On automatic elevators, the swing door must close by what mechanism if the car leaves the landing zone?

In an automatic elevator system, the swing door is designed to ensure passenger safety and efficient operation. When the elevator car leaves the landing zone, it is crucial for the door to close automatically to prevent any potential hazards, such as passengers accidentally stepping through an open door.

An automatic door closer is specifically engineered to manage the closing action of the door without requiring manual effort from the user. This mechanism ensures the door closes reliably and efficiently after the elevator has departed the landing area, minimizing the risk of accidents and providing a seamless experience for passengers.

Other options, such as a manual door closer or a spring-loaded closer, would not provide the level of efficiency and safety required in an automatic elevator system. A manual door closer relies on user intervention, which is contrary to the automatic operation of the elevator. A spring-loaded closer, while it can assist in closing the door, may not offer enough control or consistency for the function needed in automatic elevator systems. A gravity closer, designed to utilize the weight of the door to close it, would also not be ideal in maintaining the precise operation expected in such elevators. Therefore, the use of an automatic door closer is the most appropriate and effective choice.