What is 'brake fade' and when does it most commonly occur?

BRAKE FADE in air-brake vehicles primarily results from thermal degradation:

Cause: Continuous service brake application on a downgrade generates heat faster than drums/rotors can dissipate it; when drum/rotor temperature exceeds the lining's thermal rating, the lining material partially volatilizes (gasifies), creating a gas layer between drum and lining that dramatically reduces friction.

Result: Brake pedal pressure that previously produced strong deceleration now produces minimal deceleration.

Prevention: NEVER ride service brakes on downgrades; use a lower gear before the descent to use engine braking as the primary retardation; apply service brakes in brief, firm applications (snubs) to control speed — then release to allow heat to dissipate.

Recovery: Once fade occurs, the vehicle must be pulled to a side road or runaway truck ramp; the brakes require cool-down time before they function again. A vehicle with brake fade on an active downgrade is extremely dangerous.