Definition
Removable plugs fitted inside the hollow crankshaft of a reciprocating aircraft engine that collect sludge — heavy contaminants, carbon, and combustion by-products — thrown out of the oil by centrifugal force as the crankshaft spins. They are removed and cleaned during engine overhaul to prevent the trapped sludge from blocking oil passages that supply the connecting rod bearings.
Plain English
Small caps inside the hollow parts of the crankshaft that catch the gunk spun out of the engine oil so it doesn't clog the oil passages feeding the bearings. They have to be taken out and cleaned when the engine is overhauled.
Context Anchor
Seen in aircraft engine maintenance, especially when inspecting or cleaning crankshafts and internal oil passages during engine repair or overhaul.
Derivation
Sludge' comes from an old English word for thick mud or slush, used here for the heavy residue that builds up in engine oil. 'Plug' is a stopper that fills a hole. Together: stoppers that catch the muddy residue inside the crankshaft.
Why Pilots Care
Blocked oil passages from uncleared sludge can lead to inadequate lubrication and premature engine failure.
Analogy
Think of a drain cleanout cap in a plumbing line: the cap closes the opening, but it also gives maintenance personnel a place to access and clean out buildup.
Intuition Check
Do not read “sludge plugs” as plugs made of sludge or as spark plugs. They are metal closing plugs associated with areas where dirty oil residue can collect.
Example Sentence 1
During the overhaul, the technician removed the sludge plugs from the crankshaft and cleaned out the accumulated deposits before reinstalling them.
Example Sentence 2
Failure to clean behind the sludge plugs can result in oil starvation to the bearings.