FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician is preparing to hand-turn the propeller of a large radial engine that has been sitting for several days. What safety precaution addresses the risk of hydraulic lock?
- ARunning the engine at idle for one minute prior to full power application
- BPulling the propeller through several blades by hand before starting to check for excessive resistance
- CChecking magneto timing before every engine start
- DDraining the carburetor bowl before every start regardless of engine type
Show answer & explanationAnswer & explanation
Correct answer: B. Pulling the propeller through several blades by hand before starting to check for excessive resistance
Hydraulic lock occurs when oil or fuel seeps into the lower cylinders of a radial engine while sitting, and turning the engine over could bend a connecting rod. Pulling the prop through by hand before starting allows detection of abnormal resistance caused by trapped liquid before using the starter.
Why the other options are wrong
- A. Idle running after start does not prevent damage that occurs during initial rotation.
- C. Magneto timing is unrelated to detecting hydraulic lock.
- D. Draining the carburetor bowl does not address liquid trapped in lower cylinders.
Hydraulic Lock
A condition in radial engines where oil or fuel accumulates in the lower cylinders during shutdown, which can bend a connecting rod if the engine is rotated with the starter before the liquid is cleared.
- Most common in radial engines due to cylinder orientation
- Hand-turning the prop through several revolutions checks for resistance
- If resistance is felt, spark plugs should be removed to drain liquid
Memory trick: Turn it by hand before you demand it start