After coupling a trailer to the tractor, where should the tractor protection control valve be placed?

The tractor protection control valve should be placed in the normal position after coupling a trailer to the tractor because this position allows for the proper functioning of the air brakes in both the tractor and the trailer. When the valve is set to normal, it ensures that the service brakes will apply when the brake pedal in the tractor is pressed, thus maintaining control of both the tractor and the trailer while in motion.

Setting the valve to normal also ensures that the air supply to the trailer's braking system is active, allowing the airbags to remain inflated and functional during travel. This setup is crucial for safety, as it prevents the trailer from disconnecting from the tractor and ensures that the braking system is fully operational.

Other positions such as off, emergency, or service change the operation of the brake system. The off position would prevent any air brakes from functioning at all, while the emergency position would engage the emergency brakes, which is not necessary once the vehicle is coupled and ready to drive. The service position is typically used during normal braking operation but isn't the correct choice immediately after coupling. Therefore, placing the valve in the normal position maintains optimal safety and control over the vehicle combination.