Definition
A hydraulic cylinder operated by foot or hand pressure that generates the fluid pressure used to actuate the brakes or other hydraulic systems on an aircraft. When the pilot presses the brake pedal, a piston inside the master cylinder pushes hydraulic fluid through lines to the brake assemblies at the wheels, which then clamp down to slow or stop the aircraft.
Plain English
The pump-like unit at the brake pedal that turns your foot pressure into the fluid pressure that actually squeezes the brakes at the wheels.
Context Anchor
Seen in aircraft brake system descriptions, maintenance write-ups, and brake checks before taxi.
Derivation
‘Master’ here means the controlling or originating unit — the one that drives the others. The master cylinder is the source of the pressure; the brake cylinders at the wheels are the followers that respond to it.
Why Pilots Care
Reliable operation of the master cylinder is essential for controlled stopping during landing rollout and ground maneuvering.
Analogy
It works like the brake master cylinder in a car, where pushing the pedal moves fluid to apply the brakes at the wheels.
Intuition Check
Do not read “master” as meaning the best or most skilled cylinder. In this term, it means the main control cylinder that creates brake pressure.
Example Sentence 1
During the preflight inspection, the mechanic found a small fluid leak at the master cylinder and grounded the aircraft until it was repaired.
Example Sentence 2
During the preflight inspection the student checked the fluid level in the master cylinder reservoir.