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Cause of throttle ice at part throttle

Aviation Mechanic Powerplant (AMP) practice question 5 of 13 on Reciprocating Engine Induction and Cooling Systems. ACS element AM.III.J.K2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONDuring part-throttle operation, what condition at the partly closed throttle valve allows throttle ice to form?

  1. AA high-pressure area created by ram air at the intake
  2. BHeat radiated from the exhaust manifold shroud duct
  3. CA low-pressure area that lowers the air temperature
Show the answer and explanation

Correct answer: C. A low-pressure area that lowers the air temperature

  • A — incorrect Ram air at the intake does not produce the temperature drop at the throttle; the restriction at the throttle creates low pressure, not high pressure.
  • B — incorrect Heat from around the exhaust manifold is what the carburetor heat system uses to remove ice, so it cannot be the cause of ice forming.
  • C — correct A partly closed throttle establishes a much higher than normal air velocity past the throttle, producing an extremely low-pressure area that lowers the temperature of the surrounding air below freezing.

BACKGROUND

Induction system icing is generally classified as impact ice, fuel evaporation ice, or throttle ice. Part-throttle operation can lead to icing in the throttle area because the partly closed throttle limits the air available to the engine. Air velocity past the throttle rises well above normal and an extremely low-pressure area results, which lowers the temperature of the air around the throttle valve. If that temperature falls below freezing and moisture is present, ice forms on the throttle and nearby units and restricts airflow. On engines with controllable-pitch propellers, using a higher than normal brake mean effective pressure at low power decreases the icing tendency because the larger throttle opening removes part of the obstruction.

STUDY TIP

Tie throttle ice to the restriction itself: more velocity past the plate means lower pressure and lower temperature. Remember that throttle ice or any ice that reduces manifold pressure is best removed with full carburetor heat.

KEY POINTS

  • A partly closed throttle raises air velocity and creates a very low-pressure, low-temperature area.
  • With moisture present, ice forms on the throttle valve and restricts airflow.
  • Higher brake mean effective pressure at low power reduces the tendency toward throttle ice.

SOURCES

  • amp-powerplant/ch03.txt "Induction System Icing"
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MORE ON RECIPROCATING ENGINE INDUCTION AND COOLING 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.