If all AC power is lost, what system is deployed as a backup?

In the scenario where all AC power is lost, the Air Driven Generator (ADG) is deployed as a backup system. The ADG provides essential electrical power to critical systems in the aircraft, ensuring that vital equipment can continue operating during power outages. It is specifically designed to activate quickly and deliver power as needed, maintaining operational capabilities and safety for the crew and passengers.

The other systems mentioned, while also important, do not serve the same primary function in terms of providing backup electrical power. Battery systems primarily provide power for instrumentation and certain electronic devices but may not supply the entire load required. Emergency lights are crucial for visibility in the event of an evacuation or during low-light situations but do not support flight-critical functions. Pneumatic systems relate to aircraft controls and other non-electric functions, and they do not have the capacity to substitute for AC electrical power loss.