LGV Coupling and Uncoupling — Questions and Answers
Question 1: What is the first thing a driver should check before coupling a tractor unit to a semi-trailer?
- That the radio is turned off
- That the trailer is on a firm, level surface with the parking brake applied and the landing gear supports it (Correct answer)
- That the trailer doors are open
- That the trailer is fully loaded
Correct answer: That the trailer is on a firm, level surface with the parking brake applied and the landing gear supports it
Before coupling, ensure the trailer is on firm, level ground, the parking brake is applied, and the landing gear (legs) is properly supporting the trailer at the correct height for the 5th wheel to engage beneath the trailer plate.
Question 2: After the 5th wheel has engaged with the trailer kingpin, what test must the driver perform?
- A speed test
- A pull test (tug test) — gently try to pull away with the trailer brakes applied to confirm the coupling is locked (Correct answer)
- A fuel economy test
- A horn test
Correct answer: A pull test (tug test) — gently try to pull away with the trailer brakes applied to confirm the coupling is locked
The driver must perform a pull test by applying the trailer brakes and gently attempting to drive forward. If the coupling is properly locked, the tractor will not separate from the trailer. This confirms the jaws are fully closed around the kingpin.
Question 3: In what order should the air lines (susies) be connected when coupling?
- It does not matter which order (Correct answer)
- Connect the emergency (red) line first, then the service (yellow) line
- Connect the service (yellow) line first, then the emergency (red) line
- Only one line needs connecting
Correct answer: It does not matter which order
Modern best practice is to connect both lines ensuring the correct line goes to the correct coupling head — red to red (emergency) and yellow to yellow (service). The key is correct colour matching rather than sequence, though some operators specify emergency first.
Question 4: What should a driver visually check on the 5th wheel after coupling?
- Only that the trailer is straight
- That the locking jaws are closed around the kingpin, the release handle is in the locked position, and there are no gaps (Correct answer)
- That the 5th wheel is painted
- Only the 5th wheel grease
Correct answer: That the locking jaws are closed around the kingpin, the release handle is in the locked position, and there are no gaps
The driver must visually check that the locking jaws are fully closed around the kingpin (no visible gap between the trailer plate and the 5th wheel), the release handle is in the locked position, and any safety catch is engaged.
Question 5: When must the trailer landing gear (legs) be fully raised after coupling?
- They can stay down during driving
- After confirming the coupling is secure and before driving away (Correct answer)
- Only when loading the trailer
- Landing gear is only used for uncoupling
Correct answer: After confirming the coupling is secure and before driving away
The landing gear must be fully raised and secured after the coupling has been confirmed as secure. Driving with the landing gear down can cause serious damage if they contact the road surface, particularly on uneven ground or speed bumps.
Question 6: What is the purpose of the trailer service line (yellow coupling)?
- To supply emergency air to the trailer
- To supply air pressure that operates the trailer service brakes when the driver presses the brake pedal (Correct answer)
- To supply electricity to the trailer
- To provide air for the trailer tyres
Correct answer: To supply air pressure that operates the trailer service brakes when the driver presses the brake pedal
The service line (yellow) transmits air pressure from the tractor's braking system to operate the trailer's service brakes. When the driver presses the brake pedal, air pressure is sent through this line to apply the trailer brakes proportionally.
LGV Coupling and Uncoupling
What is the first thing a driver should check before coupling a tractor unit to a semi-trailer?