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Clearing a hydraulic lock

Aviation Mechanic Powerplant (AMP) practice question 7 of 13 on Reciprocating Engines. ACS element AM.III.A.R1 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONAfter removing a spark plug from a lower cylinder to clear a hydraulic lock, what must the technician do next?

  1. APull the propeller through in the direction of rotation
  2. BPull the propeller backward, opposite normal rotation
  3. CCrank the engine with the starter until oil pressure rises
Show the answer and explanation

Correct answer: A. Pull the propeller through in the direction of rotation

  • A — correct Pulling the propeller through in the normal direction of rotation lets the piston expel any liquid present through the open spark plug hole.
  • B — incorrect Turning the propeller opposite to normal rotation tends to inject liquid from the cylinder into the intake pipe and may cause another lock on the next start.
  • C — incorrect Cranking until oil pressure appears describes pre-oiling after overhaul or major maintenance, and cranking against a lock risks bending or breaking a connecting rod.

BACKGROUND

Whenever a radial engine sits shut down for more than a few minutes, oil or fuel may drain into the combustion chambers of the lower cylinders or collect in the lower intake pipes. As the piston approaches top center of the compression stroke with both valves closed, that incompressible liquid stops piston movement, and continued crankshaft rotation may blow out the cylinder or, more likely, bend or break a connecting rod. To eliminate the lock, remove either the front or rear spark plug of the lower cylinders and pull the propeller through in the direction of rotation so the piston expels the liquid. A lock caused by over-priming before an initial start is cleared the same way, rotating the crankshaft through two turns.

STUDY TIP

Never turn a propeller opposite to normal rotation to clear a lock; that pushes the liquid back into the intake pipe and sets up another lock.

KEY POINTS

  • A hydraulic lock forms when liquid collects in lower cylinders or intake pipes.
  • Clear it by removing a spark plug and turning the propeller in the normal direction.
  • Turning the propeller backward can drive the liquid into the intake pipe.

SOURCES

  • FAA-H-8083-32B ch10 "Hydraulic Lock"
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MORE ON RECIPROCATING ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.