This set combines fifth-wheel inspection, trailer air-system operation, safe turning, backing, skid prevention, and uncoupling principles.
01Coupling
02Air lines
03Trailer brakes
04Backing
05Turning
06Inspection
PRACTICE MODE
Combination Vehicles Test 3
Questions appear in a new order on each attempt. Choose one answer, review the explanation, and use Back or Next to move through the test.
Question 1 of 25Score 0
What does the trailer service line carry?
Answer: Controlled air for normal trailer brake application Pressure in the service line responds to the foot brake or trailer hand valve. It tells the trailer relay valve how strongly to apply the brakes. Study: Combination Vehicles — air lines
What does the trailer emergency line do?
Answer: Supplies trailer tanks and controls emergency braking The emergency or supply line fills trailer reservoirs and helps apply emergency brakes if pressure is lost. A major leak can also trigger tractor protection. Study: Combination Vehicles — air lines
What is the purpose of the tractor protection valve?
Answer: Keep a trailer air loss from draining the tractor system The valve closes the trailer supply during a major leak or breakaway to preserve tractor air. Test its operation as described in the CDL inspection procedure. Study: Combination Vehicles — tractor protection
Before coupling, how should trailer height compare with the fifth wheel?
Answer: Low enough that the tractor raises the trailer slightly Slight lifting creates firm contact between the trailer apron and fifth wheel. A trailer that is too high can miss the fifth wheel and create a false coupling. Study: Combination Vehicles — coupling
Before backing under a trailer, how should the tractor be positioned?
Answer: Directly in front of and aligned with the trailer Straight alignment reduces the chance of striking or damaging the landing gear and helps the kingpin enter the fifth wheel properly. Back slowly and verify alignment. Study: Combination Vehicles — coupling
What should be checked before connecting to a trailer?
Answer: The trailer is secured against movement A trailer can move when contacted unless its brakes are set or wheels are chocked as required. Secure it before coupling. Study: Combination Vehicles — coupling
Where must the fifth-wheel locking jaws close?
Answer: Around the kingpin shank The jaws must lock around the narrow shank, not rest on the kingpin head. A visual inspection is required after coupling. Study: Combination Vehicles — fifth wheel
What should the driver see between the fifth wheel and trailer apron after coupling?
Answer: No visible gap The trailer apron should lie flat on the fifth wheel with no gap. A gap can indicate a high hitch or incomplete coupling. Study: Combination Vehicles — coupling inspection
How should coupling security be tested before fully raising the landing gear?
Answer: Apply trailer brakes and gently pull forward A gentle tug against the set trailer brakes checks whether the fifth wheel holds the kingpin. It supplements, not replaces, a visual inspection. Study: Combination Vehicles — coupling check
What should be verified about the fifth-wheel release arm after coupling?
Answer: It is seated in the locked position The release arm should indicate a locked mechanism and the safety latch should be engaged where equipped. Any doubt requires recoupling and reinspection. Study: Combination Vehicles — fifth wheel
How should air and electrical lines be routed between tractor and trailer?
Answer: With enough slack for turns but not enough to drag Lines need room for normal articulation without pinching, snagging, or dragging. Secure them and check connections before moving. Study: Combination Vehicles — connections
What color usually identifies the emergency or supply air line?
Answer: Red Red commonly identifies the emergency or supply line, while blue identifies the service line. Do not rely on color alone if connectors or markings are unclear. Study: Combination Vehicles — air lines
What color usually identifies the trailer service air line?
Answer: Blue Blue commonly identifies the service line carrying brake-application pressure. Correct gladhand connection is vital to trailer braking. Study: Combination Vehicles — air lines
What should be done after connecting the trailer air lines?
Answer: Check that gladhands are sealed and lines are secure A poor seal leaks air and can impair trailer braking. Inspect the seals, listen for leaks, and keep lines from rubbing or dragging. Study: Combination Vehicles — connections
When should the landing gear be raised fully?
Answer: After a secure coupling has been confirmed Confirm the jaws, release arm, apron contact, and tug test first. Then raise the landing gear completely and secure the crank handle. Study: Combination Vehicles — coupling sequence
Why should the landing-gear crank handle be secured?
Answer: To keep it from extending or causing damage A loose handle can swing, catch, or damage equipment. Stow it in its keeper after the gear is fully raised. Study: Combination Vehicles — landing gear
Before uncoupling, how should the tractor and trailer be positioned?
Answer: In a straight line on firm, level ground when possible Straight, stable positioning reduces strain on the coupling and helps prevent trailer movement or landing-gear damage. Secure the vehicle before beginning. Study: Combination Vehicles — uncoupling
What must support the trailer before the tractor is pulled away?
Answer: The landing gear on firm ground Lower the landing gear until it firmly supports the trailer, using supports if the ground is soft. A loaded trailer can sink or tip on weak ground. Study: Combination Vehicles — uncoupling
Why should the trailer wheels be chocked during uncoupling when needed?
Answer: To prevent the trailer from moving The trailer can roll when the tractor pulls away or air pressure changes. Brakes and wheel chocks provide secure restraint as conditions require. Study: Combination Vehicles — uncoupling
When disconnecting air lines, where should gladhands be placed?
Answer: On dummy couplers or secure holders Dummy couplers keep dirt and water out of the lines. Secure storage also prevents damage while the tractor is away. Study: Combination Vehicles — uncoupling
After unlocking the fifth wheel during uncoupling, how far should the tractor initially pull?
Answer: Just far enough to clear the kingpin while the frame remains under the trailer Keeping the tractor frame under the trailer briefly provides support if the landing gear fails. Stop and verify stability before pulling completely clear. Study: Combination Vehicles — uncoupling
Why should a driver avoid backing a combination vehicle when possible?
Answer: It creates large blind spots and jackknife risk Long combinations are difficult to control in reverse and small errors grow quickly. Plan routes and parking to minimize backing. Study: Combination Vehicles — backing
When backing a trailer, what steering correction is best?
Answer: Small, slow corrections Trailers respond with a delay, and large inputs can quickly create a jackknife. Use both mirrors, pull up as needed, and get out to look. Study: Combination Vehicles — backing
If the trailer drifts too far while backing, what should the driver do?
Answer: Pull forward to reset the angle A pull-up is a safe correction, not a failure. Resetting the combination avoids a worsening angle or blind-side collision. Study: Combination Vehicles — backing
Why is driver-side backing preferred when a choice exists?
Answer: Visibility is generally better on the driver side Sight-side backing gives a better view of the trailer path. It still requires slow movement, mirror checks, and getting out to look. Study: Combination Vehicles — backing
Answer from memory so your first score shows which topics need work.
02
Review the explanations
Return to the cited handbook section whenever an idea is unclear.
03
Retake after review
Aim for consistent mastery, not memorizing the position of an answer.
FACT-CHECKED STUDY BASIS
Official handbook sources
Questions are original study material based on standard CDL manual topics. Requirements can vary by state and hazardous-material rules can change, so confirm details in your current state manual and applicable federal guidance.
Important: This independent study site is not affiliated with FMCSA, TSA, PHMSA, or any state DMV. It does not replace official guidance or professional instruction.