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4 Answers

What does “usable” fuel mean?

Asked by: 6552 views , ,
Student Pilot

Guys, as an acft example (Cessna 152), where  I'm starting to fly, at your aircraft manual I have the follow information:


- Total fuel: 26 US Gal

- Usable fuel: 24,5 US Gals

What is this 1,4 of unusable fuel?

What is the fuel capacity I need to calculate the C-152 endurance?


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4 Answers

  1. Jeff on Feb 09, 2013

    In most aircraft there is a small amount of fuel that can’t be drained from the tanks. That is the “unuseable” fuel. For calculation of endurance, use the “useable” amount. For weight and balance use the “total” amount.

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  2. Steve Pomroy on Feb 09, 2013

    Hi Cadu.

    As noted by Jeff, most aircraft fuel tanks have a small amount of fuel that can’t be drained (or, in some cases, can’t be drained at a rate that will support maximum power on the engine). But with regard to weight and balance, the unusable fuel should be included in the empty weight of the aircraft. So you shouldn’t need to account for it when you do your W&B calculations. For range, endurance, and W&B, just work with usable fuel.


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  3. John D. Collins on Feb 10, 2013

    Unusable fuel is fuel that under normal flight conditions may not be available for operation of the engine. An example is my Bonanza, it has two 40 gallon tanks with 37 gallons of usable fuel each. In level attitude, all the fuel may actually be used except a few ounces. But you can’t plan on using more than the 74 gallons, particularly when taking off or landing.

    I had occasion to run my right fuel tank to the point that the engine started to quit due to fuel exhaustion. I was doing this deliberately because I was going to have the fuel bladder worked on. I still had almost two hours of fuel remaining in the other tank. I was very careful to monitor the fuel used from the right tank by using a digital fuel totalizer, keeping the airplane in level flight, and watching the fuel flow pressure gage. I switched tanks immediately when the fuel pressure began to fluctuate and the engine never stopped. I also made sure I was near an airport at the time with plenty of altitude to glide to it in the unlikely event that the engine wouldn’t run after switching tanks.

    When I drained the right tank, only a few ounces of fuel drained out. On filling the tank after the maintenance, it took 40.5 gallons into the 40 gallon tank.

    The important message is that you should not ever plan to use the unusable fuel. Also, don’t run a tank dry if you don’t know what you are doing.

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  4. Cadu on Feb 13, 2013

    Thank you guys for all answers above.

    So, dosen’t matter how much fuel I take-off with C-152, I need to discount 1,4 US Gal to calculate my endurance. If I take-off with a half of fuel, I men, 13 US Gal, I need to discount 1,4 from this.



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