A ram air turbine, commonly called a RAT, is a small wind-powered emergency device built into most large commercial and military aircraft. It deploys into the airstream when an airplane loses its primary power sources, using the force of onrushing air to spin a turbine that generates just enough hydraulic or electrical power to keep critical flight controls and instruments working. Think of it as an aircraft’s absolute last resort for staying flyable long enough to land safely.
How a Ram Air Turbine Works
The concept is straightforward. A small propeller or fan blade, usually stored in a compartment in the fuselage belly or wing root, drops into the airflow beneath the aircraft. As the plane moves forward, air hits the blades and spins them at high speed. That spinning motion drives either a hydraulic pump, an electrical generator, or both, depending on the aircraft design.
A hydraulic RAT pressurizes fluid in a dedicated emergency hydraulic circuit, which keeps flight control surfaces (ailerons, elevators, rudder) responsive. An electrical RAT spins a generator rotor to produce electricity for core avionics, flight displays, and radios. Some aircraft use a combination, splitting the turbine’s output between hydraulic and electrical needs. Either way, the power output is modest. It covers only the bare essentials: maneuvering the aircraft, navigating, and communicating with air traffic control.
When the RAT Deploys
The RAT is strictly an emergency system. It activates when the aircraft suffers a total loss of its normal power sources, typically a dual engine failure or a complete electrical system failure. On most airliners, deployment is automatic once sensors detect that both engine-driven generators and other backup systems have failed. Pilots can also deploy it manually from the cockpit if they recognize a cascading power loss.
RAT deployment signals a serious emergency. When one drops into the airstream, the flight crew is coordinating with air traffic control, declaring an emergency, and working toward the nearest suitable runway. The turbine buys time, but it does not restore the airplane to anything close to normal operations.
What It Powers and What It Doesn’t
Because the RAT produces limited power, it feeds only the systems a crew absolutely needs to fly and land the airplane. Those essentials include basic flight controls, a core set of flight instruments, and communication radios. Everything else gets shed. Autopilot, anti-icing systems, passenger cabin services, overhead lighting, in-flight entertainment, and some cockpit displays all go dark. The airplane is flyable but stripped down to its most basic configuration.
The turbine also needs forward airspeed to keep spinning. For most airliners, the minimum effective speed is roughly 120 to 150 knots. Below that threshold, the RAT can no longer sustain useful power output. This means the crew must maintain sufficient speed throughout the approach and landing, which influences how they manage the descent.
Tradeoffs of RAT Deployment
Once deployed in flight, a RAT generally cannot be retracted until the aircraft is on the ground. The protruding turbine and its mounting hardware create aerodynamic drag, which shortens the airplane’s glide range if the engines are not running. Pilots have to factor this drag penalty into their calculations when planning where and how to land. It is a worthwhile tradeoff, since without the RAT there would be no hydraulic pressure or electrical power at all, but it does reduce the distance the airplane can cover in a powerless glide.
How the RAT Differs From an APU
Aircraft carry another backup power source called the auxiliary power unit, or APU, a small jet engine typically located in the tail. The APU runs on jet fuel and can produce substantial electrical and pneumatic (compressed air) power. Airlines use it routinely on the ground for air conditioning and engine starts, and it can serve as an in-flight backup if one generator fails.
The key difference is capability and role. The APU is a self-contained engine that can produce enough power to run most aircraft systems comfortably. The RAT produces a fraction of that output and exists solely for worst-case scenarios where the APU has also failed or is unavailable. If the APU is a backup generator for your house, the RAT is a flashlight you keep in the kitchen drawer for when everything else has gone out.
Not every airliner carries a RAT. The Boeing 737, for example, relies on dual engine generators, a standby battery system, and its APU for redundancy instead. Airbus designs, including the A320 family and A380, incorporate RATs as standard equipment. The design philosophy varies by manufacturer, but aircraft with fly-by-wire controls (where electronic signals replace direct mechanical links to flight surfaces) are especially likely to include a RAT, since losing all electrical power in a fly-by-wire airplane would otherwise mean losing the ability to control it.
Real-World RAT Deployments
The most well-known RAT deployment happened during the “Miracle on the Hudson” in January 2009, when US Airways Flight 1549 lost both engines after striking a flock of geese shortly after takeoff from New York’s LaGuardia Airport. The Airbus A320’s RAT deployed automatically, providing hydraulic power that kept the flight controls functional while Captain Chesley Sullenberger glided the airplane to a successful ditching in the Hudson River. Without the RAT, the crew would have had extremely limited ability to steer the aircraft.
Another notable case occurred in 2001 when an Air Transat Airbus A330 ran out of fuel over the Atlantic Ocean due to a fuel leak. Both engines flamed out, and the RAT deployed, giving the crew enough hydraulic and electrical power to glide the airplane to an emergency landing in the Azores. The plane covered roughly 75 miles with no engine power, relying on the RAT the entire way.
These incidents illustrate exactly what the RAT is designed to do: keep a powerless airplane controllable long enough for skilled pilots to reach a safe landing spot. It is a small, mechanically simple device, but in the rare moments when it is needed, it can make the difference between a survivable emergency and a catastrophic one.