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Meaning of an octane rating

Aviation Mechanic Airframe (AMA) practice question 8 of 12 on Aircraft Fuel Systems. ACS element AM.II.J.K6 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA gasoline resists detonation exactly as well as a reference mixture of 90 percent iso-octane and 10 percent heptane. What is the octane rating of that fuel?

  1. A10 octane, from the heptane content
  2. B90 octane
  3. C100 octane, the highest possible
Show the answer and explanation

Correct answer: B. 90 octane

  • A — incorrect The rating is set by the iso-octane percentage, not by the percentage of heptane in the reference mixture.
  • B — correct A fuel takes the octane rating equal to the percentage of iso-octane in the reference mixture that has the same critical pressure, so this fuel is rated 90 octane.
  • C — incorrect A rating of 100 corresponds to pure iso-octane and is the ceiling of this rating system, not the value for a 90 percent mixture.

BACKGROUND

Octane ratings and performance numbers describe a fuel's resistance to detonation, and fuels with high critical pressure carry the higher numbers. The reference system blends two hydrocarbons, iso-octane and heptane, in various ratios to give proportional antidetonation properties; the more iso-octane, the greater the resistance. A fuel with the same critical pressure as a given reference mixture is assigned an octane rating equal to the iso-octane percentage of that mixture. Ratings from 80 to 100 can be established this way, with 100 octane, pure iso-octane, the highest possible under the system. Above that, antidetonation is described by referencing mixtures of pure iso-octane with specific quantities of tetraethyl lead, given arbitrary numbers above 100.

STUDY TIP

Read an octane number as the iso-octane share of the reference blend, and read a performance number pair as lean rating over rich rating.

KEY POINTS

  • Octane rating equals the iso-octane percentage of the matching reference mixture.
  • Iso-octane and heptane form the reference blend; more iso-octane means more resistance.
  • 100 octane, pure iso-octane, is the ceiling of the octane rating system.

SOURCES

  • sources/ama-airframe/ch14.txt, "Octane & Performance Number Rating"
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MORE ON AIRCRAFT FUEL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.