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Turbine engine inlet duct acting as a diffuser

Aviation Mechanic Powerplant (AMP) practice question 1 of 9 on Turbine Engine Air Systems. ACS element AM.III.K.K6 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONAir enters a turbine engine inlet duct that is built with a divergent shape. What happens to that air as it travels aft toward the compressor?

  1. AVelocity drops and static pressure rises
  2. BVelocity rises and the static pressure drops
  3. CBoth the velocity and the pressure rise
Show the answer and explanation

Correct answer: A. Velocity drops and static pressure rises

  • A — correct A divergent inlet duct functions as a diffuser, decreasing the velocity of the air and increasing its static pressure before the compressor.
  • B — incorrect That is the effect of a convergent, nozzle-shaped passage, which is the opposite of a subsonic diffuser.
  • C — incorrect A diffuser trades velocity for static pressure; it cannot raise both at the same time.

BACKGROUND

The engine inlet must deliver a relatively distortion-free flow of air, in the required quantity, to the compressor inlet, and many engines add inlet guide vanes to straighten that flow. Uniform, steady airflow is needed to avoid compressor stall and excessive internal temperatures in the turbine section. To accomplish this, inlet ducts are designed to function as diffusers with a divergent shape, decreasing the velocity and increasing the static pressure of the air passing through them. Airflow through the engine must stay below Mach 1 at all times, so the duct must slow the incoming air before it reaches the compressor. The inlet also recovers much of the total pressure of the free airstream as the aircraft speeds up, an effect called ram recovery or total pressure recovery.

STUDY TIP

Tie the shape to the effect: divergent equals diffuser equals slower air at higher static pressure; the reverse shape does the reverse.

KEY POINTS

  • Inlet ducts are divergent so they act as diffusers ahead of the compressor.
  • A diffuser decreases air velocity and increases static pressure.
  • Steady, distortion-free inlet air helps prevent compressor stall and high turbine temperatures.

SOURCES

  • amp-powerplant ch03 "Turbine Engine Inlet Systems"
  • amp-powerplant ch03 "Variable-Geometry Duct"
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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.