Combination Vehicle Inspection and Coupling/Uncoupling
What the pre-trip adds
A combination pre-trip goes past the single-unit walk-around: fifth wheel jaws and mounting bolts, the apron and kingpin, the gap between tractor frame and trailer, glad hands and their rubber seals, the electrical cord and receptacle, landing gear (legs straight, crank secure, pads present), trailer frame and crossmembers, and the rear underride guard. Shiny metal on the fifth wheel plate is a warning - the trailer is riding on the plate instead of locking into the jaws.
Coupling order
Open and grease the jaws, check the mounting bolts, then back slowly so the kingpin enters the throat and nudges the tractor forward. Set the parking brake. Do the tug test twice: pull gently against the trailer spring brakes, and the trailer must not move. Then get under and look - jaws closed around the kingpin, safety latch down, no gap between apron and fifth wheel. Hook the air lines (blue to service, red to emergency), the electrical cord, raise the landing gear fully, and release the trailer brakes.
Worked example
A driver's tug tests pass, but the visual check shows a 2-inch gap between apron and fifth wheel. Reasoning: a gap means the kingpin is sitting on top of the closed jaws, not locked in them - the tractor is pushing the trailer, not towing it. Pull forward slowly with the trailer brakes set, re-approach the trailer, and repeat every check.
Traps that cost marks
Skipping the second tug; leaving landing gear partly extended; crossing blue and red so trailer brakes drag or never release; pulling the release handle before the air lines are disconnected; forgetting to chock wheels on a slope during uncoupling.