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Fuel correction when weighed with full tanks

Aviation Mechanic General (AMG) practice question 19 of 25 on Weight and Balance. ACS element AM.I.C.K7 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONAn airplane is weighed with its 30-gallon fuel tanks full. A hydrometer check shows the fuel weighs 5.9 lb/gal, and the TCDS lists 6 lb of unusable fuel as part of the empty weight. What net fuel weight is subtracted from the as-weighed total to arrive at the empty weight?

  1. A177 lb, the full fuel load
  2. B174 lb, using the standard 6 lb/gal
  3. C171 lb
Show the answer and explanation

Correct answer: C. 171 lb

  • A — incorrect Subtracting the entire 177 lb ignores the 6 lb of unusable fuel that belongs in the empty weight and must be added back.
  • B — incorrect When a hydrometer check is available, its measured 5.9 lb/gal is used instead of the 6 lb/gal standard value.
  • C — correct Full fuel is 30 gal x 5.9 lb/gal = 177 lb, which is subtracted, and the 6 lb of unusable fuel is added back, for a net subtraction of 171 lb.

BACKGROUND

When an airplane is weighed with full fuel, the full weight of the fuel must be subtracted and the unusable fuel added back in, because unusable fuel is part of the empty weight. The weight of the fuel being subtracted is based on the pounds per gallon measured with a hydrometer, which in the handbook's example was 5.9 lb/gal rather than the 6.0 lb/gal standard for Avgas. The handbook's worked example uses 30 gallons of fuel at 5.9 lb/gal, giving 177 lb subtracted at its arm, and 6 lb of unusable fuel added back at its own arm. Each fuel item is handled as its own line with weight, arm, and moment in the six-column weighing chart.

STUDY TIP

Treat the full fuel as a negative line and the unusable fuel as a positive line; net weight is the difference, but each has its own arm and moment.

KEY POINTS

  • Full fuel is subtracted using the hydrometer-measured lb/gal.
  • Unusable fuel is added back as part of the empty weight.
  • 30 gal x 5.9 = 177 lb, less 6 lb = 171 lb net.

SOURCES

  • FAA-H-8083-30B ch06 "Example Weighing of an Airplane"
  • FAA-H-8083-30B ch06 "Hydrometer"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.