Coupling and Uncoupling
What this domain covers
Coupling and uncoupling questions test the exact order of steps, not just the final result. The exam expects you to prevent high hooking, trailer creep, and dropped trailers by using chocks, alignment, a tug test, and correct air-line connection.
Coupling rules
- Inspect the fifth wheel and kingpin. The jaws must be open, greased, and free of damage.
- Chock the trailer wheels. Back under the trailer slowly with the tractor centered and the trailer at the correct height.
- The kingpin should enter the fifth wheel and the jaws should close with a click. Do not rely on sound alone.
- Perform a tug test: pull gently against the trailer with the trailer brakes set. If the tractor moves and the trailer does not, the coupling is suspect.
- Connect the air lines and electrical cord. Blue is service; red is emergency. Confirm the trailer air pressure builds.
- Raise the landing gear fully, secure the crank, and release the trailer spring brakes.
Uncoupling rules
- Park on level ground, set the tractor parking brake, and chock the trailer wheels.
- Lower the landing gear until it supports the trailer, then disconnect the air lines and electrical cord.
- Pull the fifth wheel release handle, pull forward slowly, and stop when clear.
Worked example
You back under a trailer but the trailer is too high. The kingpin rides over the fifth wheel instead of locking into the jaws. Reasoning: this is “high hooking.” The release handle may feel locked, but the jaws never closed. Lower the trailer, back under again, and confirm with a tug test. Driving away can drop the trailer.
Traps that cost marks
- Forgetting to chock the trailer before uncoupling.
- Skipping the tug test because the handle “felt” locked.
- Raising the landing gear before the air lines are connected and charged.
- Leaving the trailer at the wrong height for coupling.