If a pilot reports that a brake is spongy, what is the most likely cause?

When a pilot describes a brake as spongy, the most likely cause is air in the hydraulic system. This condition occurs because hydraulic systems rely on incompressible fluids to transmit force from the brake pedal to the braking mechanism. When air enters the system, it compresses under pressure, leading to a soft or spongy feel when the brake pedal is applied.

In practical terms, this phenomenon is a result of the hydraulic system's inability to effectively transmit the force needed to engage the brakes firmly. This can happen if there's a leak in the system or improper bleeding of the brakes, allowing air to infiltrate the hydraulic lines.

While low brake fluid can contribute to issues with braking performance, it typically results in a firmer pedal during use rather than a spongy feel. Faulty brake pads mainly affect the friction and engagement of the brakes but do not directly cause a spongy pedal sensation. Worn-out brake lines can lead to other issues such as brake fluid leaks or reduced structural integrity but are not primarily responsible for the spongy sensation experienced in the pedal.

Thus, the presence of air in the hydraulic system is the most common and direct cause of a spongy brake feel that a pilot would report.