When coupling a tractor to a semi-trailer, when should the air lines be connected to the trailer?

When coupling a tractor to a semi-trailer, it is essential to connect the air lines to the trailer after the kingpin has been securely connected to the fifth wheel. Connecting the air lines after the kingpin engagement serves multiple purposes.

First, connecting the air lines before securing the kingpin may lead to damage to the lines if there is any movement or misalignment during the coupling process. By ensuring that the kingpin is properly locked in the fifth wheel first, the tractor and trailer are securely linked before air supply lines are connected.

Additionally, only after the kingpin is engaged and the fifth wheel is locked does it become appropriate to ensure that the tractor's air supply can activate the trailer's brakes and other systems. This sequence helps ensure that there is no accidental release of the trailer while attempting to make pneumatic connections.

This process is important for maintaining safety standards, ensuring that the trailer's air brakes are functional, and helping prevent any operational difficulties once the coupling is complete.