Crabwise · Library
How can a fuel-and-time log be converted into an average flow?
Fuel used over a timed interval gives an average flow: 14.2 US gal in 95 minutes is 8.97 gal per hour, and 41 L in 70 minutes is 35.14 L per hour.
Flow from a quantity and its minutes
The fuel screen's flow mode computes FF = F ÷ (t ÷ 60): fuel used divided by the elapsed hours. The answer is per hour in the same unit as the fuel, whether that is gallons, litres or pounds. It is an average over the interval you timed, so the quantity and the minutes must start and end at the same moments.
A flow measured in cruise can then be reused in the fuel-burned and endurance modes, as long as its unit and the conditions it came from travel with it.
Worked example
Ten fuel units used in 75 minutes corresponds to an average of 8 units per hour. The calculation does not determine whether that average is appropriate for another leg.
- Average hourly flow in the chosen fuel unit
- 8
From log to average flow
- Record the fuel reading and the clock at the start of the interval.
- Record both again at the end and subtract to get fuel used and elapsed minutes.
- Enter fuel used and minutes in the flow mode.
- Note the power setting and phase of flight next to the average.
Two logged intervals
14.2 US gal used in 95 minutes gives 14.2 ÷ (95 ÷ 60) ≈ 8.968421 gal per hour. A separate log of 41 L in 70 minutes gives 41 ÷ (70 ÷ 60) ≈ 35.142857 L per hour.
- Average flow, 14.2 gal in 95 min (gal/h)
- 8.968421
- Average flow, 41 L in 70 min (L/h)
- 35.142857
The two numbers are not comparable until one is converted: 35.142857 L/h is roughly 9.28 US gal/h.
Reused in the endurance mode, the first average turns 30 US gal into 30 ÷ 8.968421 × 60 ≈ 200.704225 minutes. That figure inherits every assumption of the 95-minute interval it was measured over, including the power setting.
Common mistakes
- Dividing the whole flight's fuel by the cruise minutes only.
- Entering the elapsed time as decimal hours in the minutes field.
- Treating an average from cruise as the flow for climb.
- Reading a gauge change in pounds and entering it in the gallon-based calculation without converting.
Scope
The average carries the uncertainty of the fuel readings. It describes the interval measured, not other power settings. The FAA navigation chapter below covers fuel and time relations.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.