Combination Vehicles
What this domain tests
Combination Vehicles tests tractor-trailer coupling, air-line routing, trailer brake checks, and the handling differences caused by a long wheelbase and high center of gravity. Iowa questions often target fifth-wheel inspection, off-tracking, and rollover prevention.
Core rules
- Inspect the fifth wheel for cracks, missing or loose mounting bolts, and a properly greased plate; check that locking jaws close around the kingpin.
- After coupling, do a tug test, visually confirm the jaws are closed, and raise the landing gear fully.
- Connect the emergency (red) air line first and the service (blue) line second; when uncoupling, disconnect the service line first, then the emergency line.
- Check trailer brakes with the tractor protection valve and trailer hand valve before driving; trailer spring brakes must hold when air is lost.
- A trailer’s rear wheels track inside the tractor’s path in a turn, so swing wide enough without crossing the curb or hitting adjacent traffic.
Worked example
A 53-foot trailer turning right from a narrow lane will cut the corner if the driver starts the turn too early. The rear trailer wheels follow a shorter arc than the tractor’s front wheels. The driver should move forward until the tractor clears the corner, then turn, watching the trailer’s right side in the mirror. Starting late risks hitting a pole; starting early risks mounting the curb.
Traps
Relying on the tug test alone without a visual jaw check; forgetting to raise landing gear; crossing air lines; and assuming a loaded trailer stops as quickly as a bobtail tractor.