Answer:
In flight, the ADG (Air Driven Generator) auto-deploy system monitors the three main AC generators and the main AC buses for a complete AC power failure. If all three generators fail to provide AC power and both AC buses are not powered, the ADG will automatically deploy. The ADG is an emergency power source that is designed to provide electrical power in the event of a complete loss of AC power. It is typically activated when the normal sources of electrical power are unavailable or have failed.