What is meant by liquid lock in the cylinder of an aircraft engine, and how is it cleared?

• A. A fuel line blockage that requires purging.
• B. Water in the cylinder; drain the coolant.
• C. Oil accumulates in the cylinders below the center line of the engine and prevents the piston from moving to the top of its stroke. To clear liquid lock, remove one of the spark plugs and turn the engine crank shaft until all of the oil is forced out of the cylinder.
• D. Cooling water boils in the cylinder and must be vented.

Answer: (C)

Liquid lock happens when a noncompressible liquid, most often oil that has leaked into a cylinder, pools below the piston. Since liquids don’t compress like air, this filled cylinder can’t allow the piston to move to the top of its stroke, making the engine feel stuck or hard to turn.

To clear it, remove a spark plug from the affected cylinder and crank the engine so the liquid is expelled through the spark plug opening. Once the liquid is pumped out and air can again fill the cylinder, reinstall the spark plug and continue operation. Afterward, check for the source of the oil intrusion and fix any leaks to prevent recurrence.