Imagine looking out of an aircraft window and seeing a small propeller drop from beneath the plane. It is not an engine starting up or a strange attachment. It could be one of the aircraft's most important emergency devices: the Ram Air Turbine, known as the RAT.

Usually hidden inside a compartment in the fuselage or wing area, this compact turbine is designed to deploy when certain critical aircraft power systems fail.

How can wind power an aircraft?

A RAT works on a simple principle. As the aircraft moves through the air, the airflow rushes past its deployed blades and makes them spin. That rotation drives an electrical generator, a hydraulic pump, or a combination of systems, depending on the aircraft's design.

Hydraulic power uses pressurised fluid to operate selected equipment. Electrical power can support essential instruments, flight-control computers, navigation and communication systems. The exact equipment supplied varies between aircraft types.

When does it deploy?

The RAT is normally stowed away during routine flight. Depending on the aircraft, it may deploy automatically after specified failures or be deployed by the flight crew using an emergency control.

The aim is to keep selected essential systems available when normal electrical generation or hydraulic power is lost. Batteries may support critical equipment during the transition while the turbine deploys and reaches operating speed.

Can it save a plane?

It can be an important part of the safety system, but its capabilities have limits. A RAT provides only the power its particular design can deliver, and its output depends partly on airflow and aircraft speed. It cannot restore every onboard system or guarantee a safe landing by itself.

If an aircraft loses engine thrust, it may still be able to glide because an aircraft in flight can generate lift while moving through the air. The RAT's role is different: it helps maintain essential power so pilots can use the systems available to manage the emergency.

Commercial aircraft also have multiple layers of protection, which can include engine-driven generators, auxiliary power, batteries and redundant hydraulic systems. The RAT is one part of that wider design, not the only backup.

A small device with a serious job

Most passengers will never see a RAT deployed. But its design reveals something remarkable about aviation: engineers plan for situations that are rare but must be managed carefully.

The next time you board a flight, remember that even the air rushing past the aircraft can become a source of emergency power when needed. Sometimes, the smallest device hidden in a large machine plays a crucial role in keeping essential systems working.