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CDL combination vehicles practice test

The combination vehicles test is required for a Class A CDL. It covers how a tractor and trailer behave together, coupling and uncoupling step by step, inspecting the fifth wheel and air lines, and the rollover and jackknife risks that come with a trailer. The sample questions below come from Section 6 of the FMCSA CDL manual.

Free combination vehicles practice questions

Ten questions from the PassRig bank. Pick an answer in your head, then show the answer to read why. The full bank, timed mock exams and AI explanations live in the app.

  1. Compared with an empty rig, a fully loaded tractor-trailer is about how much more likely to roll over in a crash?

    • Equally likely
    • Twice as likely
    • Five times as likely
    • Ten times as likely
    Show answer
    Answer: D. Ten times as likely

    A loaded combination has a high center of gravity and rolls over far more easily. Keep the load low and centered, and slow down well before curves and ramps.

    Source: FMCSA CDL Manual, Section 6.1.1 (Rollover Risks)

  2. When backing a trailer, to make the trailer go left you should:

    • Turn the top of the steering wheel to the left
    • Turn the top of the steering wheel to the right
    • Use the trailer hand valve
    • Keep the wheel straight and use the mirrors
    Show answer
    Answer: B. Turn the top of the steering wheel to the right

    The trailer goes the opposite way from the top of the steering wheel. Turn right to push the trailer left, then straighten and follow it. Back toward the driver's side when you can, use pull-ups to fix your angle, and get out and look.

    Source: FMCSA CDL Manual, Section 6.1.5 (Backing with a Trailer)

  3. After coupling, how should the fifth wheel look when you inspect it with a flashlight?

    • A small gap between the upper and lower fifth wheel is normal
    • No space between the upper and lower fifth wheel, and the locking jaws closed around the shank of the kingpin
    • The jaws closed around the head of the kingpin
    • The release handle pulled out
    Show answer
    Answer: B. No space between the upper and lower fifth wheel, and the locking jaws closed around the shank of the kingpin

    Any gap means the trailer is riding high and not locked. The jaws must grip the shank, the narrow part of the kingpin, not the head. Also check that the locking lever is in the lock position and the safety latch is over it.

    Source: FMCSA CDL Manual, Section 6.4.1 (Coupling Tractor-Semitrailers: Inspect the Coupling)

  4. In a combination vehicle, which wheels should lock up last in hard braking?

    • The steering axle wheels
    • The trailer wheels
    • The drive wheels
    • It does not matter
    Show answer
    Answer: A. The steering axle wheels

    You can still steer as long as the front wheels roll. A drive-wheel lockup can cause a tractor jackknife, and a trailer-wheel lockup can cause a trailer jackknife, but losing steering control is the worst outcome, so brakes are set up so the front wheels lock last.

    Source: FMCSA CDL Manual, Section 6.1.3 and 2.18 (Braking and Skids)

  5. The trailer starts to skid and swing out in a jackknife. The best way to stop it is to:

    • Apply the trailer hand valve
    • Brake harder to lock the tractor wheels too
    • Release the brakes so the trailer wheels can regain traction
    • Accelerate to pull the trailer straight
    Show answer
    Answer: C. Release the brakes so the trailer wheels can regain traction

    Trailer jackknifes come from trailer wheels locking up. Once they roll again they will grip the road, and the trailer will straighten out behind the tractor. Do not use the trailer hand valve.

    Source: FMCSA CDL Manual, Section 6.1.3 (Trailer Skids)

  6. The service air line carries air to the trailer brakes whenever you:

    • Press the brake pedal or use the trailer hand valve
    • Push in the red trailer air supply knob
    • Start the engine
    • Pull out the yellow parking brake knob
    Show answer
    Answer: A. Press the brake pedal or use the trailer hand valve

    The service line is controlled by the foot brake and the hand valve. How hard you press sets how much pressure goes down the line and how hard the trailer brakes apply.

    Source: FMCSA CDL Manual, Section 6.2.4 (Service Air Line)

  7. The air tanks on a trailer are filled through the:

    • Service line
    • Electrical cord
    • Trailer hand valve
    • Emergency (supply) line
    Show answer
    Answer: D. Emergency (supply) line

    The red emergency line charges the trailer air tanks and holds the trailer emergency brakes released. Like the tractor's tanks, trailer tanks collect water and oil and must be drained.

    Source: FMCSA CDL Manual, Section 6.2.6 (Trailer Air Tanks)

  8. Before coupling, the fifth wheel should be tilted:

    • Up toward the rear of the tractor
    • Level with the frame
    • Down toward the rear of the tractor
    • To one side to help the trailer line up
    Show answer
    Answer: C. Down toward the rear of the tractor

    With the fifth wheel tilted down at the rear, the kingpin slides into the throat of the fifth wheel as you back under. Also check that the jaws are open and the release arm is in the unlocked position.

    Source: FMCSA CDL Manual, Section 6.4.1 (Coupling: Inspect Fifth Wheel)

  9. After raising the landing gear all the way up, you should:

    • Secure the crank handle and check for enough clearance between the tractor and the landing gear and trailer
    • Lower it halfway again for stability
    • Leave the crank hanging so it is easy to find
    • Remove the crank and store it in the trailer
    Show answer
    Answer: A. Secure the crank handle and check for enough clearance between the tractor and the landing gear and trailer

    A loose crank handle can swing and injure someone or be lost. Also make sure the tractor frame will clear the landing gear and that the rear of the tractor will clear the trailer in a tight turn.

    Source: FMCSA CDL Manual, Section 6.4.1 (Coupling: Raise Front Trailer Supports)

  10. During the inspection of a combination vehicle, the lower fifth wheel should be:

    • Loose enough to pivot freely by hand
    • Dry, with no grease on the plate
    • Tilted up at the rear while coupled
    • Securely mounted to the frame with no missing or damaged parts, and no visible gap between it and the trailer
    Show answer
    Answer: D. Securely mounted to the frame with no missing or damaged parts, and no visible gap between it and the trailer

    Look for missing bolts, cracks, and a gap between the upper and lower fifth wheel plates. The locking jaws must be around the kingpin shank, the release lever locked, and the safety latch in place.

    Source: FMCSA CDL Manual, Section 6.5.1 (Coupling System Areas)

What the combination vehicles test covers

  • Driving combination vehicles: rollover, offtracking and jackknife
  • Trailer hand valve and trailer brakes
  • Combination vehicle air brakes: service, emergency and supply lines
  • Coupling and uncoupling, in order
  • Inspecting a combination vehicle
  • Antilock brakes on tractors and trailers

Who needs it

Every Class A applicant. If you will pull a trailer, this test is required.

The test has 20 multiple-choice questions and the passing score is 80%, so you can miss 4. Your state DMV or DOT administers it; the federal rules are on the FMCSA CDL page. Study Section 6 of the CDL manual alongside these questions.

More CDL practice tests

Combination Vehicles test questions

How many questions are on the CDL combination vehicles test?

20 questions. 16 correct answers reach the 80% pass mark.

Do I need the combination test for a Class B licence?

No. Class B covers single vehicles. The combination vehicles test is a Class A requirement, because Class A means you tow a trailer over 10,000 pounds.

What is the hardest part of the combination vehicles test?

The coupling and uncoupling sequence and the air line questions. Learn the order of the steps and what each air line does before test day.