off fuel and not Block Fuel so do not need taxi fuel.
3. An airline is scheduled to operate a commercial flight under IFR to an aerodrome with a single runway using a MRJT aircraft. A valid En-route Alternate (ERA) has been selected for contingency fuel purposes.
Given the following planning data, what is the minimum Block Fuel for the intended flight?
- Taxi Fuel 700 kg
- Take-off and climb 1500 kg
- Cruise Fuel 9600 kg
- Holding Fuel Flow (Dest + Alt) 4000 kg/hr
- Descent 500 kg
- Instrument Approach 1200 kg
- Alternate Fuel 2500 kg
- Taxi Fuel 700 kg
- Trip Fuel (1500 + 9600 + 500 + 1200) 12800 kg
- Contingency (0.03 x 12800) 384 kg (use)
- Contingency (5 ÷ 60 x 4000) 333 kg
- Alternate Fuel 2500 kg
- Final Reserve (0.5 x 4000) 2000 kg
18384 kg
This time we need to use the 3% contingency as we have an ERA nominated and we must also be careful to add up all the components of trip fuel: take-off and climb, cruise, descent, and approach.
It is possible that with this type of question that 5 mins could be greater 0200Kg than 3%.