Coupling and Uncoupling Twin Trailers
Coupling and Uncoupling Twin Trailers
This domain tests the order in which you join a tractor to a semitrailer and a semitrailer to a dolly or second trailer, and the checks that prove each connection is actually locked. Expect questions on the tug test, locking jaws, glad hands, and landing gear.
Coupling rules
- Inspect the fifth wheel before backing under: jaws open, no cracks, mounting bolts tight, plate greased. Look at the kingpin for bends, wear, and a missing collar.
- Set the tractor parking brake, then back straight so the kingpin enters the throat squarely.
- After the jaws close, do a tug test: pull forward gently against the locked trailer brakes.
- Repeat the tug test after the landing gear is raised, and get out and look: jaws closed around the kingpin, release handle in the locked position.
- Connect the red emergency line, the blue service line, and the electrical cord; confirm nothing is crossed, chafed, or dragging.
- Raise the landing gear fully, secure the crank, then test the trailer brakes before pulling away.
Uncoupling rules
- Park on firm, level ground, set the parking brake, and chock the wheels if the surface is not level.
- Lower the landing gear until it carries the trailer's weight before releasing the fifth wheel.
- Disconnect the air lines and electrical cord first, then release the locking jaws, then pull the tractor out slowly.
Worked example
You back under, the jaws lock, the first tug test holds, and you raise the landing gear. When you pull away the trailer brakes drag. Reasoning: either the trailer parking brake valve is still applied, or the red and blue lines are crossed so the emergency circuit never receives supply air. Check glad hands red-to-red and blue-to-blue, release the valve, retest. A tug test only proves the coupling, not the air plumbing.
Traps
- Skipping the second tug test or the outside visual check.
- Pulling out with the landing gear still down.
- Releasing the fifth wheel before the air lines are disconnected.