If you're pursuing a Class A CDL, the Combination Vehicles knowledge test is mandatory. It covers the skills and systems specific to operating a tractor-trailer — from coupling and uncoupling to preventing jackknifes on slippery roads.
Test Format
- Questions: 20 multiple-choice questions
- Passing score: 80% (16 out of 20)
Key Topic Areas
Coupling and Uncoupling
Expect several questions on the proper step-by-step procedure to safely couple a tractor to a semi-trailer and uncouple them. Know the order: position tractor, check trailer height, back slowly, connect air lines, raise landing gear, secure the connection, and always tug-test.
Air Lines (Glad Hands)
The emergency (supply) line is red. The service line is blue. You must connect the emergency line first and disconnect it last. Cross-connecting these lines is a common mistake — and a common test question.
Landing Gear
Landing gear must be fully raised before driving. Low gear is used when the trailer is loaded; high gear is used for light or empty trailers. Never drive with the landing gear partially lowered.
Fifth Wheel Inspection
After coupling, the fifth wheel jaws must be fully closed and locked around the kingpin. Pull the release handle and confirm it won't open. Check that the trailer sits flat on the fifth wheel with no gap.
Anti-Jackknife and Skid Prevention
Jackknifing occurs when the tractor and trailer fold at the coupling point — usually caused by excessive braking, especially of the drive axles. To prevent it: brake smoothly, avoid locking the wheels, and steer into a skid. ABS systems help but don't eliminate the risk.
Rearward Amplification
When a combination vehicle swerves or changes lanes quickly, the rear trailer swings wider than the tractor (the "crack the whip" effect). This risk increases with double and triple trailer combinations. Take lane changes slowly and avoid sudden maneuvers.
Off-Tracking
The rear wheels of a trailer track inside the path of the front wheels during turns. The longer the combination, the more off-tracking occurs. Account for this by turning wider than you normally would — but not so wide that you enter the wrong lane.
Study Tips
- Memorize the air line colors (red = emergency, blue = service)
- Practice the coupling/uncoupling steps in order — they appear as sequence questions
- Know what causes jackknifing and how to recover
- Take several Combination Vehicles practice tests before your exam