Combination Vehicles
Combination Vehicles
This domain tests coupling, air-line routing, off-tracking, and rollover control. Combination vehicles are less stable and need more room than a straight truck.
Coupling and inspection
Inspect the fifth wheel for open jaws, a missing safety latch, or a damaged kingpin. After coupling, do a tug test with the trailer brakes set, raise the landing gear, and connect the air lines. The emergency line is red and supplies trailer air; the service line is blue and applies the trailer brakes. Charge the trailer tanks and test the trailer brakes before moving.
Driving
Combination vehicles off-track: the trailer wheels cut inside the tractor's path. Swing wide before a turn, but do not cross the centerline or crowd the curb. Slow down before curves because a high center of gravity increases rollover risk. Brake earlier than in a car; the trailer can push the tractor in a quick stop.
Worked example
A 53-foot trailer makes a right turn from a narrow lane. If you start the turn too late, the trailer's rear wheels will climb the curb. Start from a wider position, keep the tractor out of the opposite lane, and let the trailer track through the turn without swinging over the sidewalk.
Uncoupling
Chock the trailer wheels, lower the landing gear, disconnect the air lines and electrical cord, then pull the fifth-wheel release handle. Do not pull the pin while the trailer is resting on the tractor.
Traps that cost marks
- Swapping the red and blue air lines.
- Forgetting the tug test after coupling.
- Turning too early and clipping the curb.
- Pulling the fifth-wheel release under load.