Plain English Meaning

A tiny, carefully sized hole that lets air in or out slowly on purpose. Because the hole is small, pressure inside the instrument lags behind pressure outside, and the instrument uses that lag to show what is happening.

Full Definition

A small, precisely sized opening inside a pneumatic flight instrument (such as a vertical speed indicator) that allows air to flow into or out of a sealed chamber at a controlled, predictable rate. The deliberate restriction of flow creates a measurable pressure difference between the chamber and the outside static pressure, and that difference is what drives the instrument's indication.

Context Anchor

Seen in discussions of vacuum-driven flight instruments, especially where airflow is used to or stabilize an instrument.