What is the first way ELS operates?

The first way ELS (Emergency Lighting System) operates is automatically after aircraft power loss. This is a critical safety feature designed to ensure that emergency lighting is activated in case of a failure in the aircraft's normal power supply. When the main power is lost, the ELS is engineered to switch on without any manual intervention, thereby providing immediate illumination to guide passengers and crew to exits and safe areas. This automatic response is essential in emergency situations, as it enhances the overall safety of the aircraft by ensuring visibility even when the normal lighting systems are non-operational.

This approach contrasts with the other methods listed. For instance, manual activation from the AFT CSA panel or through a switch in the flight deck requires human action, which may not be reliable during critical situations. Using an external battery as a power source would not be the primary mode of operation in the event of main power loss, making automatic activation the most effective and immediate response for the ELS.