The Ram Air Turbine is a small wind-driven backup system used as an emergency power source for aircraft during emergencies. This spring-loaded unit normally sits stowed inside a compartment in the fuselage or wing and deploys when a power loss occurs. Once extended into the slipstream, the RAT derives power from the airstream at aircraft speeds above 130 knots, producing the electricity needed to control and land the aircraft.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is RAT in aircraft?
A Ram Air Turbine (RAT) is used as an alternate or emergency hydraulic or electrical power source, and it is at the heart of an aircraft's emergency power system. This turbine is connected to a hydraulic pump or electrical generator installed in an aircraft.
The RAT is a small wind-turbine that normally sits stowed inside the wing or fuselage. It is called a ram air turbine because it rotates to extract sufficient power from the airstream, using the ram effect of forward speed. In the emergency power system the RAT is an alternate hydraulic power source and an alternate electrical power source and it is connected to a hydraulic pump and to an electrical generator. The moment loss of power includes loss from engines or loss from backup batteries, the RAT is deployed, becoming the emergency hydraulic and electrical power source that lets flight surfaces rendered inoperable due to lack of power move again. Because it is an air-driven generator, the device is alternatively known as an air-driven generator.
What is the purpose of RAT in aircraft?
The purpose of the Ram Air Turbine is to provide an aircraft with emergency electrical and hydraulic power in the event of total failure. When primary and secondary sources are lost, the RAT extends into the slipstream; the extracted power is used to land the aircraft and to control it.
An air-driven generator, the RAT will provide electrical generation so that the emergency bus-bar powers instruments and electronics. Vital systems include flight instrumentation, assuring control of the aircraft is maintained even after failure of the four engines. The turbine delivers alternate hydraulic pressure, eliminating the need to rely on internal sources. In this way, the RAT keeps vital controls and flight instrumentation alive until landing is completed.
Where is the Ram Air Turbine located?
On most commercial jets the RAT is normally stowed inside a compartment within the belly fairing, the lower section of the aircraft's fuselage. When deployed, the small propeller-type unit drops into the slipstream beneath the wing root or slightly aft of the right wing landing gear, depending on the model. On the EA-18G Growler and several fighters, the assembly is tucked on the top of the fuselage, allowing the turbine blade to extend upward into the airflow. Regional jets like the CRJ series store the unit under the nose, just below the first-officer's foot-well, so that it falls forward and downward when required. Whether belly-mounted, roof-mounted or nose-mounted, every installation keeps the RAT clear of control surfaces and engines so that, once it is flung into the airstream, the turbine can spin freely and drive the emergency hydraulic pump or generator without interference.
What are the parts of a ram air turbine?
The parts of a ram air turbine are listed below.
- RAT has turbine blades and hub
- RAT consists of two hydraulic fluid distribution modules
- RAT consists of a variable pitch propeller mounted on the end of a pivoted support leg
- RAT consists of a small turbine
- RAT includes a mounting bracket
The turbine has two or four blades attached to a central hub. These blades are designed to capture oncoming air and rotate quickly. The support leg contains the shaft transmitting turbine drive to the gearbox through bevel gears. The gearbox is attached to the mounting bracket at the leg pivot point. The gearbox transmits turbine torque to the two hydraulic pumps through a system of gears. The support leg contains pipes forming the Green hydraulic fluid loop supplying the blade pitch change system. Green pump pressurizes propeller blade pitch-control system in an event of very low temperature in RAT housing. The mounting bracket is attached to the aircraft structure and has fitted two hydraulic fluid distribution modules. A Green and Yellow system distributor module is attached to the mounting bracket. The distributor modules contain the necessary components for distributing and filtering the hydraulic fluid for each system. The support leg includes a cam which operates the unlocking rollers of the fairing doors when ejection commences.
How does a ram air turbine work?
The RAT works by utilizing the natural airflow produced by the aircraft's movement through the air, generating power from the airstream passing over it as the plane flies. When the unit is deployed, the airflow's force starts spinning the turbine blades, and those rotating turbine blades drive a hydraulic pump or electric generator. The turbine can be connected to a generator directly or via a hydraulic motor. Hydraulic power produced by the pump is also used to power an electrical generator when required.
Most RATs are spring loaded; a loaded spring pushes the device out into the breeze so that the turbine rotates even if no electrical command is available. Because a complete electrical failure leaves no electrical trigger to extend the unit, the mechanical spring ensures automatic deployment. Batteries help power instrumentation during the first seconds after power loss, but once the RAT is out, the airstream alone keeps the turbine blades spinning and the emergency generator turning.
There is no separate duct around the blades. The RAT works in the free airstream, so ram pressure on the exposed propeller-type turbine is the sole energy source. If the spring fails or the turbine seizes, the RAT cannot extend or turn, and power is no longer generated. Normal specifications require only forward speed to keep the turbine turning, and the device continues to supply hydraulic or electrical power for as long as the aircraft keeps moving through the air.
When does a ram air turbine deploy?
Automatic deployment of RAT occurs after loss of all main engine-driven power sources. On many types, RAT deploys automatically if AC BUS 1 and AC BUS 2 are both lost, or if both engines are failed and centre system pressure is low. On the B-777, all five generators plus the APU must fail before the turbine is needed.
Once the aircraft is on the ground with engines shut down, hydraulic demand is supplied by pumps driven from the ground-power cart; RAT does not deploy on the ground. If the crew needs the turbine while the aircraft is still on the ramp, a guarded manual switch permits deployment, but standard procedure reserves this for flight-deck use in the air.
How fast does a ram air turbine spin?
In normal flight the RAT blades often reach speeds between 5,000 and 7,000 revolutions per minute, while the nominal turbine speed is 5,200 rpm. Because the rate of spin of the RAT in the no-load condition is closely related to true airspeed, a self-governing device limits the turbine's speed so that constant speed is reached quickly.
For airliners the minimum airspeed is often around 120-150 knots, and extension is prevented below 100 knots because additional drag appears. Below this minimum speed power is difficult to generate.
What does the ram air turbine power?
The RAT powers a generator. When all normal and back-up electrical channels fail, the ram air turbine drives a 12-kVA generator. This machine feeds electricity through an on-board resonant converter to an emergency bus-bar that powers electronics and instruments. The same emergency bus-bar connects to C1 TRU that powers the captain's flight instruments and to C2 TRU that powers the first officer's flight instruments, so attitude, altitude, air-speed and heading data remain available on both sides of the flight deck.
Hydraulic energy is created at the same moment. The turbine turns a gearbox that drives two hydraulic pumps which pressurize the Green system and the Yellow system. The pressurized fluid is routed to primary flight control components connected to the center hydraulic system and to the flight control actuation lines, giving pilots full authority over pitch, roll and yaw. Ram air turbines can supply electrical and hydraulic power simultaneously, but the total load is limited to the energy extracted from the airstream by ram pressure.
Power produced by RAT depends on the generator rating and on aircraft speed. At normal glide speed the unit supplies roughly 12 kVA of continuous three-phase 115 VAC, enough to keep systems alive and to maintain the autopilot. Ram air turbine generates power from the airstream only; if forward speed drops, delivered watts and hydraulic flow drop in step, so the crew must still reach an airfield while energy remains.
How big is a ram air turbine?
Most RATs measure about 25 to 35 inches (63.5 to 88.9 cm) in diameter, with 80 centimetres (31 in) the common size on commercial transports. The Airbus A380 carries the largest RAT in service: its propeller measures 1.63 metres (64 in), roughly twice the width of a standard unit. Despite the wide blade, a typical large RAT weighs only a few tens of kilograms; the whole assembly is intentionally small so that it can be stowed flush with the fuselage and deploy rapidly when needed.
What affects ram air turbine efficiency?
RAT efficiency is affected by air speed. The faster the aircraft moves through the air, the more wind energy is available to spin the RAT, and hence, the more power it can generate. As the aircraft slows down the RAT's output diminishes. For airliners, the minimum airspeed is often around 120-150 knots, below which electrical and hydraulic loads must be strictly limited.
Worn turbine blades make the RAT less effective at capturing air energy, so maintenance addresses this by overhauling or replacing components at intervals. Increasing the number of blades does not affect the maximum efficiency of the turbine, but it does affect the tip speed ratio where maximum efficiency is obtained. More blades give a steep power curve that peaks at low tip speed ratios, and the turbine with eight blades will rotate slower (lower RPM) than the one with two blades.
Does a ram-air turbine create drag? The RAT increases drag by about 1%. A turbine with a larger area than the smallest possible turbine for a specified power extraction will experience lower drag, because it can extract the required energy at a lower aircraft speed and lower turbine rotational speed, reducing the swirl and wake losses that accompany high-speed rotation.
What is involved in ram air turbine maintenance?
A common maintenance task for RAT is checking hydraulic lines and fittings associated with the RAT pump; leakage is fixed by replacing seals or lines. After deployment tests they inspect for fluid leakage around the RAT pump and the connecting lines and any seepage triggers immediate repair. Maintenance crews use RAT test rigs or hydraulic mules to simulate the conditions needed for RAT operation and to verify correct pump pressure and generator output. Periodically the RAT test unit is inspected for loose fasteners, hose fittings, damaged hoses, and worn data/power cables. Worn parts are replaced to keep the test rig accurate.
Instructions direct a general visual inspection of RAT hub assemblies with affected part numbers and serial numbers for missing balance washers. Hamilton Sundstrand SB 729548-29-18 is followed to replace retention screws and thereby prevent damage caused by separation of balance washers during flight or back-drive operation on the ground. MPD Task 292500-06-1 (10-month interval) was added to extend the RAT and check for correct adjustment of the locking rod, guaranteeing the uplock assembly remains secure and Inspection SB A300-29-6053 supports this check. Uncommanded RAT extensions have the potential to injure maintenance personnel working in the vicinity of the RAT, so the deployment area is treated as outside the usual maintenance operation zone and personnel are briefed on RAT extension logic. If an electrical generator shows lower output, worn bearings or brushes are overhauled or replaced at prescribed intervals.
What are the types of RAT in aviation?
The types of RAT in aviation are listed below.
- Hydraulic RAT: These turbines drive a hydraulic pump, supplying pressurized fluid to critical systems like flight controls.
- Electric RAT: These drive an electrical generator to provide power directly to essential avionic, communication, and navigation systems. These are common on many regional jets and modern aircraft like the Boeing 787.
- Hybrid (Hydraulic/Electric) RAT: These systems are connected to both a hydraulic pump and an electrical generator, providing both types of power simultaneously, which is common on aircraft like the Boeing 777.
Military aircraft must be capable of surviving sudden and complete loss of power, so their RATs are sized to cover emergencies that include a complete loss of primary electrical generation and loss of hydraulic systems. Because Boeing had historically used triple or quad hydraulic systems, their RATs were originally designed to deliver hydraulic flow alone; later models added integral generators so the same turbine can recover both hydraulic and electrical capability. Civil designs follow the same division: a minimal RAT that pressurises the centre hydraulic system, or a larger unit whose hydraulic output first spins a generator through a power-transfer unit. Whatever the type, each RAT incorporates a self-governing device that holds blade pitch and rpm within limits, guaranteeing that hydraulic pressure or generator frequency stays within certification values as aircraft speed varies.
What is the difference between a ram air turbine and an APU?
The difference between a ram air turbine and an APU is that a ram air turbine is a wind-driven emergency unit that flips out into the slipstream to generate electricity and hydraulic pressure only when the normal power sources have failed. An auxiliary power unit (APU) is a self-contained, fuel-burning turbine engine, often mounted in the rear of the fuselage or the main-wheel well, which provides bleed air and electrical power on the ground and, when required, in flight. The APU is not directly connected to the propulsion system and does not provide thrust but instead it starts the main engines, drives the air-conditioning packs, and lets the aircraft power itself independently of ground services. While the ram air turbine is a last-resort, airspeed-dependent device that uses no fuel, the APU is a small jet engine that draws fuel from the aircraft's fuel system and operates whenever the main engines are shut down or additional power is needed.
Do all airplanes have RATs?
No, not all airplanes have RATs. Older Boeing designs did not have RATs; the 737, 747 up to -400, and 737-800 do not have a RAT. Airbus has RATs on all its current models, including the A320, A330, and A350 series. Newer designs and those meant for ETOPS flights have RATs, so the Boeing 757, 787 Dreamliner, and Concorde have a RAT. Military aircraft like the F-16 must be capable of surviving complete loss of power and therefore carry a RAT.
I am of the view that a RAT is a vital part of the security structure for big technical jetliners and long-range industry planes, where a number of vital subsystems rely on electrical and hydraulic power.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
When was RAT first used?
The first RAT was used as early as 1913.
Who makes ram air turbines?
Collins Aerospace, a subsidiary of Raytheon Technologies, stands as the dominant producer, supplying the B787, A320 and A380 Ram Air Turbines and offering electric, hydraulic and hybrid RAT designs. Honeywell Aerospace manufactures the Part Number 2780222 7 RAT. Safran is another active producer and ranks top by geographic reach. ATGI designs and manufactures the world's most robust Ducted Ram Air Turbine (HiRAT) for commercial, military, emergency power, ISR and electronic-warfare applications. Additional key industry participants include Lockheed Martin, Rolls-Royce and Airbus, all of which are patent filers in the RAT sector.
