Combination Vehicles
Combination vehicles test coupling and uncoupling, air-line routing, fifth-wheel inspection, and the handling differences of a tractor-trailer.
Coupling
Inspect the fifth wheel, kingpin, and locking jaws. Back under the trailer squarely, listen for the click, and perform a tug test with the trailer brakes set. Connect the emergency (red) and service (blue) lines without crossing them, plug in the electrical cord, raise the landing gear, build air, and check the trailer brakes and lights. After a pull test, verify the jaws are closed around the kingpin.
Driving
A combination off-tracks in turns and has rearward amplification: the trailer swings wider than the tractor in a quick lane change. Brake earlier than with a single unit. Turn wide but not into oncoming traffic, and watch the trailer's rear wheels in the mirror. Do not use the trailer hand valve for parking; use the spring brakes.
Worked example
A 53-foot trailer makes a right turn from a two-lane road. The driver pulls forward until the tractor's rear axle passes the corner, then turns slowly. The trailer's rear wheels still cut inside the tractor's path by several feet, so the driver watches the right mirror and stops if a sign or curb is in the track. Reasoning: the trailer follows a shorter arc, and the mirror shows the actual clearance.
Traps
Skipping the tug test; crossing the glad hands; forgetting to raise the landing gear; and cutting a corner because the tractor cleared it.