Definition
A removable threaded plug fitted to a reservoir, sump, or fluid-holding component that seals the opening through which fluid is added. Common locations include hydraulic reservoirs, oil tanks, brake master cylinders, and shock strut servicing ports.
Plain English
The cap or screw-in plug that closes the hole you pour fluid into when topping something up.
Context Anchor
Seen during preflight checks and maintenance when inspecting or adding oil, fuel, hydraulic fluid, or other serviced contents.
Derivation
“Filler” comes from “fill,” meaning to put material into a space. “Plug” means a stopper used to close an opening. Together, the term points to a stopper for the opening used to fill something.
Why Pilots Care
If a filler plug is loose, missing, or cross-threaded, the system can leak fluid in flight or allow contamination to enter, leading to system failure. Confirming filler plugs are secure after servicing is part of careful preflight and maintenance practice.
Intuition Check
Do not think of this as an electrical plug. In this context, a filler plug is a physical stopper or cap for a fill opening.
Example Sentence 1
After topping up the hydraulic reservoir, the mechanic torqued the filler plug and wiped the area clean to make any future leaks easy to spot.
Example Sentence 2
After adding hydraulic fluid, the technician replaced and torqued the filler plug to prevent leaks.