Usable vs. Unusable Fuel: What It Means for Planning, Weight and Balance, and Fuel Sampling
Updated: Sep 26
The difference between usable and unusable fuel seems like a technicality until you realize it directly affects how far you can actually fly, how your weight-and-balance numbers work, and where water and contamination hide in your fuel system. Every pilot plans with usable fuel, but understanding what unusable fuel is, why it exists, where it sits in the system, and how it factors into weight calculations separates pilots who truly understand their aircraft from those who just read numbers off a chart.
This post covers usable and unusable fuel in practical depth: the precise definitions, where fuel gets trapped in the system, why unusable fuel is already in your empty weight, the fuel sampling and sump procedures, the reality of running a tank dry, and what the fuel gauge accuracy regulation actually requires.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
Usable Fuel: What You Can Actually Use
Usable fuel is the portion of total fuel that can be reliably delivered to the engine under all normal flight conditions.
The definition:
Fuel that can reach the fuel pickup points
Under all normal attitudes (climb, cruise, descent, turns)
In turbulence and maneuvering
Available for engine operation
What makes fuel usable:
It can flow to the pickup point
It's not trapped by system design
It remains available during maneuvering
It sustains engine operation reliably
Why usable fuel is what matters for planning:
This is the fuel you can actually burn
Range and endurance are based on usable fuel
Reserve requirements use usable fuel
Never plan with total fuel
The planning number:
All flight planning uses usable fuel
The POH specifies usable fuel capacity
Endurance = usable fuel ÷ burn rate
Range = endurance × groundspeed
Unusable Fuel: What Stays Behind
Unusable fuel is the small amount that cannot be reliably delivered to the engine.
The definition:
Fuel that can't reach the pickup under adverse conditions
Trapped by system design
Determined during certification
Not available for planning
Where unusable fuel sits:
In tank low points below the pickup
In fuel lines and passages
In filters and gascolators
In the bottom of the system
Trapped by tank shape
Why unusable fuel can't be used:
The pickup can't draw it under all conditions
During climbs/descents/turns, it moves away from the pickup
Drawing it risks air entering the lines
Certification deems it unreliable
The certification determination:
The aircraft is tested in various attitudes
Climb, descent, steep turns, etc.
The lowest fuel that allows continuous operation is found
Fuel below that is "unusable"
Where Fuel Gets Trapped in the System
Understanding the fuel system components helps explain unusable fuel:
Fuel tanks:
The tank shape leaves low points
Fuel below the pickup can't be drawn
Wing tanks especially (fuel moves with attitude)
Some fuel always remains
The fuel pickup:
Located to draw fuel in normal attitudes
Positioned above the very bottom (to avoid drawing sediment)
Fuel below the pickup is unusable
Design balances usability with contamination avoidance
Fuel lines:
Fuel in the lines between tank and engine
Some remains in the plumbing
Part of the unusable quantity
The gascolator (fuel strainer):
A low point in the system
Collects water and sediment
Has a drain for sampling
Some fuel remains here
Filters:
Fuel trapped in filter media
Small quantity
Part of unusable fuel
The sump:
Low points where water and debris collect
Drained during preflight
Fuel here may be unusable
Why Unusable Fuel Is in Your Empty Weight
A key concept often misunderstood: unusable fuel is included in the aircraft's empty weight.
The empty weight inclusion:
Standard empty weight includes unusable fuel
Also includes full operating fluids (oil, hydraulic)
The unusable fuel is "always there"
It's part of the aircraft's baseline weight
Why this matters:
You don't add unusable fuel to your weight calculation
It's already in the empty weight
Only add usable fuel you load
Avoid double-counting
The weight-and-balance implication:
Empty weight (includes unusable fuel) + usable fuel + payload = total weight
Don't separately account for unusable fuel
It's in the empty weight figure
The POH empty weight already includes it
The practical calculation:
Start with empty weight (unusable fuel included)
Add the usable fuel you're loading
Add passengers, baggage
The unusable fuel is already counted
The distinction from usable fuel weight:
Usable fuel: you add it to the calculation (varies by load)
Unusable fuel: already in empty weight (constant)
This prevents confusion in weight-and-balance
Fuel Sampling and Sumps
Unusable fuel areas are where contamination collects, making sampling critical:
Why sample fuel:
Water collects at low points
Sediment and debris settle
Contamination affects engine operation
Sampling detects problems before flight
The sump drains:
Located at fuel system low points
Each tank has sump drains
The gascolator has a drain
Sample from all drains
The sampling procedure:
Use a clear fuel sampler cup
Drain fuel from each sump
Check for water (settles at the bottom, clear/beads)
Check for sediment or debris
Check the color (correct fuel grade)
Check for proper smell
Water in fuel:
Water is denser than fuel
Settles at the bottom (sump)
Appears as clear bubbles or a distinct layer
Must be drained until clear fuel appears
Fuel color check:
100LL: blue
Jet A: clear/straw
Confirms correct fuel grade
Wrong color indicates a problem
The contamination connection:
The unusable fuel areas (sumps, gascolator) trap contamination
This is where water and debris collect
Regular sampling is essential
Especially after fueling or rain
Running a Tank Dry
A debated practice related to usable/unusable fuel:
The concept:
Some pilots intentionally run a tank dry
To use all usable fuel from that tank
Then switch to another tank
Maximizes known fuel use
The arguments for:
Confirms exactly how much usable fuel that tank held
Uses all available fuel from the tank
Removes uncertainty about remaining fuel
Some POHs address the technique
The arguments against:
Risk of engine stoppage if switching is delayed
Air can enter the system
Restart procedures needed
Stress and workload
Not appropriate in all situations (low altitude, etc.)
The safety considerations:
Never run a tank dry at low altitude
Be ready to switch immediately
Know your restart procedure
Consider the risks vs. benefits
Many instructors advise against it
The unusable fuel connection:
Running dry uses fuel down to unusable
The engine stops when usable fuel is depleted
Unusable fuel remains
This confirms the usable quantity
The Fuel Gauge Accuracy Regulation (and a Common Myth)
There's a persistent hangar myth that fuel gauges are "only required to be accurate when they read empty." That's false, and it's worth correcting, because it encourages pilots to distrust an instrument that's actually supposed to work.
What the regulation actually requires:
Under the pre-2017 FAR 23.1337(b) — how most of the current GA fleet was certificated — the main requirement is a means to indicate the quantity of usable fuel in each tank during flight. That's a throughout-the-range requirement. The gauge is supposed to tell you how much usable fuel you have, at any point in the flight, not just at empty.
Where the myth comes from:
FAR 23.1337(b)(1) adds that the gauge must read "zero" during level flight when the remaining fuel equals the unusable fuel supply. This is an additional requirement layered on top of the main one — the word the regulation uses is "in addition." Its purpose is to make clear that the gauge indicates usable fuel, not total fuel. A gauge that reads zero while the tank still holds usable fuel doesn't comply. Pilots and mechanics who read only that "zero" clause and skipped the stem above it concluded "accurate only at empty," which misreads the regulation.
The current rule:
In the 2017 Part 23 rewrite, the "zero" language was removed entirely. The current FAR 23.2430 requires the fuel system to provide the flightcrew a means to determine the total usable fuel available. Same intent — you must be able to know your usable fuel.
How accurate is "accurate"?
The regulations don't specify a numeric tolerance for GA fuel gauges. As a ballpark, TSO-C55 (not mandatory for GA) specifies 3% of full scale for the entire sender-to-gauge system. The practical reality is that typical light-aircraft float-type senders in long, thin wing tanks are genuinely imprecise — fuel sloshes with attitude and turbulence, so the reading wanders. The FAA's position is that the intent of the rule is to prevent fuel-exhaustion accidents, so a gauge too inaccurate to be useful is a discrepancy to be repaired, not tolerated.
The practical takeaway:
The gauge is required to indicate usable quantity and should be reasonably accurate. But because real-world GA senders are imprecise, never rely on the gauge as your only fuel information. Calculate fuel using known starting quantity, burn rate, and time, and use the gauge as a cross-check against that calculation. When the two disagree, investigate before you fly.
Practical Pilot Considerations
For flight planning:
Use usable fuel for all calculations
Never include unusable fuel in range/endurance
Know your aircraft's usable fuel capacity
Base reserves on usable fuel
For weight and balance:
Empty weight includes unusable fuel
Add only usable fuel to the calculation
Don't double-count unusable fuel
Use POH figures
For preflight:
Sample all sump drains
Check for water and contamination
The unusable fuel areas trap contaminants
Drain until clean fuel appears
For fuel management:
Know how much usable fuel each tank holds
Monitor consumption against a calculated burn
Don't rely on running tanks dry
Maintain reserves in usable fuel
Common Misconceptions
"I should add unusable fuel to my weight calculation."
No — unusable fuel is already in the empty weight. Add only usable fuel.
"Unusable fuel is wasted."
It's a necessary design and safety consideration, not waste. It prevents air ingestion and accommodates tank design.
"Fuel gauges only have to be accurate when empty."
A myth. FAR 23.1337(b) requires indication of usable quantity during flight; the "zero" clause is an additional requirement, not the whole of it. The current FAR 23.2430 requires a means to determine total usable fuel available.
"I can use unusable fuel in an emergency."
No — unusable fuel can't be reliably delivered to the engine. It's not available even in emergencies.
"Total fuel capacity is what I plan with."
No — plan with usable fuel. Total capacity includes unusable fuel you can't use.
On the Written Test and Checkride
Usable/unusable fuel appears on tests. The most commonly tested topics:
Definitions of usable and unusable fuel
Planning with usable fuel only
Unusable fuel in empty weight
What the fuel gauge regulation actually requires
Fuel sampling and contamination
Weight and balance implications
Quick Reference
Usable Fuel:
Can reach the engine under all normal conditions
Available for planning
POH specifies the quantity
Range/endurance based on this
Unusable Fuel:
Trapped in the system
Can't reliably reach the engine
Determined by certification testing
Not for planning
Where Fuel Gets Trapped:
Tank low points (below pickup)
Fuel lines
Gascolator
Filters
Sumps
Empty Weight:
Includes unusable fuel
Also includes full oil and operating fluids
Add only usable fuel to calculations
Don't double-count unusable fuel
Fuel Sampling:
Sample all sump drains
Check for water (settles, beads at bottom)
Check for sediment
Check color (100LL blue, Jet A clear/straw)
Drain until clean
Running a Tank Dry:
Uses fuel to the unusable point
Risk of engine stoppage
Never at low altitude
Be ready to switch
Debated practice
Fuel Gauge Accuracy:
Myth: "only accurate at empty" — false
FAR 23.1337(b): must indicate usable fuel quantity during flight (throughout range)
FAR 23.1337(b)(1): additionally reads zero at unusable fuel (shows usable, not total)
2017 rewrite → FAR 23.2430: means to determine total usable fuel available
No numeric tolerance specified (TSO-C55 ~3% as a ballpark, not mandatory)
GA float senders are imprecise in practice
Use calculated fuel (time × burn rate) as primary; gauge as cross-check
Planning:
Use usable fuel only
Never include unusable in range/endurance
Know your aircraft's values (POH)
Key Principle:
Plan with usable fuel only; unusable fuel is trapped in the system and already counted in empty weight. Fuel gauges must indicate usable quantity but GA senders are imprecise, so use calculated fuel as your primary method and the gauge as a cross-check. Sample the sumps where contamination collects.
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:
Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.