Coupling and Uncoupling
Coupling and Uncoupling
This domain tests the sequence and the verification steps - the exam loves the order of operations and the two checks that prove a coupling is locked.
Coupling, in order
- Inspect the fifth wheel: jaws open, locking lever open, no cracks, plate lightly greased.
- Inspect the kingpin and apron for bends, cracks, and missing bolts.
- Set the trailer height so the tractor raises it slightly as you back under; too high and the kingpin rides over the fifth wheel.
- Back under slowly and squarely until you hear the jaws lock.
- Set the tractor parking brake, then do the tug test: pull forward gently twice and feel for resistance.
- Get out and look: jaws closed around the kingpin, locking lever in place, no gap between apron and fifth wheel.
- Connect the air lines - blue emergency, red service - and the electrical cord. Do not cross them.
- Raise the landing gear fully, then test the trailer brakes and check for air leaks.
Uncoupling, in order
Park on firm, level ground; set the parking brake; chock the wheels if there is any slope; lower the landing gear to the ground; disconnect and secure the air lines and cord; release the fifth-wheel locking lever; pull out slowly.
Worked example. You hear the jaws click, but on the tug test the trailer rolls forward with the tractor and you later see daylight between the apron and the fifth wheel. That gap means the kingpin never seated - usually because the trailer was set too high. Lower the trailer, back under again, and repeat both the tug test and the visual check.
Traps that cost marks. Relying on the click alone; skipping the visual check; raising the landing gear before the air lines are connected; pulling out with the landing gear still down; uncoupling on a slope without chocks.