Understanding The Risk Of Trailer Rollaway

All drivers should be familiarised with the operation of the park and shunt valves on a trailer, but when they apply the trailer park brakes, are they fully aware of why this is so critical?

To be clear, there are situations where a trailer can roll either backwards or forwards with potentially fatal consequences. The situation can arise from the potential assumption that, when the air lines are disconnected, the trailer brakes apply automatically. Whilst this is strictly true, it should not be relied upon due to the way that the trailer air/spring brakes work. If a trailer is left for any length of time and the trailer park brake has not been applied, rollaway can occur when re-coupling if the driver does not apply the tractor park brakes (hand control) or carries out split coupling.

Don-Bur has produced an useful infographic to explain how trailer air brakes work and how this stuation could arise.

What is "Split Coupling?"

Split coupling is the frowned-upon practice of reversing the tractor fifth wheel under the trailer bed plate just enough to support the trailer but not enough to couple positively with the trailer kingpin. This provides the driver with more room between the tractor cab and the trailer front bulkhead in order to couple up the "Suzie" lines with greater ease. Once all lines have been connected, the driver then continues reversing the tractor until the fifth wheel clamps onto the kingpin.

Recommended Coupling Procedure

Slowly reverse the tractor unit in a straight line towards the front of the trailer.
Apply the tractor unit parking brake, stop the engine and remove the keys.
Check the trailer parking brake is applied.
Make any necessary adjustments to the trailer coupling height and slowly reverse the tractor unit under the trailer until the 5th wheel jaws engage.
Apply the tractor unit parking brake, stop the engine and remove the keys.
Carry out a visual check that the 5th wheel jaws have engaged correctly and fit the security “dog clip” or other safety device.
Carry out a second test that the 5th wheel jaws have engaged by selecting a low forward gear and with the trailer brakes still applied slowly pulling forward.
Apply the tractor unit parking brake, stop the engine and remove the keys.
Connect the service airline (yellow) and electrical connections.
Connect the emergency airline (red) and watch for any unexpected movement. (If the trailer moves, immediately disconnect the emergency airline (red) and check that the trailer parking brake has been applied.
Wind up the landing legs and secure the handle.
Fit the number plates and check that the lights work.
Carry out visual and functional vehicle checks, and release the trailer handbrake before setting off.

Published: Tue 30 May 2017

Updated: Thu 11 Apr 2024

Author: Richard Owens

Published by: Don-Bur