VFR fuel planning: taxi, trip, contingency and reserve
Most fuel planning mistakes come from adding up the wrong things, or from using a figure from the manual that the aircraft does not give on the day. The fuel you need is a sum of five parts, each with a reason. This guide explains each part, shows how to work out your endurance, and shows how the planner puts them together.
The five parts
Taxi fuel is the fuel burned from engine start to take-off. It depends on the aerodrome and the traffic, and it is the part you cannot use for the flight.
Trip fuel is the fuel to fly from take-off to the destination, along the route you planned, with the winds of the day. It is the part that comes straight from the nav log: the sum of the fuel burned on each leg, including the climb and the descent.
Contingency fuel is a margin for what the plan does not know, such as a wind that is stronger than the forecast, a detour around weather or a longer approach. It is usually a share of the trip fuel. AirPlotter starts at 5 % of the trip fuel.
Alternate fuel is the fuel to fly from the destination to the alternate aerodrome, for the case that you cannot land at the first one. You plan it when you plan an alternate.
Final reserve is the fuel you still have on landing. It is not for the plan. It is there so that a flight that has used everything else can still reach the ground. AirPlotter starts at 30 minutes, and it shows the recommended reserve under the field: 30 minutes by day and 45 minutes when you land at night. It warns you when the reserve you chose is lower.
The sum of the five is the fuel you need on board. The rules that apply to your flight set the minimum, and they depend on the aircraft, the licence and the type of flight, so check them.
Endurance
Endurance is how long you can fly with the fuel you have. Take the fuel on board, subtract the taxi fuel, and divide by the cruise fuel flow. With 120 litres on board, 5 litres of taxi and a cruise fuel flow of 30 litres an hour, that is (120 − 5) / 30 = 3.8 hours, or 3 hours and 50 minutes.
Endurance is the longest flight you can make, with nothing set aside. Your plan uses less than that, because the final reserve and the contingency are fuel you plan not to use.
An example
A flight of 64 NM takes 40 minutes at a cruise fuel flow of 30 litres an hour, with the climb and descent included in the trip fuel. Say the nav log gives a trip fuel of 20 litres.
| Part | Litres |
|---|---|
| Taxi | 5 |
| Trip | 20 |
| Contingency, 5 % of the trip | 1 |
| Alternate, a leg of 15 minutes | 7.5 |
| Final reserve, 30 minutes | 15 |
| Needed on board | 48.5 |
With 120 litres on board, you have a large margin. With 45 litres, the plan does not work, and the answer is more fuel, a closer alternate, or a different flight.
How the planner does it
In AirPlotter, you enter the fuel flows of your aircraft once, together with its usable fuel and the taxi fuel. Then you add the fuel on board to the plan. If you leave it empty, the planner assumes full usable fuel. The nav log then shows, for every leg, the fuel burned and the fuel left, and the fuel card sets the parts you need against what you have on board, with the endurance that leaves you.
You can change the final reserve and the contingency in the fuel card of the nav log. The planner computes the final reserve at your cruise fuel flow, which is on the cautious side when the real flow at a holding speed is lower.
Habits that help
- Plan the fuel with the wind of the day. A headwind leg burns more than the tailwind leg saves. Wind correction angle and ground speed shows why.
- Use your own fuel flows. The figure in the manual is a reference. The flow you measure in cruise on your aircraft is the one to enter.
- Look at the fuel left at each leg, not only at the end. A leg where it drops faster than you expected is the place to change the plan.