AeroTech
Think of the rat as your last chance. The rat will provide power (hydraulic or electric) to the Bare Minium items on that aircraft to fly and hopefully land.
So what do you need to fly? Airspeed indicator, altitude indicator, Airtfical horizon, then you'll need to talk to someone so you'll need a radio.
Everything else from then on depends on the aircraft type, gear will free fall, you might need hydraulics to power flight controls, etc.
So depending on the type of aircraft you want to talk about, you will have different setups, different requirements, etc.