Definition
A manually adjustable valve, fitted with a tapered (needle-shaped) plunger, used in a venturi-driven pneumatic instrument system to limit and stabilize the suction applied to gyroscopic instruments. By restricting airflow through a small, precisely sized opening, it holds the vacuum at the value required for correct gyro operation, regardless of small variations in venturi suction.
Plain English
A small, finely adjustable valve that controls how much suction reaches the gyro instruments, keeping it at the right level so the instruments work properly.
Context Anchor
Seen in discussions of venturi tube systems and vacuum-driven flight instruments, where airflow must be controlled so the instruments operate correctly.
Derivation
Called a needle valve because the moving part inside is a slim, tapered rod shaped like a needle. Turning it in or out changes the size of the opening very gradually, which is why it can set suction precisely rather than just on or off.
Why Pilots Care
Correct setting protects delicate gyros from over-vacuum damage while still supplying enough suction for accurate attitude and heading indications.
Analogy
It works like turning a faucet very slightly to control water flow, except this valve controls air flow and pressure instead of water.
Intuition Check
Pressure-reducing does not mean the valve destroys pressure or creates suction by itself. It means the valve restricts airflow in a controlled way so the pressure after the valve is lower than before it.
Example Sentence 1
The pressure-reducing needle valve was adjusted on the ground to set the gyro suction to the value listed in the aircraft manual.
Example Sentence 2
During the preflight check the instructor verified that the pressure-reducing needle valve was set to prevent excessive vacuum on the attitude indicator.