Combination vehicles — a tractor pulling one or more trailers — present unique driving challenges. This endorsement is required for Class A CDL holders who will operate tractor-trailers, doubles, or triples.
The most critical concept is the coupling and uncoupling procedure. Improper coupling can cause the trailer to separate from the tractor while driving. You must understand the fifth wheel mechanism, kingpin, locking jaws, and the specific step-by-step procedures for safely coupling and uncoupling.
Combination vehicles are susceptible to jackknifing (when the trailer pushes the tractor sideways) and trailer swing. Understanding the causes of these dangerous situations — primarily excessive braking on slippery surfaces — and how to prevent them is heavily tested on the CDL exam.