Coupling, Uncoupling and Safe Operation of Combination Vehicles
What this domain tests
Coupling and uncoupling questions test sequence, not strength. The exam wants you to prove the fifth wheel is locked, the trailer brakes work, and the landing gear is stowed before you move. Combination-vehicle handling questions test off-tracking and smooth control.
Coupling sequence
- Set the trailer brakes and chock the wheels; check kingpin and fifth-wheel height.
- Back under squarely in low gear; listen for the lock, then do two tug tests with the trailer brakes applied.
- Connect the emergency (red) and service (blue) air lines, the electrical cord, and check for chafing.
- Raise the landing gear fully; test trailer brakes, lights, and ABS warning.
Uncoupling and safe operation
- Chock the wheels, lower the landing gear to the ground, disconnect lines, release the fifth-wheel jaw, and pull forward slowly.
- Swing wide in turns; the trailer wheels track inside the tractor’s path, so watch mirrors and roadside objects.
- Use smooth steering, braking, and acceleration to avoid trailer sway and jackknife conditions.
Worked example: After backing under, a driver does one tug test, connects the lines, and leaves the landing gear down. The missing second tug test may not reveal a partially closed jaw; the landing gear can be destroyed when the tractor pulls out. The correct sequence is two tugs, lines, gear up, then a trailer-brake test.
Traps: standing between the units during coupling; trusting the audible click without a tug test; crossing the red and blue lines; and forgetting that a trailer with no brake test may not hold on a grade.