SkyDrill

Core airstream share of high-bypass turbofan thrust

Aviation Mechanic Powerplant (AMP) practice question 6 of 9 on Turbine Engine Air Systems. ACS element AM.III.K.K2 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONOn a high-bypass turbofan, the secondary airstream accelerated by the outer part of the fan blades and ducted overboard produces 80 percent of the thrust. Approximately what share of the engine's thrust comes from the primary (core) airstream?

  1. AAbout 20 percent
  2. BAbout 50 percent, an even split
  3. CAbout 80 percent, same as fan air
Show the answer and explanation

Correct answer: A. About 20 percent

  • A — correct If the ducted fan airstream produces 80 percent of the thrust, the remaining core flow accounts for roughly the other 20 percent.
  • B — incorrect An even split would contradict the stated 80 percent contribution of the fan airstream.
  • C — incorrect The 80 percent figure belongs to the fan (secondary) airstream, not to the core flow.

BACKGROUND

In a high-bypass turbofan the fan is at the forward end of the compressor, inside an inlet cowl that is sometimes bolted to the front of the engine. Air accelerated by the outer part of the fan blades becomes the secondary airstream, or fan flow, which is ducted overboard without passing through the main engine. Air passing through the inner part of the fan blades becomes the primary airstream, or core flow, through the engine itself. The secondary fan flow produces 80 percent of the thrust in high-bypass engines. Fan discharge air may leave through short dual exhaust nozzles right behind the fan, or be carried all the way aft in closed ducts to a mixed exhaust nozzle in what is called a ducted fan.

STUDY TIP

Fix the 80/20 split in mind and be ready to state it from either side; also be able to say which airstream is ducted around the core and which passes through it.

KEY POINTS

  • Fan flow (secondary air) is ducted overboard and makes 80 percent of high-bypass thrust.
  • Core flow (primary air) passes through the engine and makes roughly the remaining 20 percent.
  • Fan discharge exits either through short ducts behind the fan or through closed ducts to a mixed nozzle.

SOURCES

  • amp-powerplant ch03 "Turbofan Engine Inlet Sections"
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON TURBINE ENGINE AIR 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.