Definition
A small, deliberately narrowed opening placed in a fluid or air line to limit the rate of flow through it. In aircraft systems, restricted orifices are used to slow the movement of hydraulic fluid, pneumatic air, or oil so that components extend, retract, or actuate at a controlled, predictable speed.
Plain English
A small hole in a line, made on purpose to slow down how fast fluid or air can pass through it. It acts like a built-in throttle, keeping things from moving too quickly.
Context Anchor
Seen in the principle of operation for the vertical speed indicator in the pitot-static instrument system.
Derivation
‘Restricted’ comes from the Latin restringere, meaning ‘to hold back.’ ‘Orifice’ comes from the Latin orificium, meaning ‘an opening.’ Together the term literally describes ‘a held-back opening’ — a passage made smaller on purpose to limit what passes through it.
Why Pilots Care
Maintains correct fuel or air metering to prevent engine flooding, starvation, or pressure imbalances.
Analogy
Think of pinching a garden hose. The water still flows, but at a steady, slower rate. A restricted orifice does the same thing inside a line — permanently and by design.
Grounding Statement
In a climb or descent, the restricted orifice creates a short pressure delay inside the instrument so the instrument can sense vertical movement.
Intuition Check
Restricted does not mean closed or blocked here. It means deliberately narrowed to slow the pressure change at a known rate.
Example Sentence 1
A restricted orifice in the landing gear line slows the gear as it extends, preventing a hard mechanical stop.
Example Sentence 2
A blocked restricted orifice in the fuel vent line can cause tank pressure problems during flight.