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Calculating turbofan bypass ratio

Aviation Mechanic Powerplant (AMP) practice question 10 of 13 on Turbine Engines. ACS element AM.III.B.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 turbofan engine's fan delivers 108 pounds per second of bypass airflow, while 18 pounds per second passes through the engine core. What is the bypass ratio?

  1. A7:1
  2. B6:1
  3. C0.17:1
Show the answer and explanation

Correct answer: B. 6:1

  • A — incorrect This comes from dividing total airflow of 126 pounds per second by the core flow; the ratio uses bypass flow only, not total flow.
  • B — correct Bypass ratio is the fan bypass flow divided by the core flow: 108 divided by 18 equals 6.
  • C — incorrect This inverts the ratio by dividing core flow by bypass flow, which would describe a fan smaller than the core.

BACKGROUND

A turbofan engine turns a large fan at the front that produces about 80 percent of the engine's thrust. Air driven by the fan generally does not pass through the internal working core of the engine. The amount of air flow in pounds per second from the fan bypass compared to the core flow of the engine is the bypass ratio. The handbook's worked example uses 100 pounds per second of fan flow against 20 pounds per second of core flow, which gives a 5:1 bypass ratio. Turbofans are described as low bypass or high bypass depending on how much air is bypassed around the core.

STUDY TIP

Always put bypass flow on top and core flow on the bottom; an answer smaller than 1 means you flipped the fraction. Do not add the two flows together before dividing.

KEY POINTS

  • Bypass ratio equals fan bypass flow divided by core flow, both in pounds per second.
  • 108 lb/sec bypass over 18 lb/sec core gives a 6:1 bypass ratio.
  • Turbofan bypass ratio determines whether an engine is called low bypass or high bypass.

SOURCES

  • FAA-H-8083-32B ch01 "Gas Turbine Engines, Types & Construction"
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MORE ON TURBINE ENGINES

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.