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.
Plain English
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.
Context Anchor
Seen in discussions of vacuum-driven flight instruments, especially where airflow is used to run or stabilize an instrument.
Derivation
‘Metered’ comes from the Latin ‘metrum’ meaning ‘measure’ — a metered flow is one that is measured or controlled, not free. Paired with ‘leak,’ it describes a leak that is intentional and calibrated, not accidental.
Why Pilots Care
Prevents rough idling or engine stumble by providing the exact fuel needed when airflow through the venturi is too low for normal metering.
Analogy
It is like a small pinhole made on purpose to let a precise amount of air pass through, rather than a crack that appeared by accident.
Intuition Check
Do not read leak as a failure here. A metered leak is intentional and controlled.
Example Sentence 1
The vertical speed indicator relies on a metered leak to slowly equalize pressure inside its case with the surrounding static pressure.
Example Sentence 2
At low throttle settings the metered leak keeps the mixture from becoming too lean.