It’s common for pilots to mention their airplane’s fuel capacity and follow it with the disclaimer that it’s “usable.” Like saying “75 gallons usable.” Even by the fuel caps there will usually be a label explaining the amount of usable fuel in that tank which is different than the total fuel capacity of the airplane. It isn’t very common to reference “usable” fuel outside of aviation and so it’s a common question to ask: Why do aircraft have an unusable amount of fuel?
Aircraft have a certain amount of unusable fuel because in certain flight attitudes some of the total fuel can become inaccessible, particularly with low fuel levels. Counting some of the total fuel as unusable ensures that the entirely of your usable fuel (which is shown on your fuel quantity indicators) is accessible in all phases of flight.
Unusable fuel is really a safety mechanism to ensure that the engine is properly fed fuel in all phases of flight. It’s not a reserve tank of any sort, although you can sometimes access this unusable fuel. Understanding what causes fuel to be unusable can help you be a safer pilot in fuel planning and not assume that you have more fuel than you really do.
Why do aircraft have an unusable amount of fuel?
Deeming part of a fuel tank “unusable” is partially a safety mechanism and it is a result of the engineering challenge of providing an engine with a constant fuel supply despite changes in aircraft pitch, roll, and raw, as well as turbulence and varying G-forces.
Drawing Fuel from an Aircraft Fuel Tank is Complicated
Aircraft fuel tanks aren’t like vehicle fuel tanks. With the fuel tank in your car, there is generally one low point within the tank from which the fuel can be extracted. Even if you are in a hilly area, the changes in pitch (which change that center of gravity in the fuel tank) are going to be varied, relatively shallow, and temporary. So it’s easier in a car to design a fuel tank that will use essentially 100% of the available fuel because of gravity.
In an airplane, unless you are taxing or in stable cruise flight, the fuel tank’s center of gravity is going to constantly be changing due to the aircraft turning, climbing, descending, and going through turbulence. Engineers must design a fuel tank to have access to the most amount of fuel knowing that the fuel will have a changing center of gravity throughout different phases of flight.
It’s not as simple as just putting the fuel extraction point in the center or corner of a fuel tank with a sloped tank feeding fuel to that point (like the drain on your shower or bathtub) since the airplane is constantly moving.
While you might be able to access all of the usable and unusable fuel during one particular flight attitude, engineers design an unusable fuel level so that your usable fuel is accessible in all normal flight attitudes.
Aircraft Fuel Tank Design is Intricate
Another reason that fuel becomes unusable is that aircraft fuel tanks can have multiple sections and baffles to make fuel more stable during flight, which can make extracting every last drop difficult, if not impossible in certain phases of flight. Aircraft fuel tanks are designed to minimize fuel movement within the tank that could impact the aircraft’s flight characteristics.
Different separators, dividers, and baffles are often built into the design of the fuel tank to ensure that fuel does not gather inertia during turns or turbulence. This is yet another reason that not all of the fuel will be usable during flight.
Fuel Tanks Must Be Able to Supply Minimum Fuel Pressure and Flow
Another reason that aircraft have unusable fuel is that FAR 23.955 regulates how much fuel pressure and flow a fuel system must be able to provide in certain phases of flight. For gravity-fed fuel systems like the Cessna 172, there must be enough fuel in the tank to supply the required gravity-created pressure to give the engine its required fuel flow, even at low fuel levels.
FAR 23.955 (a) reads “The ability of the fuel system to provide fuel at the rates specified in this section and at a pressure sufficient for proper engine operation must be shown in the attitude that is most critical with respect to fuel feed and the quantity of unusable fuel.”
Subsections (a)(4)(b) and (a)(4)(c) mandate that the fuel flow rate for gravity systems must be able to provide 150 percent of the takeoff fuel consumption of the engine, and fuel flow rate for pump systems must be 125 percent of the fuel flow required by the engine at the maximum takeoff power.
For both gravity fed and pump systems, there has to be enough fuel in the tank to supply the required pressure as outlined above. Unusable fuel is partially there to ensure that there is enough pressure for the usable fuel to comply with this regulation.
Related Questions
There are several related questions on this topic, most of which can impact the safety of your flight. Too many accidents still occur because of improper fuel planning, so it’s an area worth studying. Read on!
Is unusable fuel is the same thing as fuel reserve requirements?
While it would be easy to think that unusable fuel functions as a reserve, that is definitely not the case. VFR and IFR fuel reserve requirements are completely separate from unusable fuel. You should treat your unusable fuel as just that – unusable.
So if you have total usable fuel in your airplane of 75 gallons, you’ll need to calculate your reserve requirements (enough to arrive at destination plus: 30 minutes during daytime VFR, 45 minutes for nighttime VFR, and 45 minutes for IFR (after flying to the alternate, if required)) within that usable fuel.
What portion of the fuel tank is unusable?
While it will vary on each aircraft, here are a few examples to give an idea of what percent of the fuel tank might be unusable. Remember though that the usable fuel for your particular aircraft will always be in the POH and often times placarded elsewhere around the airplane (next to the fuel caps, and sometimes by the fuel quantity indicators).
These are just some illustrative numbers to provide some perspective. Don’t use these as a substitute for fuel planning in your particular aircraft.
Cessna 182A: Total fuel of 65 gallons, 55 usable
Cessna 182T: Total fuel of 92 gallons, 87 usable
Cessna 172S: Total fuel of 56 gallons, 53 usable
Cessna 152: Total fuel of 26 gallons, 24.5 usable
Beechcraft A36 Bonanza: Total fuel of 80 gals, 74 usable
Is fuel unusable to avoid water and sediment getting into the engine?
This isn’t really the reason there is unusable fuel. While water is heavier than aviation fuel and will sink to the bottom of a fuel tank, that doesn’t necessarily mean that the fuel draining point should be “uphill” of that low point to avoid water (which would render all fuel below it potentially unusable).
In fact, when you drain the sumps before each flight, you are taking that fuel sample from low points within the fuel tanks where any water would reside. So you’re already trying to filter out water before each flight.
Furthermore, the fuel then goes through a strainer and fuel filter to remove any water and sediment before going into the engine. All of that to say, unusable fuel is not a mechanism to prevent sediment and water from entering the engine as there are other safeguards in place.
Can you ever use part of your unusable fuel?
It’s worth noting that you might be able to still access some (or even all) of that unusable fuel in your tanks in certain flight attitudes depending on the airplane and its fuel tank design. While you might be able to access that unusable fuel in level flight, it might not be accessible in a turn or climb, hence why the manufacturer has deemed it unusable.
However that definitely does NOT mean that you should count this as usable or as any type of reserve in any case. Always treat unusable fuel as completely unusable and abide by the fuel reserve requirements. Running out of fuel is probably the most avoidable accident there is, so don’t let it happen to you.
Does empty weight include any fuel?
Remember that the empty weight of your airplane does include the unusable fuel (along with full oil, among other things). It does not include any usable fuel. So if you are calculating the weight and balance for full fuel, you would just need to use the total usable fuel and leave the unusable out of it since it’s already factored in.
While you’re on the subject… here are other articles on Airplane Academy related to fuel:
