Definition
A condition in a hydraulic brake system in which the brake pedal feels soft, springy, or compressible when pressed, rather than firm. It is caused by air trapped in the hydraulic brake lines. Because air compresses and hydraulic fluid does not, the trapped air absorbs pedal travel before full braking pressure reaches the wheel brake assemblies, reducing braking effectiveness. The condition is corrected by bleeding the brakes to remove the air.
Plain English
Brakes that feel soft or squishy when you press the pedal instead of firm. This happens because air has gotten into the brake fluid lines, and the air squashes down before the brakes can fully grab.
Context Anchor
Seen during aircraft brake checks, taxi operations, and maintenance troubleshooting of hydraulic brake systems.
Derivation
From 'sponge' — something soft that compresses when squeezed. The pedal feels like pressing on a sponge rather than something solid, because trapped air behaves the same way under pressure.
Why Pilots Care
Spongy brakes reduce stopping power and may fail to hold the aircraft on the ramp or during landing rollout.
Analogy
Like stepping on a wet sponge that squishes instead of a solid board.
Intuition Check
Spongy does not mean the brake parts are made of soft material. Here it means the brake control feels soft because pressure is not being transferred firmly through the brake system.
Example Sentence 1
During the preflight brake check, the pilot noted spongy brakes and wrote up the aircraft for maintenance before flight.
Example Sentence 2
After the mechanic bled the lines, the spongy brakes firmed up and gave reliable stopping.