top of page

Understanding Usable vs. Unusable Fuel in Aviation

Fuel management is one of the most critical aspects of aviation safety and performance. Pilots and aircraft engineers must not only ensure that an aircraft carries enough fuel for the planned flight but also understand how much of that fuel is usable versus unusable. While the terms might sound similar, they have distinct meanings that directly affect flight planning, aircraft certification, and safety margins.



Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >


What Is Usable Fuel?

Usable fuel refers to the portion of total fuel onboard that can be safely delivered to the engine(s) under all normal operating conditions. This is the fuel a pilot can rely on during flight for power generation.


In simpler terms, it’s the fuel that can be pumped from the tanks, through the fuel lines, and into the engine to sustain normal operation.


Usable fuel includes:

  • Fuel in tanks that can reach the fuel pickup points under all attitudes and flight conditions (climb, cruise, descent, etc.)

  • Fuel that is not trapped by the design of the fuel system

  • Fuel that remains available even when the aircraft is banking, pitching, or in turbulence


The usable fuel quantity is the number pilots use in flight planning, performance calculations, and legal fuel reserve requirements.


What Is Unusable Fuel?

Unusable fuel is the small amount of fuel that cannot be safely used by the engine. It remains trapped in tanks, fuel lines, filters, or low points of the system due to:

  • Tank design (shape, structure, or placement of fuel outlets)

  • Residual fuel that can’t reach the fuel pickup because of gravity or suction limitations

  • Contamination or debris that prevents full drainage


Unusable fuel is accounted for during aircraft certification and is specified in the aircraft’s Pilot’s Operating Handbook (POH) or Aircraft Flight Manual (AFM). Even though it can’t be used in flight, it’s still part of the aircraft’s total fuel weight and affects takeoff weight and balance calculations.


Example: Usable vs. Unusable Fuel in Practice

Let’s say an aircraft has:

  • Total fuel capacity: 50 gallons

  • Unusable fuel: 2 gallons


That means the aircraft has:

  • Usable fuel: 48 gallons


When planning a flight, the pilot must base endurance, range, and reserve calculations on the 48 usable gallons, not the total 50. The extra 2 gallons will remain in the system but are not available for engine operation.


Why Does Unusable Fuel Exist?

It might seem inefficient to have unusable fuel onboard, but it’s a necessary safety consideration. Factors include:

  • Design constraints: Fuel tanks have specific shapes to fit inside wings or fuselage sections, making it impossible to drain 100% of the fuel.

  • Safety margin: A small amount of fuel helps prevent air from entering the lines when fuel levels are low.

  • System complexity: Aircraft with multiple tanks or fuel pumps may have uneven drainage, leaving traces of fuel behind.


Certification and Testing

During the aircraft certification process, engineers determine unusable fuel through rigorous testing. The aircraft is placed in various flight attitudes (climb, descent, steep turns, etc.) to identify the lowest fuel quantity that allows continuous engine operation without interruption. Any fuel that remains beyond that point is deemed unusable fuel.


This ensures the POH fuel capacity figures are conservative and safe, giving pilots reliable data for flight planning.


Pilot Considerations

For safe flight operations:

  1. Always plan using usable fuel quantities. Never include unusable fuel in range or endurance calculations.

  2. Know your aircraft’s specific values. These are listed in the POH/AFM fuel system section.

  3. Account for unusable fuel in weight calculations. Even though it can’t be burned, it adds weight.

  4. Check for contamination. Unusable fuel can trap water or debris, making regular fuel sampling critical.


Conclusion

The distinction between usable and unusable fuel may seem minor, but it’s vital for safe and accurate flight operations. Pilots must always plan based on usable fuel, while understanding that unusable fuel is a built-in safety and design consideration. By knowing how much fuel your aircraft can truly use—and what’s left behind—you ensure both compliance and safety on every flight.



Study Full Aviation Courses:

wifiCFI's full suite of aviation courses has everything you need to go from brand new to flight instructor and airline pilot! Check out any of the courses below for free:


Study Courses:


Checkride Lesson Plans:


Teaching Courses:



 
 
bottom of page