Under what condition does the ELS operate automatically?

The correct answer is that the Emergency Lighting System (ELS) operates automatically once aircraft power is lost. This system is designed to ensure that emergency lighting is available to guide passengers and crew safely to exits during a critical situation, such as a power failure.

When there is a loss of power, the ELS activates automatically to provide illumination in the cabin and along the escape routes. This feature is crucial because in scenarios where cabin lights may fail, the ELS ensures that all emergency exits remain visible, enhancing the safety and evacuation process for everyone on board.

The other conditions reference actions that may be taken by crew members or pilots during their duties but do not directly trigger the automatic operation of the ELS. For example, the activation by cabin crew or at the request of the pilot is a manual intervention, and it is not the primary function of the ELS. Similarly, while emergency landings may require lighting, the ELS specifically responds to loss of power rather than other operational conditions.