CDL - Commercial Driver's License Combination Vehicles Practice Test 1

Question 1

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

Accepted Answer

A tractor jackknife caused by the drive wheels locking up

Answer

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.

Question 2

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

Accepted Answer

Red

Answer

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.

Question 3

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?

Accepted Answer

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

Answer

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.

Question 4

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

Accepted Answer

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

Answer

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.

Question 5

What is 'rearward amplification' in a combination vehicle?

Accepted Answer

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

Answer

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.

CDL Practice Test

CDL Practice Test

CDL - Commercial Driver's License Combination Vehicles Practice Test 1