Combination Vehicles (CDL Class A)
Combination Vehicles is the Class A knowledge area covering tractor-trailers, doubles/triples, and truck-trailer rigs. Florida's test checks inspection, coupling and uncoupling, trailer air systems, and safe control of long vehicles.
Coupling and the fifth wheel
Check that the fifth-wheel jaws are open, the release lever is open, and the safety latch is free. Inspect the kingpin, apron, mounting bolts, and pivot pins. Set the trailer height so the fifth wheel contacts the apron and lifts it slightly; too low can push the tractor back, too high can damage the kingpin and fifth wheel. Back slowly with the kingpin centered until the jaws close. Do not trust the click. Apply the trailer brakes and do a gentle tug test, then visually confirm the jaws are closed around the kingpin.
Air lines, landing gear, and uncoupling
Connect the red emergency line to the emergency glad hand and the blue service line to the service glad hand. Crossed lines can release trailer spring brakes or cause drag. Charge the trailer system and check that the trailer brakes apply and release. Raise the landing gear fully and secure the crank. To uncouple: level ground, chock trailer wheels, lower the landing gear to firm contact, disconnect and secure the lines and electrical cord, pull the release lever, and pull the tractor forward slowly. If the trailer does not separate, stop and find the cause; never jerk it free.
Handling and off-tracking
Rear wheels of a combination follow a shorter path than the front wheels, so a right turn can put the trailer's rear wheels over a curb. Turn wide enough but not so wide that you hit traffic in the next lane. Doubles and triples amplify this rearward swing, so watch the last trailer in the mirrors.
Worked example
You back under a trailer and hear a click. The release lever looks closed, so you raise the gear and drive off; the trailer drops onto its frame at the first bump. The click only shows the jaws passed the kingpin. The missed steps were the tug test and the visual check of the jaws and release lever. Correct order: couple, tug test, visual check, then landing gear, air lines, and brake check.
Traps that cost marks
- Mixing up red (emergency) and blue (service) glad hands.
- Not knowing the red trailer air supply knob: in charges the trailer, out closes the line and sets trailer spring brakes.
- Using the trailer hand valve for parking.
- Turning so sharply that the trailer's rear wheels off-track into a curb or vehicle.
- Raising the landing gear only partway.