Definition
A condition in a radial or inverted reciprocating engine in which oil or fuel has drained by gravity into the lower cylinders and collected in the combustion chamber or intake port. Because liquid cannot be compressed, the trapped fluid blocks the piston from completing its travel and can bend or break the connecting rod, crack the cylinder, or damage the crankshaft if the engine is started or turned through with the fluid still present.
Plain English
Liquid has pooled inside the bottom cylinders of certain engines, and because liquid will not squeeze down like air, the piston cannot push through it. Trying to start or turn the engine in that state can break engine parts.
Context Anchor
Most often discussed during engine starting and maintenance checks, especially on engines where oil can drain into lower cylinders while the aircraft is parked.
Derivation
Hydraulic comes from the Greek hydor, meaning water, and refers to anything involving liquid. Lock here means jammed or held in place. Together it describes the engine being mechanically held still by a trapped liquid.
Why Pilots Care
Attempting to start an engine with hydraulic lock can bend connecting rods or cause other serious damage.
Analogy
It is like trying to push a sealed syringe when the end is blocked and the syringe is full of liquid. The liquid has nowhere to go, so forcing it can break something.
Intuition Check
Hydraulic lock is not a normal locked position or a problem only in the aircraft hydraulic system. Here, it means trapped liquid inside an engine cylinder is stopping engine movement.
Example Sentence 1
Before the first start of the day on the radial, the mechanic pulled the propeller through several blades by hand to check for hydraulic lock.
Example Sentence 2
If hydraulic lock is detected, the lower spark plugs must be removed to drain the fluid before attempting another start.