RAT means Ram Air Turbine. We use the RAT when we losted all of the electric and hydraulic power in the aircraft. Therefore the RAT is very important equipment for the our aircraft.
Aircrafts have electric generators working as connected to engines and APU. In addition there are batteries for providing electrical power in the aircraft. There are pumps driven by engines and driven by electric system for providing hydraulic power in the hydraulic systems of aircraft.
If we losted engines, electric generators and engine driven pumps could disable. (This equıpments could disable because of another reasons.)
Subsequently electric system is fed by generators in the APU. Thus electrical power availables for primary systems of aircraft. And electrical pumps in the hydraulic systems are used for providing to hydraulic power.
If APU fails or not available, batteries become active. Electric and hydraulic power become available status in the same way.
Last remedy for providing electric and hydraulıc power is RAT when batteries be discharged.
Ram Air Turbine extends when all of these occur. So if we losted hydraulic and electric power completely, RAT becomes active. RAT produces electrical and hydraulic power for primary systems by using movement energy providing through wind. RAT feeds emergency situation generator in the electric system and hydraulic pump attached to one from the hydraulic systems. (This situation can change according to aircraft)
So RAT has been feeding primary hydraulically and electrically equipments as limited in these cases. (These primary equipments can be those: Primary flight display, engine and warning display, navigation display and some controls in the cockpit. Flight controls, some communication and navigation systems.)
RAT located in the under of aircraft fuselage or under of wing. RAT is a vitally important equipment for aircrafts.
RAT can become active as automatically or manually. Flight crew can control RAT by using overhead panel.
So there are backup of many systems in the aircraft. We can understand reliability and safety of aircrafts because of these cases.