Combination Vehicles Practice Test 1 Flashcards

Which type of jackknife is considered the most dangerous because it is the most difficult to correct?

Answer: A tractor jackknife caused by the drive wheels locking up

When the drive wheels lock up, the tractor can spin around the trailer — this tractor jackknife is the most dangerous because once the angle becomes severe, there is almost no way to recover. Trailer jackknifes, while serious, can sometimes be corrected by releasing the brakes.

On a combination vehicle, what color is the emergency (supply) air line glad hand?

Answer: Red

Federal regulations require the emergency (supply) air line glad hand to be red and the service (control) air line to be blue. This color coding prevents connecting the lines to the wrong couplers, which would leave the trailer without proper brake control.

After coupling a tractor to a semi-trailer, what is the correct way to perform the 'tug test' to confirm the fifth wheel is locked?

Answer: With the trailer brakes applied, gently pull forward with the tractor and check that the trailer does not separate

The tug test is performed with the trailer brakes set — you then gently pull forward with the tractor. If the fifth wheel is properly locked around the kingpin, the trailer will not move. If it separates, the fifth wheel was not latched correctly.

When uncoupling a semi-trailer, you should lower the landing gear BEFORE disconnecting the air lines. Why?

Answer: To support the trailer's front end and prevent it from dropping and damaging the air lines and electrical cord

If you disconnect the air and electrical lines before the landing gear is down and bearing weight, the front of the trailer can drop suddenly, tearing apart the lines and potentially injuring anyone nearby or damaging equipment.

What is 'rearward amplification' in a combination vehicle?

Answer: The magnification of sway toward the rear of a combination, so the last trailer swings more violently than the tractor

Rearward amplification means that a lane-change or swerving motion grows progressively larger toward the rear of the vehicle. A triple-trailer combination can see the last trailer sway several times more than the tractor, making rollovers or loss of control far more likely at highway speeds.

What is the correct use of the trailer hand valve (also called the trolley valve) in a combination vehicle?

Answer: Use it only to test that the trailer brakes are working before driving, not as a brake while in motion

The trailer hand valve applies only the trailer brakes, with no front-axle braking to stabilize the rig. Using it while driving can lock up the trailer wheels and cause a trailer jackknife. Its correct use is a quick pre-trip check to confirm the trailer brakes respond.