FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
A pilot reports that during a cold-weather start, the engine required excessive cranking and then ran very rough for several minutes after starting, eventually smoothing out. Which of the following is the most likely contributing factor to these symptoms?
- AReduced oil viscosity due to high temperatures.
- BAn overly rich mixture setting during starting.
- CImproper operation of the carburetor accelerating pump.
- DInsufficient atomization of fuel due to low air temperature.
Show answer & explanationAnswer & explanation
Correct answer: D. Insufficient atomization of fuel due to low air temperature.
In cold weather, low air temperatures reduce fuel vaporization and atomization. This results in a less combustible air-fuel mixture entering the cylinders, making the engine harder to start and causing it to run rough until sufficient engine heat is generated to improve fuel vaporization.
Why the other options are wrong
- A. Reduced oil viscosity occurs with high temperatures, not low, and would not directly cause rough running after starting, though it could affect oil pressure.
- B. An overly rich mixture would typically cause excessive black smoke, spark plug fouling, and potentially difficulty starting, but less likely to smooth out as described without adjustment.
- C. An accelerating pump primarily provides an enriched mixture during rapid throttle changes; it's not the primary factor for overall cold starting difficulty and prolonged rough running.
Cold Weather Starting Issues
Challenges in starting and initial operation of reciprocating engines in low temperatures, primarily due to poor fuel atomization.
- Low temp reduces fuel vaporization.
- Results in lean, non-combustible mixture.
- Causes hard start, rough running until warm.
Memory trick: Cold air makes fuel lazy; it won't vaporize, leading to a weak, rough start.