Definition
The maintenance procedure of removing trapped air from an aircraft's hydraulic brake system by flushing fluid through the lines until only solid, bubble-free fluid remains. Air in the lines compresses under pedal pressure, producing a spongy brake feel and reduced braking effectiveness, so it must be purged after any service that opens the system.
Plain English
Pushing fresh hydraulic fluid through the brake lines to force out any air bubbles, so the brakes feel firm and work properly when the pedals are pressed.
Context Anchor
Seen during aircraft maintenance, especially after brake parts are replaced, brake fluid is added, or the brake pedals feel soft.
Derivation
The term comes from the medical sense of 'bleeding' — letting fluid out of a closed system through a small opening. In brakes, a bleeder valve is opened to let fluid (and the unwanted air with it) escape until the system runs clean.
Why Pilots Care
Air trapped in brake lines produces a soft pedal and weaker braking force, increasing landing rollout distance and reducing safety margins.
Grounding Statement
Brake fluid passes pressure well, but trapped air squeezes down like a cushion, so the brake pedal may move without giving full braking force.
Intuition Check
Bleeding the brakes does not mean draining the brakes empty. It means removing trapped air while keeping the system properly filled with brake fluid.
Example Sentence 1
After replacing the left brake caliper, the technician bled the brakes until the pedal felt firm and no air bubbles appeared in the discharge line.
Example Sentence 2
A spongy brake pedal during the preflight check led the owner to schedule bleeding of brakes before the next flight.