Definition
A service valve installed on a pressurized component, such as an oleo strut, accumulator, or pneumatic system, that allows compressed air or nitrogen, and sometimes hydraulic fluid, to be added during servicing. It seals against system pressure when not in use and opens when a charging hose or servicing tool is connected.
Plain English
The fitting a mechanic connects a charging hose to when topping up a strut, accumulator, or pneumatic system with air, nitrogen, or fluid. It stays closed and pressure-tight on its own and opens only when something is screwed onto it.
Context Anchor
Seen during aircraft servicing and maintenance, especially when checking or refilling an installed oxygen system.
Derivation
From 'fill,' meaning to put something into a container, and 'valve,' a device that controls the flow of fluid or gas. The name describes its job: the valve through which the system is filled.
Why Pilots Care
Pilots and mechanics inspect filler valves for leaks, missing caps, and proper servicing. A leaking or improperly serviced filler valve can lead to a flat strut, low accumulator pressure, or a pneumatic system that won't hold charge -- any of which can ground the aircraft.
Analogy
Similar in principle to the valve stem on a car or bicycle tire: it stays sealed under pressure and only opens when you connect the right tool to add air.
Intuition Check
A filler valve is not just a cap or an open hole. It is a controlled valve that lets servicing flow in and then seals the system afterward.
Example Sentence 1
After noticing the nose strut was low, the mechanic removed the protective cap from the filler valve and connected a nitrogen servicing cart.
Example Sentence 2
During the preflight, the pilot checked that the filler valve cap was secure and showed no signs of leakage.