Definition
A small adjustable valve with a tapered, needle-shaped point that fits into a matching seat, used to precisely meter the rate of fluid or air flow through a passage. In gyroscopic instrument systems, a needle valve regulates the volume of suction or air supplied to the gyro instruments.
Plain English
A fine-tuning valve. Turning it in or out lets a little more or a little less air or fluid through, so you can set the flow rate exactly where you want it.
Context Anchor
Seen in diagrams and explanations of air-driven gyroscopic instruments, where controlled airflow is part of how the instrument operates.
Derivation
Named for the slim, needle-shaped tip on the end of the valve stem. The narrow point lets very small movements produce very small changes in flow, which is what makes the adjustment so precise.
Why Pilots Care
Correct needle-valve adjustment keeps gyro rotors at the designed RPM so attitude and heading indications remain accurate and responsive.
Analogy
Like the fine adjustment knob on a kitchen tap -- a small turn changes the flow just a little, so you can dial it in exactly.
Intuition Check
Do not think of “needle” here as an indicator needle on the instrument face. In “needle valve,” the needle is the tapered part inside the valve that controls flow.
Example Sentence 1
The mechanic adjusted the needle valve in the suction line to bring the gyro pressure back into the green arc.
Example Sentence 2
During preflight, a partially blocked needle valve was found to be starving the heading indicator of the required airflow.