LGV Coupling and Uncoupling 2 — Questions and Answers
Question 1: What is the correct first step when uncoupling a semi-trailer?
- Release the 5th wheel lock immediately
- Apply the trailer parking brake and lower the landing gear until they are firmly supporting the trailer weight (Correct answer)
- Disconnect the air lines first
- Drive forward quickly to separate
Correct answer: Apply the trailer parking brake and lower the landing gear until they are firmly supporting the trailer weight
The first step is to apply the trailer parking brake and lower the landing gear until they take the weight of the trailer. This ensures the trailer is stable and supported before the tractor is removed. Never release the coupling before the trailer is independently supported.
Question 2: What could happen if a driver fails to perform a proper tug test after coupling?
- Nothing — tug tests are optional
- The coupling may not be fully locked, and the trailer could separate from the tractor while driving (Correct answer)
- The engine might overheat
- The brakes would work better
Correct answer: The coupling may not be fully locked, and the trailer could separate from the tractor while driving
Without a tug test, the 5th wheel jaws may appear closed but not be fully locked around the kingpin. A partial engagement could release under load, causing the trailer to separate from the tractor at speed — an extremely dangerous situation that has caused fatal incidents.
Question 3: What should a driver check about the trailer kingpin before coupling?
- Only its colour
- That it is not worn, bent, or cracked, and that the trailer plate is not damaged (Correct answer)
- That it is rusty — rust means it is strong
- Nothing — the kingpin is maintenance-free
Correct answer: That it is not worn, bent, or cracked, and that the trailer plate is not damaged
The kingpin should be inspected for wear, bending, cracking, or damage. The trailer coupling plate should also be checked for cracks or distortion. A worn or damaged kingpin may not lock securely in the 5th wheel jaws.
Question 4: Why is it important to ensure the trailer is at the correct height before coupling?
- For aesthetic reasons
- If too high, the kingpin may ride over the jaws; if too low, it may not engage properly or could damage the trailer plate (Correct answer)
- Height does not matter for coupling
- Only to make it easier to connect the air lines
Correct answer: If too high, the kingpin may ride over the jaws; if too low, it may not engage properly or could damage the trailer plate
The trailer must be at the correct height for the 5th wheel to slide smoothly under the trailer plate and for the kingpin to drop into the jaws. Too high and the kingpin may not engage; too low and the 5th wheel could strike and damage the trailer underside.
Question 5: When should the electrical connections (suzi coils) be connected during coupling?
- Electrical connections are not needed
- After the mechanical coupling is confirmed secure and air lines are connected (Correct answer)
- Before approaching the trailer
- Only when carrying refrigerated loads
Correct answer: After the mechanical coupling is confirmed secure and air lines are connected
Electrical connections (for lights, ABS, EBS) should be connected after the mechanical coupling and air lines are confirmed secure. This ensures the trailer lights and electronic brake systems are functional before driving.
Question 6: What is a drawbar coupling and how does it differ from a 5th wheel?
- There is no difference
- A drawbar couples a full trailer to a rigid vehicle using a pin and jaw, allowing the trailer to run on its own wheels at both ends (Correct answer)
- A drawbar is only used on cars
- A drawbar is another name for the kingpin
Correct answer: A drawbar couples a full trailer to a rigid vehicle using a pin and jaw, allowing the trailer to run on its own wheels at both ends
A drawbar coupling connects a full trailer (with axles at both ends) to a rigid vehicle using an eye/pin arrangement. Unlike a 5th wheel where the tractor carries trailer weight, a drawbar trailer supports its own weight. Maximum combination length is 18.75 metres.
LGV Coupling and Uncoupling 2
What is the correct first step when uncoupling a semi-trailer?