Now we'll talk about a RAT on a plane. A ram air turbine (RAT) is a small wind turbine that is connected to a hydraulic pump, or electrical generator, installed in an aircraft and used as a power source. The RAT generates power from the airstream by ram pressure due to the speed of the aircraft.

Modern aircraft generally use RATs only in an emergency. In case of the loss of both primary and auxiliary power sources the RAT will power vital systems (flight controls, linked hydraulics and also flight-critical instrumentation). Some RATs produce only hydraulic power, which is in turn used to power electrical generators. In some early aircraft (including airships), small RATs were permanently mounted and operated a small electrical generator or fuel pump.

Modern aircraft generate power in the main engines or an additional fuel-burning turbine engine called an auxiliary power unit, which is often mounted in the rear of the fuselage or in the main-wheel well. The RAT generates power from the airstream due to the speed of the aircraft. If aircraft speeds are low, the RAT will produce less power. In normal conditions the RAT is retracted into the fuselage (or wing), and is deployed manually or automatically following complete loss of power. In the time between power loss and RAT deployment, batteries are used.

On the B787, the RAT extends automatically if any of the following occur:

Ram Air Turbine (RAT) Generator

• both engines are failed

• all three hydraulic system pressures are low

• loss of all electrical power to captain’s and first officer’s flight

instruments

• loss of all four EMPs and faults in the flight control system occur on

approach

• loss of all four EMPs and an engine fails on takeoff or landing