Observed fuel burn rate calculator
The fuel flow in the flight manual describes a new aircraft flown precisely at a test condition. What your aircraft actually burns is worth measuring, and it is usually the more useful number.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The result is the average rate across everything the engine did: start, taxi, run up, climb, cruise and descent. That makes it higher than a cruise figure from the manual, and more honest for planning a whole flight.
Build up several of these over time. One flight is a data point, an average of ten is a planning figure you can trust.
When this is useful
- After refuelling, using the uplift and the hours flown since the last fill.
- Checking whether an engine is burning more than it used to, which can be an early sign of something worth investigating.
- Replacing a manual figure with a measured one for your own flight planning.
How it is calculated
Divide the fuel used by the time flown. The only difficulty is measuring both accurately, and the accuracy of the answer depends entirely on that.
burn rate = fuel used / time flown
Worked example
- Fuel used
- 85 litres
- Time flown
- 2 hours 10 minutes
The average burn rate was 39.2 litres per hour.
Check: 2 hours 10 minutes is 2.1667 hours. 85 divided by 2.1667 gives 39.23 litres per hour. Multiplying back, 39.23 multiplied by 2.1667 returns 85 litres, so the arithmetic closes.
Limitations
- It is only as accurate as the refill. Both fills have to be done the same way, to the same tab or to the filler neck, and on ground that is level enough for that to mean something.
- Time has to be measured the same way each time. Engine hours and airborne time are different, and mixing them across flights produces a meaningless average.
- A short flight is dominated by start, taxi and climb, so the rate will be well above the cruise figure.
Common mistakes
- Comparing an observed rate that includes taxi and climb against a manual cruise figure and concluding something is wrong.
- Using the fuel gauge reading rather than the actual uplift.
- Drawing a conclusion from a single flight.
Sources
- Pilot's Handbook of Aeronautical Knowledge, aircraft performance , US Federal Aviation Administration