One of the most dramatic pieces of equipment on some airliners is normally invisible. If particular combinations of electrical or hydraulic power are lost, a small turbine can deploy into the airflow beneath the fuselage or wing. It is called a Ram Air Turbine, or RAT. Despite the popular description of it as a device that “powers the plane,” its real job is more precise: provide a limited emergency source of hydraulic and/or electrical power so essential systems remain available while the crew restores normal generation or completes a safe diversion and landing.[1][2]
The short answer
A RAT is a deployable wind-driven turbine. Forward motion forces air through its blades, causing it to rotate. Depending on aircraft design, that rotation can drive a hydraulic pump, an electrical generator, or a hydraulic system that in turn powers an emergency generator. It is deliberately much smaller than the aircraft’s normal power sources and is intended to support essential loads rather than every galley, entertainment system and cabin service.
Why an airliner needs emergency power
Modern transport aircraft depend on electrical and hydraulic power for flight controls, instruments, navigation, communications, pumps and computers. Normal power comes from engine-driven generators and pumps, with auxiliary and battery sources providing additional capability. Certification requires failures to be considered so that no single foreseeable loss leaves the aircraft without the functions necessary for continued safe flight and landing.
The RAT is one layer, not the whole safety system
An aircraft with a RAT does not normally jump directly from full electrical power to a tiny windmill. There may be multiple engine generators, an APU generator, batteries, transformer rectifiers, static inverters and separate hydraulic systems. The RAT sits deeper in the redundancy architecture. Its value appears when several normal sources have become unavailable simultaneously.
How airflow becomes useful power
The physics is straightforward. Air approaching the turbine has kinetic energy relative to the moving aircraft. Aerodynamic forces on the blades create torque, turning a shaft. That mechanical rotation can drive a pump or generator. Extracting energy creates drag, so the RAT is not deployed during normal flight. Emergency reliability matters more than the small performance penalty once it is required.
A RAT needs airspeed
Because power comes from airflow, output depends on aircraft speed and atmospheric conditions. Emergency procedures and system design account for this. The turbine must provide sufficient useful power across the relevant part of the emergency flight envelope, but output at low speed cannot equal output at high speed indefinitely. This is one reason batteries remain important during transitions and near the ground.
Electrical RAT architectures
Some aircraft use the RAT to drive an electrical generator directly. Certification material describes a typical transport-category architecture with two active engine-generator channels and a third passive backup electrical channel provided by a RAT. The turbine remains stowed in normal operation and deploys automatically and/or manually after loss of the main engine-driven generation.[2]
Hydraulic RAT architectures
On other aircraft, the RAT primarily pressurises a hydraulic system. That hydraulic power may keep selected flight-control actuators available and may also drive an emergency electrical generator through a hydraulic motor. Airbus A330 regulatory material, for example, discusses RAT deployment in the context of providing hydraulic power for emergency electrical generation.[1] The exact architecture must therefore be described aircraft by aircraft.
Why it is wrong to say every RAT does the same thing
RAT diameter, deployment logic, rated output and the systems supplied differ between types. Some aircraft do not use a RAT at all because their emergency-power architecture uses other solutions. Even within one manufacturer, different families can route RAT power differently. The universal concept is emergency energy extraction from the airstream, not one universal wiring diagram.
Automatic deployment
On many types, a defined loss of normal power causes automatic deployment. The logic is designed to recognise a severe failure condition without requiring the crew to spend precious seconds diagnosing every source. Manual deployment is also commonly available. Exact trigger logic is aircraft-specific and belongs in approved flight-crew documentation rather than generic internet instructions.
Deployment is generally irreversible in flight
Once deployed, many RAT installations cannot simply be retracted from the cockpit. The turbine remains in the airflow until maintenance personnel reset and stow it on the ground. This simplifies emergency reliability: the mechanism is optimised to deploy when needed rather than repeatedly cycle like normal landing gear.
Why the turbine does not overspeed
A turbine exposed to high-speed airflow needs speed control. RAT designs use aerodynamic blade governing and/or mechanical regulation to keep rotational speed within an intended range as aircraft speed changes. Without governing, a turbine sized to produce adequate low-speed power could overspeed destructively at high airspeed.
What stays powered
Emergency electrical distribution sheds non-essential loads. Essential flight instruments, selected flight-control computers, communications, navigation and other safety-critical equipment receive priority according to the aircraft architecture. Cabin entertainment, many galley loads and comfort systems are expendable. The purpose is not to preserve a normal passenger experience; it is to preserve control and situational awareness.
Load shedding
Electrical systems are divided into buses so equipment can be disconnected in stages. If generation falls, automatic logic can remove lower-priority loads and protect essential buses. A RAT may therefore have a surprisingly modest power rating compared with the aircraft’s normal generation because the emergency network has been deliberately reduced to what is necessary.
Batteries bridge the gap
RAT deployment takes time and useful output depends on airflow. Aircraft batteries can supply critical DC loads during the transition and may support equipment not directly powered by the RAT system. Batteries are also finite energy stores, which is why emergency procedures manage them carefully. The RAT’s advantage is that it can keep extracting energy from airflow as long as sufficient speed remains.
What about the APU?
The auxiliary power unit is a small gas turbine that can often generate electrical power independently of the main engines. In some in-flight failures, crews may start the APU and restore a much larger source of normal-style electrical power. The RAT then remains a backup or bridge. APU start capability, altitude limits and system configuration vary by aircraft.
Why engine failure does not normally deploy the RAT
A twin-engine airliner losing one engine still has the other engine’s generator and usually multiple remaining system paths. RAT deployment is generally associated with more extensive loss of power sources, not the routine certified one-engine-inoperative case. This distinction matters because online explanations often imply the RAT appears whenever an engine stops.
Flight controls and the RAT
Large airliners use powered flight controls because aerodynamic loads are too high for direct human muscle power. Multiple hydraulic or electrical channels provide redundancy. In a severe power-loss scenario, RAT-supported hydraulic pressure or electrical generation can preserve a reduced but adequate subset of control capability. Which surfaces remain powered depends entirely on the aircraft.
Emergency generation is designed around safe landing
EASA material concerning the A330 discusses emergency generation capability in terms of providing sufficient duration and power for safe landing and go-around considerations.[1] That illustrates the certification objective: emergency systems are not designed to recreate normal cruise indefinitely; they are sized so the aircraft retains the functions needed to reach a safe outcome.
Why landing configuration can be demanding
Extending flaps and slats, lowering landing gear and moving flight-control surfaces can create significant hydraulic and electrical demand. Emergency architecture therefore has to account for the phase when the aircraft is slowest and system demand may increase. Procedures may sequence configuration changes differently under emergency power to protect available capacity.
Testing the RAT
Maintenance programmes include checks of deployment mechanisms, pumps or generators and associated controls. A backup that remains stowed for years must still work immediately when required. Testing may use specialised ground equipment to spin or load the system without relying on actual flight airflow. Exact maintenance procedures are restricted to approved technical data and qualified personnel.
Why reliability is unusual
Emergency equipment faces a paradox: it may be used extremely rarely but must have high confidence of operation. Engineers reduce unnecessary complexity, protect components from contamination and corrosion, monitor system status and require periodic maintenance. Reliability is achieved through design assurance and maintenance rather than frequent real-world use.
The RAT creates noise and vibration
A deployed turbine is an aerodynamic machine attached to the airframe, so occupants may hear additional noise or feel vibration. That does not mean the turbine is malfunctioning. The exact acoustic signature depends on installation location and aircraft structure. During a real emergency, passenger comfort is secondary to maintaining essential power.
Drag is acceptable
Any device extracting energy from the airflow must increase drag. The RAT therefore slightly worsens glide or fuel performance compared with a clean aircraft. But the safety value of retaining essential controls and instruments overwhelmingly outweighs that penalty. Emergency flight planning and procedures account for the changed configuration.
Common misconception: the RAT can restart the engines
The RAT is not normally an engine starter. Its role is to supply essential power. Engine relight attempts depend on fuel, ignition, starter or windmilling capability and aircraft procedures. The turbine can help keep the systems needed to manage the situation alive, but it does not function like a giant external starter motor.
Common misconception: it powers everything
Normal airliner generators can supply very large electrical loads. A RAT is intentionally limited. Emergency buses isolate the equipment necessary for safe flight and shed comfort or commercial loads. If the cabin suddenly becomes darker and many systems are unavailable, that may be exactly what the emergency architecture was designed to do.
Why some aircraft use different emergency systems
Aircraft certification specifies safety objectives rather than mandating one identical component for every design. Manufacturers can meet emergency-power requirements using different combinations of batteries, permanent-magnet generators, hydraulic accumulators, APUs and RATs. The chosen architecture reflects aircraft size, flight-control design and electrical philosophy.
The engineering lesson
The RAT is a physical expression of redundancy. Instead of assuming multiple engine-driven systems can never fail together, designers provide a source that depends on something more fundamental: the aircraft moving through the air. That independence is what makes the system valuable.
Conclusion
A Ram Air Turbine is not a miniature engine and it does not return an aircraft to normal operation. It converts airflow into limited emergency hydraulic or electrical power, keeping essential systems alive after major failures of normal sources. Its small size hides a sophisticated role in the aircraft’s redundancy architecture. Most passengers will never see one deployed—and that is exactly how an emergency backup should spend almost all of its life.
Sources / Technical References
- [1] EASA Airworthiness Directive material, Airbus A330 emergency electrical generation and RAT deployment — https://ad.easa.europa.eu/blob/easa_ad_2014_0273_superseded.pdf/AD_2014-0273_1
- [2] EASA/CATA certification material describing RAT emergency electrical architecture — https://www.easa.europa.eu/en/downloads/138330/en
- [3] FAA, 14 CFR Part 25 transport-category electrical-system requirements — https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-25
- [4] Airbus technical and safety publications — https://aircraft.airbus.com/
Disclaimer: RAT architecture and deployment logic differ by aircraft. This is general education and not a substitute for aircraft flight, maintenance or emergency procedures.
