FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
A pilot flying a fixed-pitch propeller airplane through visible moisture near freezing temperatures notices a gradual, unexplained loss of RPM and manifold pressure, followed by engine roughness. What is the most likely cause?
- ACarburetor icing restricting the airflow through the venturi
- BVapor lock in the fuel lines from excessive fuel pump pressure
- CDetonation caused by using too low an octane fuel grade
- DSpark plug fouling from an excessively rich mixture at altitude
Show answer & explanationAnswer & explanation
Correct answer: A. Carburetor icing restricting the airflow through the venturi
Carburetor ice forms when moist air is cooled by the venturi effect and fuel vaporization, even at temperatures well above freezing, restricting airflow and causing a gradual loss of RPM (in fixed-pitch props) or manifold pressure, along with engine roughness as ice breaks off unevenly. Applying carburetor heat is the correct corrective action.
Why the other options are wrong
- B. Vapor lock is caused by fuel boiling from heat, not associated with cold, moist conditions.
- C. Detonation typically causes rough running from high cylinder temperatures/pressures, not gradual RPM loss in icing conditions.
- D. Spark plug fouling causes roughness but not the characteristic gradual power loss tied to moisture and cold temperatures.
Carburetor Icing
Ice forms in the carburetor venturi due to the cooling effect of fuel vaporization and pressure drop, even in temperatures well above freezing with high humidity.
- Can occur at temperatures up to 70°F with high humidity
- Symptoms: gradual RPM or manifold pressure loss, roughness
- Corrective action: apply full carburetor heat
Memory trick: Carb ice sneaks in on humid, cool days, not just freezing ones