A pilot reports that a reciprocating engine runs rough after extended ground operations at low power but smooths out after a brief application of high power. A technician suspects spark plug fouling. What is the most likely cause and appropriate corrective action?
- AWater contamination in the fuel; correct by draining the fuel sumps before the next flight
- BCarbon fouling from rich mixture at high power; correct by leaning the mixture during cruise flight
- COil fouling from worn piston rings; correct by replacing the magneto immediately
- DLead fouling from prolonged low-power ground operation; correct by leaning the mixture during ground operations and periodically clearing the engine with a brief power application
Show answer & explanationAnswer & explanation
Correct answer: D. Lead fouling from prolonged low-power ground operation; correct by leaning the mixture during ground operations and periodically clearing the engine with a brief power application
Extended low-power ground operation with a rich idle mixture allows lead bromide deposits (from the tetraethyl lead in avgas) to build up on spark plug electrodes, causing intermittent misfire; the standard correction is to lean the mixture during ground operations per the AFM and periodically run the engine up briefly to help clear the deposits.
Why the other options are wrong
- A. Water contamination would typically cause rough running independent of power setting duration, not this pattern.
- B. Carbon fouling is more associated with rich mixtures and low compression, not this specific lead-fouling scenario.
- C. Oil fouling relates to worn rings/guides, not the described low-power ground symptom, and doesn't require magneto replacement.
Lead Fouling
Deposits of lead bromide on spark plug electrodes caused by extended low-power/idle operation with leaded avgas, leading to intermittent misfire that often clears with a brief high-power run.
- Most common during prolonged ground idle or descent at low power
- Leaning the mixture during ground ops reduces lead deposit formation
- A brief high-power 'clearing' runup can help burn off deposits
Memory trick: Lead fouling = 'idle too long, lead builds strong' — lean it out to keep it clean