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Weight of oil from specific gravity

Aviation Mechanic Powerplant (AMP) practice question 5 of 9 on Engine Lubrication Systems. ACS element AM.III.G.K1 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONWater weighs approximately 8 pounds per gallon. Using that value, about how much does 5 gallons of an engine oil with a specific gravity of 0.9 weigh?

  1. AAbout 36 pounds
  2. BAbout 40 pounds
  3. CAbout 7.2 pounds
Show the answer and explanation

Correct answer: A. About 36 pounds

  • A — correct Specific gravity 0.9 means the oil weighs 0.9 times as much as an equal volume of water, so 0.9 x 8 = 7.2 pounds per gallon, and 5 gallons weigh about 36 pounds.
  • B — incorrect This uses water's weight of 8 pounds per gallon and ignores the specific gravity of 0.9 entirely; the oil is lighter than water.
  • C — incorrect 7.2 pounds is the weight of a single gallon of this oil, not the weight of 5 gallons.

BACKGROUND

Specific gravity is a comparison of the weight of a substance to the weight of an equal volume of distilled water at a specified temperature. The handbook uses the approximation that water weighs about 8 pounds to the gallon. An oil with a specific gravity of 0.9 therefore weighs about 7.2 pounds to the gallon. Mechanics use this relationship when computing the weight of oil aboard an aircraft, since oil quantity is measured in gallons or quarts but weight and balance work is done in pounds. Specific gravity is one of several oil properties, along with viscosity, viscosity index, flash point, and pour point, that describe a grade of engine oil.

STUDY TIP

Practice the two-step conversion: multiply water's weight per gallon by the specific gravity to get pounds per gallon, then multiply by the number of gallons. Watch for answers that give the per-gallon figure when the question asks for a total.

KEY POINTS

  • Specific gravity compares a substance's weight to an equal volume of distilled water.
  • Water is taken as roughly 8 pounds per gallon, so oil at 0.9 specific gravity is about 7.2 pounds per gallon.
  • Multiply pounds per gallon by the number of gallons to get total weight.

SOURCES

  • sources/amp-powerplant/ch06.txt "Specific Gravity"
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON ENGINE LUBRICATION SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.