CDL Combination Vehicles TestDriving Combination Vehicles SafelyMedium
A commercial driver is operating a combination vehicle on a steep downgrade. The driver has downshifted and is using engine braking, but the vehicle is still gaining speed. What is the appropriate next action?
- AUse a series of short, firm brake applications to control speed.
- BEngage the parking brake to assist in slowing the vehicle.
- CShift to a higher gear to reduce engine RPMs.
- DApply the service brakes continuously with light pressure.
Show answer & explanationAnswer & explanation
Correct answer: A. Use a series of short, firm brake applications to control speed.
When engine braking is insufficient, a series of short, firm brake applications (stab braking or controlled braking) allows the brakes to cool between applications, preventing overheating and brake fade. Continuous light braking can build up excessive heat.
Why the other options are wrong
- B. Engaging the parking brake while moving will lock the drive wheels and cause a dangerous skid or loss of control.
- C. Shifting to a higher gear would reduce engine braking, causing the vehicle to gain even more speed.
- D. Continuous light braking can cause brake fade by overheating the brakes.
Downgrade Braking (Engine & Service)
On steep downgrades, use engine braking first. If speed still increases, use short, firm service brake applications to control speed and prevent brake fade.
- Engine braking is primary for downgrades.
- If insufficient, use 'stab braking' or controlled braking.
- Avoid continuous light braking to prevent overheating.
Memory trick: Downhill run, engine's done? Stab the brakes, have some fun!