Combination Vehicle Basics and Trailer Hookup
A combination vehicle is a tractor pulling a trailer, and the candidate must understand the parts that make the pair work as one unit: the fifth wheel (the horseshoe-shaped coupling plate on the tractor), the kingpin (the pin under the trailer that locks into the fifth wheel), the landing gear (trailer support legs), the air lines (service and emergency/supply), the electrical cord, the glad hands that connect the air lines, and the trailer's spring brakes. Know that the service line carries air to apply the trailer brakes, while the emergency (supply) line charges the trailer tanks and triggers emergency braking if it separates. Traps in questions: confusing the two air lines, thinking the electrical cord supplies air, assuming the tractor's parking brake holds the trailer, or forgetting that the trailer's spring brakes stay set until air pressure releases them. Another trap is the pivot point - in a tractor-trailer the pivot is the fifth wheel, not the rear axle, which is why the trailer cuts corners. Rule of thumb: before moving, do a tug test - pull gently against the trailer with the trailer brakes set (or the tractor brakes applied) to confirm the kingpin is locked. Worked example: if the trailer has no air and the spring brakes are set, you must supply air through the emergency line to release them before the trailer will roll.