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Clearing a hydraulic lock

Aviation Mechanic General (AMG) practice question 13 of 26 on Ground Operations and Servicing. ACS element AM.I.F.R8 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA radial engine has been shut down for more than 30 minutes. While pulling the propeller through by hand with the ignition off, the mechanic feels abnormal resistance and the propeller stops abruptly. What is the correct action?

  1. ARemove a lower-cylinder spark plug and pull the prop through.
  2. BPull the propeller through in the opposite direction to clear it.
  3. CUse the starter to rotate the engine through the resistance.
Show the answer and explanation

Correct answer: A. Remove a lower-cylinder spark plug and pull the prop through.

  • A — correct The symptoms indicate a hydraulic lock; the handbook says to remove either the front or rear spark plug from the lower cylinders and pull the propeller through to clear the liquid.
  • B — incorrect Reverse rotation pushes the liquid into the intake pipe, from where it is drawn back into the cylinder and can cause a lock on the next start.
  • C — incorrect Force must never be used when a lock is detected; enough force on the crankshaft can bend or break a connecting rod.

BACKGROUND

Before starting a radial engine that has been shut down for more than 30 minutes, the ignition switch is checked off and the propeller is turned three or four complete revolutions by hand to detect a hydraulic lock. Liquid in a cylinder is indicated by the abnormal effort required to rotate the propeller or by the propeller stopping abruptly. Force is never used when a lock is detected, because the crankshaft can exert enough force to bend or break a connecting rod. To eliminate the lock, either the front or rear spark plug is removed from the lower cylinders and the propeller is pulled through. The lock is never cleared by pulling the propeller backward, since this injects the liquid into the intake pipe and it is drawn back into the cylinder on the next start.

STUDY TIP

Treat a hydraulic lock as a three-part rule: never force it, never reverse it, and clear it by pulling a lower plug.

KEY POINTS

  • Check for hydraulic lock by hand-turning a radial engine that has sat more than 30 minutes.
  • Never force the propeller when a lock is felt; a connecting rod can bend or break.
  • Clear the lock by removing a lower-cylinder spark plug, never by reverse rotation.

SOURCES

  • FAA-H-8083-30B ch01 "Reciprocating Engines" (hydraulic lock)
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.