CDL Combination Vehicles TestCoupling and UncouplingMedium
A driver is uncoupling a combination vehicle. After ensuring the landing gear is fully down and supporting the trailer, and the air and electrical lines are disconnected, what is the next step before pulling the fifth wheel release handle?
- AChock the trailer wheels to prevent movement.
- BRelease the tractor's parking brake.
- CPump the brake pedal to drain all air from the system.
- DCheck the clearance between the tractor frame and the landing gear.
Show answer & explanationAnswer & explanation
Correct answer: A. Chock the trailer wheels to prevent movement.
Chocking the trailer wheels is a crucial safety step during uncoupling, typically performed after the landing gear is down and before releasing the fifth wheel. This prevents the trailer from rolling away once it's separated from the tractor, especially if the ground is not perfectly level or if the trailer brakes fail for any reason.
Why the other options are wrong
- B. The tractor parking brake is released after the fifth wheel is released and the tractor is ready to pull away.
- C. Pumping the brake pedal to drain air is part of a full system check, not a direct step before releasing the fifth wheel in uncoupling.
- D. Checking clearance is done *before* lowering the landing gear to ensure adequate space, not after it's already down and supporting the trailer.
Trailer Wheel Chocking (Uncoupling)
During uncoupling, after the landing gear is down, chocking the trailer wheels is an essential safety measure to prevent unintentional trailer movement once the tractor is separated.
- Prevents rolling on slopes or uneven ground.
- Backup for trailer brake failure.
- Performed before releasing fifth wheel.
Memory trick: Gear, Disconnect, Chock, Release.