# Calculate Block Fuel based on Altitude, Temp, Taxi Fuel, Time for climb/approach and landing

I'm preparing for my EASA exams and one of the questions I came across for the "Flight Performance and Planning" exam sounds like this:

For planning a VFR flight, the following data are given:
Pressure Altitude = 6.500 ft
Temperature = ISA-20
Power setting = 2300 RPM
Taxi Fuel = 2 USG
Additional time for climb = 7 min
Additional time for approach and landing = 10 min
The reserve fuel has to be 30% of trip fuel.

Determine the minimum block fuel:

A. 47.3 USG
B. 50.4 USG
C. 43.8 USG
D. 39.2 USG

The correct answer is B. 50.4 USG.

The exercise also gives the following chart as a help:

My thinking was the following:

1. Based on the chart and for the given OAT, Altitude I can detect the Fuel Flow of 12,4 GPH
2. If I am subtracting 7 min climb time + 10 min approach and landing time, I get a Trip Time of 2h 26min
3. Based on The Trip Time and the Fuel Flow I can calculate that the Trip Fuel is ~28 USG
4. 30% of the Trip Fuel would be about 8,4 USG, representing the Reserve Fuel
5. Adding Taxi Fuel + Trip Fuel + Reserve Fuel = 38 USG

Now, of course, something I am calculating wrong or terrible wrong, for sure, the missing part is the fuel needed for T/O and Landing... but I just don't know how to get that.

• I think you are correct in climb and descent fuel being the missing pieces. Climbing at full power for 7 minutes isn't too hard, just interpolate the fuel flow roughly between sea level and 6000'. Descent is a little harder because most pilots don't descend at full power. Nor do they cruise at full power... Jan 17, 2023 at 21:11
• @MichaelHall, even if I add the Fuel needed for T/O and Landing, the numbers don't add up to 55.4 USG. For T/O and Landing I calculated 17 minutes x 0.206 USG/minute = ~3.5 USG Jan 17, 2023 at 21:35
• I think the "tricky" part of this is the climb/approach time. The "overhead to overhead" reference gave me a clue that maybe they meant you'd climb, then fly 2:43, then descend, so you would add rather than subtract the climb/landing times. Jan 18, 2023 at 3:55

I think Max R (in a since-deleted answer, "2:43 + :7 + :10 = a convenient 3.0 hrs") is right about adding the times to get 3 hours. Then,

• 3 hours x 12.4 GPH = 37.2 gal
• 37.2 Gal x 1.3 = 48.36 gal
• Add 2 gallons taxi fuel = 50.36 gal
• Rounded gives 50.4 gal

Apparently they don't want you to multiply 1.3 x the taxi gas. (Although if you do, you get a higher number than any of the answers, which hints that you should try excluding those 2 gallons from the 1.3x multiplier and recalculating.)

I'll add that, from an actual flight planning perspective, using the fuel flow at cruise altitude times some number of minutes as the calculation for fuel used in climb & in descent seems a little non-sensical. As a way to make the math in the question easier, it works, but there's no obvious reason why fuel used in the climb is equal to 7 minutes of fuel at cruise. You can certainly pick some values where it just happens to work out like that, but that isn't how real flight planning works.

Especially if you want to claim 1/10th of a gallon precision in your answer!

• Ah! I did that (deducted the taxi fuel from the reserve) in one of my trial-and-errors, but then did the math wrong! I would edit out your reference to my answer, and then I will delete mine. Jan 18, 2023 at 3:51
• @MaxR Edited to incorporate your formulation of the 3 hours total, thanks.
– Ralph J
Jan 18, 2023 at 4:37