Flight Planning & Fuel Calculations

High Performance Aviation

November 20, 2011

It’s All Going According to Plan

By Hank Gibson, CFI, CFII, MEI

Fuel Calculations

How much fuel does the airplane you are flying hold? This is a very important question that most pilots take for granted. Let’s keep things simple and use a Cessna Skyhawk for example. It holds 53 usable gallons of fuel. At an average cruise power setting, the airplane burns roughly 9 gallons an hour. That all adds up to approximately 5 hours and 48 minutes of fuel till the tanks run dry. Let’s say that you own the airplane and you and a co-worker fly it 4 days a week for work. With both of you in the front seats, you can take full fuel and don’t have to worry about weight and balance. The trip takes about four and a half hours, there and back, so you never have to worry about filling up until you get back home.

How much fuel can you take on this flight? This is, quite possibly, an even more important question than the one of fuel capacity. Let’s go back to our Skyhawk example. One day, your boss calls and tells you that a new guy is coming aboard and he will be accompanying you on your flights. The first, most obvious question is, how much does he weigh? Every pilot thinks about that. But now, suddenly, you can’t take full fuel.

The night before, you call the FBO and have them fill up the airplane with only 25 gallons much lower than normal. After a glass of milk, you’re off to bed with no second thoughts. Early in the morning, you sump the tanks after some coffee and a donut, then you and your co-workers are off. You press the direct to key shortly after lift-off, turn the autopilot on and sit back to enjoy the sunrise. (side-note: doing the math, 25 gallons only gives you 2 hours and 42 minutes of fuel; 2 hours and 12 minutes before the FAA mandated VFR reserve).

After a long day on location, all of you hop back into the airplane for the ride home. You never check the fuel before leaving because there is always enough. After a chocolate bar and some water, it’s off into the night. Half an hour into the flight, the engine sputters, you stall the airplane while trying to find a place to land in the dark, overcompensate when a wing dips with the yoke, spin and die, killing two others with you.

Harsh? Yes. Real? Absolutely. Fuel plays into more than just weight and balance. Fuel is a pilot’s life blood and when he doesn’t take the time to do fuel calculations for a flight, it is asking for trouble. Weight and balance questions should always be followed by fuel burn questions. Will we have enough fuel to make the trip like normal? Do we need to fill up elsewhere? Do we need an extra fuel stop? Should I run the engine on 65% power instead of 75%?

All that to say, planning must still go into flights. It is so easy to fall into the trap of getting in the airplane, pressing the direct to key to the destination and just flying, letting the G1000 “do all the work.” The last question to ask before getting into the airplane should always be, have I calculated the fuel needed for our flight?

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