CDL Air Brakes TestUsing Air BrakesHard
A driver is preparing to descend a long, steep downgrade in a heavy commercial vehicle equipped with air brakes. To prevent brake fade, what is the most appropriate braking strategy?
- ADisengage the clutch and coast down the grade, using brakes only as needed.
- BShift to a lower gear and apply the brakes only for emergencies.
- CApply the brakes continuously and lightly to maintain a slow speed.
- DUse the engine brake and apply the service brakes in short, firm 'snubs'.
Show answer & explanationAnswer & explanation
Correct answer: D. Use the engine brake and apply the service brakes in short, firm 'snubs'.
Brake fade occurs when brakes overheat. Using a low gear and the engine brake helps control speed, while short, firm 'snubs' of the service brakes allow them to cool between applications, preventing excessive heat buildup.
Why the other options are wrong
- A. Coasting is extremely dangerous and illegal; it removes engine braking entirely and can lead to runaway vehicle.
- B. Relying solely on the engine brake might not be enough for heavy vehicles on steep grades, and reserving service brakes only for emergencies could lead to loss of control.
- C. Continuous light braking causes constant friction and heat buildup, leading to brake fade.
Braking on Downgrades (Preventing Fade)
To safely descend long, steep grades with air brakes, use a low gear, engine brake, and short, firm applications (snubs) of the service brakes to maintain a safe speed and prevent overheating.
- Use a low gear to maximize engine braking.
- Apply service brakes firmly until speed drops by 5 mph.
- Release brakes to allow cooling, then repeat.
- Never 'ride' the brakes or coast.
Memory trick: Downhill: 'Gear Low, Snub Slow!'