Definition
An engine-driven vacuum pump that uses engine oil for internal lubrication and sealing. Oil is drawn into the pump, lubricates the steel vanes and rotor, helps seal the pumping chambers, and is then discharged with the exhaust air through an oil separator before being returned to the engine sump.
Plain English
A vacuum pump that runs with engine oil flowing through it to keep the moving parts lubricated and to seal the chamber. Because oil passes through it, the air it discharges has to go through a separator before the oil is sent back to the engine.
Context Anchor
Seen in aircraft instrument system maintenance, especially when comparing oil-lubricated vacuum pumps with dry vacuum pumps.
Derivation
Called 'wet' because engine oil is continuously present inside the pump during operation, in contrast to a 'dry-type' pump which uses self-lubricating carbon vanes and runs without oil.
Why Pilots Care
Failure or oil loss can disable attitude and heading indicators, creating a serious loss of reference in instrument conditions.
Intuition Check
“Wet” does not mean the pump uses water. In this term, “wet” means oil-lubricated.
Example Sentence 1
The technician traced the oil residue on the firewall to the discharge line of the wet-type vacuum pump and inspected the oil separator for blockage.
Example Sentence 2
A failing wet-type vacuum pump can cause the attitude indicator to become sluggish.