What is the function of the emergency air line in a trailer?

The function of the emergency air line in a trailer is crucial for safety and operational control. This line is designed to supply air to the trailer's air tanks and controls the emergency brakes. When a trailer becomes disconnected from the towing vehicle, the emergency air line allows the trailer's brakes to engage automatically. This prevents the trailer from rolling away, enhancing safety on the road.

In addition to providing air to the brake system, the emergency air line also serves as a critical safety feature. It ensures that, in case of a disconnection, the brakes will activate to bring the trailer to a stop, rather than allowing it to pose a hazard. This is essential for both the trailer's security and that of other road users.

The other options don't align with the primary purpose of the emergency air line, as they refer to different systems or functionalities that are not related to braking or safety. For example, the refrigeration unit and height adjustment are governed by separate systems and lines, while draining water from the air system is a maintenance function unrelated to emergency brakes.