Combination Vehicles
Combination Vehicles tests coupling, uncoupling, inspection, and control of a tractor-trailer. The exam focuses on the fifth wheel, air lines, landing gear, trailer brakes, and off-tracking.
Core rules
- Before coupling, check the fifth wheel, kingpin, trailer height, and air lines. Back under the trailer, listen for the lock, and perform a tug test.
- Connect the air lines and electrical cord, then check that trailer brakes and lights work. Raise the landing gear fully and secure the crank.
- Inspect the coupling after a short distance. Make sure the fifth wheel jaws are closed around the kingpin and the safety latch is engaged.
- Turn wide enough to keep the trailer wheels from hitting the curb, but do not swing into another lane. Trailer wheels track inside the tractor's path.
- Control speed before curves; a combination rolls over at a lower speed than a car.
Worked example
After backing under a trailer, the driver locks the fifth wheel and connects the air lines. The driver gently pulls forward against the trailer brakes. The tractor does not move, so the coupling is holding. Reasoning: the tug test reveals a high or unlatched fifth wheel before the driver reaches highway speed, when a separation would be catastrophic.
Traps
Forgetting to open the trailer air-line shutoff valves; leaving the landing gear partly down; and turning too sharply or too wide because the driver is not watching the trailer's rear wheels in the mirrors.