Definition
A threaded plug containing a desiccant, screwed into the spark plug holes of a piston engine that will be out of service for an extended period. The desiccant absorbs moisture from the air inside the cylinders, protecting internal steel surfaces from corrosion. Most dehydrator plugs include a colored indicator that changes shade when the desiccant becomes saturated and needs replacement.
Plain English
A small device that takes the place of a spark plug while an engine is parked for a long time. It soaks up moisture inside the cylinders so the metal parts don't rust. A color window on the plug shows when it has soaked up all it can and needs to be swapped out.
Context Anchor
Seen in aircraft engine storage, preservation, and maintenance procedures, especially when spark plugs are removed and the engine needs protection from moisture.
Derivation
Dehydrator comes from Latin de- (away) and Greek hydor (water) -- literally 'something that takes water away.' That fits its job: pulling moisture out of the cylinder air to prevent rust.
Why Pilots Care
Prevents internal cylinder rust during periods of inactivity, protecting engine longevity and ensuring safe operation upon return to service.
Analogy
It is similar to putting a moisture-absorbing packet in a sealed container, except the container is part of the engine and the plug also closes the opening.
Intuition Check
A dehydrator plug is not a spark plug and it is not used to run the engine. It is a temporary storage item used to help keep the engine interior dry.
Example Sentence 1
Before putting the aircraft into winter storage, the mechanic removed the spark plugs and installed dehydrator plugs in each cylinder.
Example Sentence 2
After three months of storage the blue desiccant in the dehydrator plug had turned pink, indicating moisture had been absorbed.