Texas CDL Section 6
Combination Vehicles
Questions and Correct
Answers
1. What is the purpose of the tractor protection valve?
Answer: It automatically closes (pops out) if the trailer air
pressure drops between 20–45 psi, preserving enough air in the
tractor to stop the tractor.
2. When coupling a tractor to a semi-trailer, what position
should the fifth wheel be in?
Answer: Tilted down toward the rear of the tractor so the tractor
can slide under the trailer and engage the kingpin.
3. After you couple the tractor and trailer, how can you check
that the fifth wheel locking jaws are fully closed around the
kingpin?
,Answer: Visually check that the locking jaws have closed around
the shank of the kingpin, and then pull the tractor forward gently
with the trailer brakes still applied to test the connection.
4. What is the “crack-the-whip” effect?
Answer: A rearward amplification effect that causes the last trailer
in a combination to swing out more in a turn. It is most severe
with triple trailers.
5. Why are empty trailers more likely to jackknife than loaded
trailers?
Answer: An empty trailer has less weight on the drive axles,
making it easier for the tractor drive wheels to lock up and for the
trailer to swing around.
6. What should you do if the trailer breaks away from the
tractor?
Answer: The emergency air line will lose pressure, automatically
applying the trailer emergency brakes. The tractor protection
valve will pop out, keeping air in the tractor.
7. What are glad hands?
Answer: Coupling devices used to connect the service and
emergency air lines between the tractor and trailer.
8. Why should you connect the air lines before backing under
a trailer?
, Answer: So you can supply air to the trailer and release the spring
(emergency) brakes, allowing you to back under smoothly.
9. When uncoupling, what is the last thing you should do
before pulling the tractor away?
Answer: Lower the landing gear, make sure it is firmly on the
ground, and disconnect all air and electrical lines, securing them
on the tractor.
10. How can you prevent a rollover when driving a
combination vehicle?
Answer: Keep cargo as low as possible and reduce speed before
entering curves or turns. Avoid sudden steering movements.
11. What is the shank of the kingpin?
Answer: The narrow part of the kingpin that the fifth wheel
locking jaws must grip securely.
12. What does the trailer air supply control (red) knob do?
Answer: It supplies air to the trailer’s emergency and service lines.
Pulling it out charges the system; pushing it in applies the trailer
emergency brakes.
13. When you are not towing a trailer, why should you couple
the glad hands together or use dummy couplers?
Combination Vehicles
Questions and Correct
Answers
1. What is the purpose of the tractor protection valve?
Answer: It automatically closes (pops out) if the trailer air
pressure drops between 20–45 psi, preserving enough air in the
tractor to stop the tractor.
2. When coupling a tractor to a semi-trailer, what position
should the fifth wheel be in?
Answer: Tilted down toward the rear of the tractor so the tractor
can slide under the trailer and engage the kingpin.
3. After you couple the tractor and trailer, how can you check
that the fifth wheel locking jaws are fully closed around the
kingpin?
,Answer: Visually check that the locking jaws have closed around
the shank of the kingpin, and then pull the tractor forward gently
with the trailer brakes still applied to test the connection.
4. What is the “crack-the-whip” effect?
Answer: A rearward amplification effect that causes the last trailer
in a combination to swing out more in a turn. It is most severe
with triple trailers.
5. Why are empty trailers more likely to jackknife than loaded
trailers?
Answer: An empty trailer has less weight on the drive axles,
making it easier for the tractor drive wheels to lock up and for the
trailer to swing around.
6. What should you do if the trailer breaks away from the
tractor?
Answer: The emergency air line will lose pressure, automatically
applying the trailer emergency brakes. The tractor protection
valve will pop out, keeping air in the tractor.
7. What are glad hands?
Answer: Coupling devices used to connect the service and
emergency air lines between the tractor and trailer.
8. Why should you connect the air lines before backing under
a trailer?
, Answer: So you can supply air to the trailer and release the spring
(emergency) brakes, allowing you to back under smoothly.
9. When uncoupling, what is the last thing you should do
before pulling the tractor away?
Answer: Lower the landing gear, make sure it is firmly on the
ground, and disconnect all air and electrical lines, securing them
on the tractor.
10. How can you prevent a rollover when driving a
combination vehicle?
Answer: Keep cargo as low as possible and reduce speed before
entering curves or turns. Avoid sudden steering movements.
11. What is the shank of the kingpin?
Answer: The narrow part of the kingpin that the fifth wheel
locking jaws must grip securely.
12. What does the trailer air supply control (red) knob do?
Answer: It supplies air to the trailer’s emergency and service lines.
Pulling it out charges the system; pushing it in applies the trailer
emergency brakes.
13. When you are not towing a trailer, why should you couple
the glad hands together or use dummy couplers?
