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Workplace Transport and Driving
LGVs and Trailers

Guidance for drivers on coupling and Un-coupling L.G.Vs
It is common practice for LGV drivers to leave the tractor unit with its engine running and the parking brake not applied when coupling or uncoupling the air lines to the semi-trailer. This is unsafe practice. Although this advice applies to LGVs with semi-trailers, it applies equally to other vehicle combinations with similar braking systems.
 
Accidents when coupling the air lines
Accidents occur during coupling up because it is common practice for trailers to be parked on the emergency brake by disconnecting the emergency airline ('dropping the red line') rather than by applying the trailer parking brake.
This is unsafe practice because connecting the air lines to a parked semi-trailer, when its parking brake is not applied, releases its brakes under air pressure and the vehicle combination can then move, particularly on an incline. This unsafe practice has resulted in drivers being:
  • run over by the moving vehicle
  • crushed between the vehicle and another object
  • often as they attempt to get into the moving tractor cab to apply the parking brake.
Accidents due to parked trailers moving.
Drivers and others have been injured when they were hit or run over by trailers which were left parked. These accidents occurred because the trailer ran away or moved on a slope because its parking brake was not applied when it was disconnected from the towing vehicle. Sometimes movement occurred after the semi-trailer had been parked for a substantial period of time.
 
It is true that the emergency brakes are applied under air pressure when the emergency air line is disconnected from the trailer. However, on trailers with manually applied (ratchet) parking brakes, and some with spring brakes, the emergency brakes cannot be relied on as a parking brake. Air pressure will inevitably leak from the braking system over time and loss of air pressure can affect the ability of the emergency brakes to hold the semi-trailer in its parked position.
 
Coupling procedure: safety checklist
The vehicle will not move while it is being coupled if the semi-trailer parking brake is applied. However, when the semi-trailer parking brake is released the vehicle can move if the tractor unit parking brake is not applied.
 
Here are the key steps for safe coupling:
  • check that the trailer parking brake is applied
  • reverse the tractor unit slowly under the semi-trailer and listen for the locking mechanism to engage
  • check that the fifth wheel locking mechanism is fully engaged (usually by trying to drive forwards in a low gear)
  • apply the parking brake on the tractor unit
  • visually check the engagement of the fifth wheel and, depending on the type of mechanism, put on any safety clip
  • reconnect the air lines (turning on any air taps that may be fitted to older vehicles) and electrical supplies to the semi-trailer
  • wind up the semi-trailer landing legs and secure the handle
  • Release the trailer parking brake; and fit the number plate, as well as any necessary warning plates, and check the lights etc.
The procedure for uncoupling is generally the reverse of the coupling procedure but always remember to set the tractor unit parking brake before leaving the cab.
 
Also, before uncoupling check that:
  • The ground is firm and level enough to support both the landing legs
  • The parking brake is set on the trailer
  • After uncoupling, check that the landing legs are not sinking into the surface.
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