What is essential to do after placing the tractor-trailer in a stationary position?

Engaging the emergency brakes after placing the tractor-trailer in a stationary position is crucial for safety. Doing so ensures that the vehicle remains securely in place, especially on inclines or uneven terrain. This action prevents the trailer from rolling or shifting, which could pose risks to the driver, surrounding vehicles, or pedestrians nearby.

While securing the load, checking mirror adjustments, and confirming the air supply are important operational practices, they are not the immediate priorities once the vehicle comes to a stop. Securing the load is an essential task but is typically performed before the vehicle is stationary. Adjusting mirrors is necessary for safe driving, and ensuring air supply functionality is vital for operating air brakes, but these actions pertain more to vehicle operation and safety while in motion rather than immediately after stopping. Engaging the emergency brakes is a direct response to maintaining control of the stationary vehicle, thus making it the most essential action in this scenario.