Inspecting Doubles and Triples Combinations
Inspecting Doubles and Triples Combinations
This domain tests whether you inspect every unit and every connection in a multi-trailer combination, not just the tractor and the first trailer. Questions focus on what is checked at each coupling and how the air brake test is performed on a combination.
What the walk-around must cover
- Every trailer and every converter dolly, front to back, in the same order each time.
- Every coupling: fifth wheel jaws and locking pins, kingpin, pintle hook and its safety latch, dolly drawbar, safety chains.
- Every air line and electrical cord: glad-hand seals present, no chafing, not crossed, supported so they cannot drag.
- Landing gear fully raised with the crank secured.
- Lights on each trailer: clearance, marker, brake, and turn.
Air brake test on the combination
- Build full pressure, release the parking brakes, and watch the gauge for leakage.
- Apply the trailer brakes and confirm each unit holds.
- Disconnect an emergency line and confirm that trailer's spring brakes set. This is the breakaway protection test and it must be done on every trailer in the combination.
Worked example
A driver inspects the tractor and first trailer, hooks the second trailer, and drives off. At highway speed the dolly separates from the pintle hook. Reasoning: the pintle latch and safety chain check was skipped, and the driver's inspection routine stopped at the first trailer. Because the emergency line parted, the dolly's spring brakes locked, but the separation still happened. The plan, not the equipment, failed.
Traps
- Inspecting only the tractor's coupling and assuming the rest are fine.
- A quick glance at the lines instead of checking each glad-hand seal for leaks.
- Not knowing that a disconnected emergency line should lock that trailer's brakes.