Understanding the Ram Air Turbine (RAT) in Aircraft

What is a Ram Air Turbine (RAT)?

A Ram Air Turbine (RAT), also known as an air driven generator (ADG), is a small but critical emergency power system found on many aircraft. Picture a miniature wind turbine deploying from the aircraft’s fuselage or wing. It uses the force of oncoming ‘ram air’—generated by the plane’s forward motion—to spin its propeller-like blades.

The RAT’s primary function is to provide emergency power as a last resort when all other onboard sources fail. In the rare event of a complete engine and auxiliary power unit (APU) failure, an aircraft loses its main electrical and hydraulic systems. This is when the RAT deploys—automatically or manually—to power the most critical flight instruments and hydraulic controls.

Once deployed, the spinning turbine connects to a hydraulic pump or an electrical generator. It doesn’t power the entire aircraft. Instead, it generates just enough energy for the pilots to command the vital systems needed to fly and control the plane. This lifeline of power allows the flight crew to navigate and execute a safe landing, turning a potential catastrophe into a manageable emergency.

How Does a Ram Air Turbine Work?

A RAT’s operation follows a precise sequence, triggered only in critical situations. During normal flight, it remains securely stowed within the aircraft’s fuselage or wing.

Once the command is given, a release mechanism, often spring-loaded, pushes the RAT out into the slipstream surrounding the aircraft. The force of the air rushing past the plane—known as ram air—immediately begins to spin the turbine’s propeller-like blades. The principle is similar to how a windmill harnesses wind, but the RAT operates in a much higher-speed environment, allowing it to generate significant power from a relatively small device.

The spinning turbine converts the air’s kinetic energy into mechanical energy. This power is then channeled to an integrated hydraulic pump or an electrical generator. The output is directly proportional to the aircraft’s speed: the faster the plane flies, the faster the blades spin and the more power is generated. As long as the aircraft is moving through the air, it has a source of power.

This emergency power isn’t for non-essentials like cabin lighting or entertainment. Instead, it’s routed directly to the most vital flight systems. These typically include the hydraulic actuators for primary flight controls—ailerons, rudder, and elevators—and the essential flight deck instruments that provide pilots with critical data on altitude, speed, and heading. By powering these key functions, the RAT allows the flight crew to maintain control and execute a safe landing.

Military Aircraft Equipped with Rats

In military aviation, Rats do more than just power flight controls; they sustain mission-critical systems. This capability is vital in combat, where it can mean the difference between losing and recovering a damaged aircraft. Historically, their applications were even more specialized, powering everything from electronic warfare pods and the M61A1 Vulcan cannon to the firing circuits for British nuclear weapons like Yellow Sun and Red Beard.

• F-16 Fighting Falcon

• F/A-18 Hornet

• A-10 Thunderbolt II

• Eurofighter Typhoon

Civilian Aircraft Incidents Involving RAT Deployment

The RAT’s life-saving capability is no mere theory.

• Air Transat Flight 236 (2001): After a fuel leak caused a dual-engine flameout over the Atlantic, the RAT on its Airbus A330 powered essential controls, enabling the aircraft to glide for over 120 km to a safe landing.

• Air Canada Flight 143 (1983): Known as the “Gigli Glider,” this Boeing 767 ran out of fuel mid-flight. The RAT supplied hydraulic pressure, allowing pilots to control the aircraft during its unpowered descent.

• US Airways Flight 1549 (2009): Following a bird strike that disabled both engines on an Airbus A320, the RAT instantly powered the fly-by-wire controls, critical for the successful ditching in the Hudson River.

Specifications and Performance of Ram Air Turbines

The power a Ram Air Turbine generates isn’t one-size-fits-all; its specifications are carefully matched to the aircraft it protects. Typically, these units produce between 5 and 25 kilowatts (kW) of emergency power. This range is dictated by the aircraft’s size and its specific emergency needs. A large wide-body airliner, for instance, requires a more powerful RAT for its complex flight controls than a smaller regional jet does. Consequently, the turbine’s physical size and blade design are scaled to guarantee sufficient power generation upon deployment.

Rats are designed in three main categories to meet different system needs:

• Hydraulic RAT**: Directly drives a hydraulic pump to restore pressure to essential flight controls.

• Electric RAT**: Connects to a generator to power critical cockpit instruments, navigation systems, and communications.

• Hybrid RAT**: Powers both a hydraulic pump and an electrical generator, providing a comprehensive backup for modern fly-by-wire aircraft.

An aircraft’s speed is the critical factor governing a RAT’s performance. The turbine’s power output is directly proportional to the ram air pressure from the plane’s forward motion. The faster the aircraft glides, the more power the RAT generates. This direct relationship makes maintaining an optimal airspeed during an emergency descent crucial. It ensures a steady supply of power, giving the flight crew the resources they need to control the aircraft and land it safely.

Safety and Regulatory Aspects of Rats

The RAT is a key component of aviation safety, embodying the principle of redundancy. For modern aircraft with fly-by-wire controls—which are entirely dependent on electrical and hydraulic power—it serves as a final safety net, ensuring pilots can maintain control even after a total power failure.

Global aviation authorities like the FAA and EASA strictly regulate aircraft design and safety. Their rules mandate that commercial aircraft must have an independent emergency power source for essential flight instruments and controls. As a proven and widely adopted solution, the RAT allows manufacturers to meet these critical airworthiness standards, making it integral to an aircraft’s certification.

To be approved for use, a RAT must pass exhaustive testing to guarantee its reliability. This certification process involves demonstrating successful deployment under a wide range of flight conditions and proving it can consistently produce the specified power level across different airspeeds and altitudes. This rigorous process ensures that in a real-world emergency, the RAT will perform its life-saving function exactly as designed.

These high safety standards extend to both manufacturing and ongoing maintenance. Manufacturers like Collins Aerospace must adhere to stringent aerospace quality and production protocols. Once an aircraft enters service, its RAT is placed on a strict, regulator-approved maintenance schedule. This requires airlines to conduct regular inspections and functional checks, verifying the system’s readiness and ensuring this critical backup is prepared to deploy at a moment’s notice.