SkyDrill

Air-fuel ratio expressed as a decimal

Aviation Mechanic Powerplant (AMP) practice question 6 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.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

QUESTIONA reciprocating engine is operating on a mixture of 12 pounds of air to 1 pound of fuel. Expressed as a decimal air-fuel ratio, this mixture is:

  1. A0.067, the chemically correct value
  2. B12.0
  3. C0.083
Show the answer and explanation

Correct answer: C. 0.083

  • A — incorrect 0.067 is the chemically correct (stoichiometric) ratio of 0.067 pound of fuel to 1 pound of air, which is a 15:1 mixture, not a 12:1 mixture.
  • B — incorrect This simply repeats the whole-number form of the ratio; the decimal form is the weight of fuel per pound of air, which is far less than one.
  • C — correct Dividing 1 pound of fuel by 12 pounds of air gives 0.083, and the handbook states that a 12:1 ratio and a 0.083 air-fuel ratio describe the same mixture.

BACKGROUND

The composition of an air-fuel mixture is described by the mixture ratio, always expressed by weight because the volume of air varies greatly with temperature and pressure. A mixture with a ratio of 12 to 1 is made up of 12 pounds of air and 1 pound of fuel. The same mixture may be written as a decimal by dividing the fuel weight by the air weight, so 12:1 equals 0.083. Mixtures as rich as 8:1 and as lean as 16:1 will burn in a cylinder, and maximum power is developed at approximately 12 parts of air to 1 part of gasoline. The chemically perfect, or stoichiometric, mixture is 0.067 pound of fuel per pound of air, a 15:1 ratio.

STUDY TIP

Practice converting between the two forms until it is automatic: the decimal is always one divided by the whole-number air figure. Memorize the anchor values 0.083 (best power, 12:1) and 0.067 (stoichiometric, 15:1).

KEY POINTS

  • Mixture ratios are stated by weight, not volume, because air volume changes with temperature and pressure.
  • A 12:1 mixture and a 0.083 air-fuel ratio are the same mixture; maximum power occurs near this value.
  • The chemically correct stoichiometric mixture is 0.067, which is 15 pounds of air to 1 pound of air.

SOURCES

  • amp-powerplant/ch02.txt "Air-Fuel Mixtures"
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON ENGINE FUEL AND FUEL METERING 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.