Definition
A valve in a power brake system that meters hydraulic pressure from the aircraft's main hydraulic system to the brake assemblies in proportion to the force the pilot applies to the brake pedals. It does not transmit pedal force directly to the brakes; instead, it regulates how much hydraulic system pressure is allowed to reach each brake.
Plain English
A valve that controls how hard the brakes are applied on a large aircraft. When the pilot presses the brake pedal, the valve opens just enough to send hydraulic pressure from the aircraft's hydraulic system to the brakes -- the harder the press, the more pressure it lets through.
Context Anchor
Encountered in aircraft hydraulic brake system descriptions, maintenance manuals, and troubleshooting for weak brakes, dragging brakes, or uneven braking.
Derivation
Power means the work is done by the aircraft's hydraulic system rather than by the pilot's leg. Control valve means a valve that regulates flow or pressure rather than just opening or closing. Together: a valve that lets the hydraulic system do the braking work, under the pilot's control.
Why Pilots Care
It delivers consistent, power-assisted braking force so the pilot can stop the aircraft safely without excessive pedal effort.
Analogy
It works like the throttle on a garden hose nozzle. You're not pushing the water yourself -- the city water pressure does that. You're just controlling how much gets through.
Intuition Check
“Power brake” does not mean the engine is used to brake the aircraft. Here, “power” means hydraulic pressure helps create the braking force, while the pilot’s pedal movement controls how much pressure is sent.
Example Sentence 1
When the pilot applied the brakes during landing rollout, the power brake control valve metered hydraulic pressure to each main wheel in proportion to pedal pressure.
Example Sentence 2
During the preflight inspection the mechanic verified that the power brake control valve was free of leaks.