How can you catch a per-hour versus per-minute fuel-flow mismatch?

The hour hidden in the rate

Crabwise's fuel field is labelled fuel flow per hour, and the time field takes minutes. The fuel-burned formula F = FF × t ÷ 60 does the hour-to-minute conversion for you. Problems start when a rate is copied from a source that states it per minute, or when a burn is multiplied by hand without the division.

The app accepts gallons, litres or pounds and returns the result in the same unit as the flow. It does not know which one you meant, so a rate in litres per hour must not be mixed with a fuel quantity in US gallons.

Worked example

Ninety minutes is one and a half hours, so 8 units per hour gives 12 units. The number 720 would come from multiplying by minutes without converting the rate’s time basis.

Fuel used in the chosen quantity unit

12

Check the rate before multiplying

1. Read the full label of the rate, including the quantity unit and the time unit.
2. Convert a per-minute rate to per hour by multiplying by 60 before entering it.
3. Enter the elapsed time in minutes.
4. Compare the answer with one hour at the same rate; 50 minutes must give a little less than one hour's burn.

Litres per hour over 50 minutes

With 34 L/h and 50 minutes, fuel burned is 34 × 50 ÷ 60 ≈ 28.333333 L. Dropping the division gives 1700, sixty times too large. Converted on the unit screen, 28.333333 L is about 7.484875 US gal.

Fuel burned in 50 min (L)

28.333333

Result without the ÷ 60 step

1700

Fuel burned in US gallons

7.484875

For a weight figure, the unit screen's avgas density of 6.0 lb per US gal turns the 7.484875 gal into about 44.909249 lb. Jet-A uses 6.7 lb per US gal, so the fuel type has to be chosen before converting.

Common mistakes

• Entering a rate in pounds per hour and reading the answer as gallons.
• Converting the time to hours and then also letting the formula divide by 60.
• Correcting the time unit while leaving a litres-versus-gallons mismatch in place.
• Converting gallons to pounds with the avgas density when the aircraft burns Jet-A.

What the check covers

A consistent unit chain removes factor errors, not uncertainty in the flow itself. The NIST conversion table and the FAA navigation chapter below are the references for the units and the fuel-time relation.

Primary sources and related work

• NIST SI conversion factors
• FAA Pilot’s Handbook: navigation and basic flight calculations