An aircraft experiences a gradual, progressive loss of manifold pressure while flying through light snow with an outside air temperature of 28°F. A mechanic later determines the cause was impact ice. Which of the following best describes impact icing?
- AIce that forms on the propeller blades due to accumulation of supercooled liquid water in flight
- BIce that forms inside fuel tank vent lines, blocking pressure equalization
- CIce that forms on the induction system air filter, screen, or scoop when visible moisture strikes surfaces at or below freezing, restricting airflow
- DIce that forms in the carburetor venturi due to the cooling effect of fuel vaporization and pressure drop
Show answer & explanationAnswer & explanation
Correct answer: C. Ice that forms on the induction system air filter, screen, or scoop when visible moisture strikes surfaces at or below freezing, restricting airflow
Impact icing occurs when visible moisture, such as snow, sleet, or supercooled droplets, strikes induction system components (air filter, screen, or scoop) that are at or below freezing, causing ice to build up and restrict airflow into the engine. This differs from fuel evaporation (carburetor) icing, which is caused by the vaporization and pressure drop in the venturi.
Why the other options are wrong
- A. Propeller icing is an airframe/propeller concern, not induction system impact icing.
- B. Fuel vent line icing is a separate fuel system issue unrelated to induction airflow restriction.
- D. This describes fuel evaporation (carburetor) icing, a different mechanism from impact icing.
Impact Icing
Ice that forms on induction system components (air filter, screen, or scoop) when visible moisture strikes surfaces at or below freezing temperature, restricting engine air intake.
- Caused by visible moisture like snow or freezing rain
- Occurs on filters/screens/scoops, not the venturi
- Alternate air source can bypass a blocked filter
Memory trick: Impact ice = snowflakes 'crashing and freezing' onto the filter screen.