Every airliner and many military jets carry a small propeller that the passengers hope never to see. It sits folded inside the fuselage or a wing fairing, and it only comes out on the worst day of a crew's career: when every engine-driven generator and the auxiliary power unit have stopped.
It is called a ram air turbine, or RAT. It is a small wind turbine that swings out into the airflow and spins, driven by nothing more than the aircraft's own speed, to power a hydraulic pump or an electrical generator. With it, a crew keeps the flight controls and the instruments they need to land.
Kurzinfo
- What it is: A small wind turbine driving a hydraulic pump or electrical generator
- When it is used: On modern aircraft, mainly in emergencies after loss of all primary and auxiliary power
- Einsatz: Manually or automatically; batteries bridge the gap until it spins up
- Typical output: About 5 to 70 kW on large airliners; small models as little as 400 W
- Largest: Airbus A380, about 1.63 m in diameter
- Military examples: F-102 Delta Dagger, F-104 Starfighter, Me 163 Komet; also used in EA-18G jamming pods
How a RAT works
A RAT is essentially a windmill run backwards. In normal flight it is stowed in a bay. When the aircraft loses its main power sources, it is deployed, either by the crew or automatically, and the airflow from the aircraft's forward speed spins its blades.
That rotation drives a hydraulic pump, an electrical generator, or both. The power is limited, so it is reserved for what matters most: flight controls, the hydraulics linked to them and flight-critical instruments. In the seconds between the power failure and the RAT spinning up, batteries keep things alive.
There is a catch built into the physics. Because the RAT is powered by airspeed, it produces less as the aircraft slows down, which is exactly when the crew is trying to land. That is why RATs are carefully sized for the lowest speed at which the aircraft must still be controllable.
A very old idea
Small permanently mounted air turbines are among the oldest power sources in aviation. The Messerschmitt Me 163 Komet rocket fighter of WWII used a small nose-mounted turbine as its only source of electrical power, and Cold War fighters such as the Convair F-102 Delta Dagger and the F-104 Starfighter carried RATs for emergencies.
Not every fighter uses one. The F-16, for instance, relies on an emergency power unit fuelled by hydrazine instead. Some military systems use RATs in normal operation: the AN/ALQ-99 jamming pods carried by the EA-18G Growler are powered by their own nose turbines, so they can hang on a standard pylon without a dedicated power supply.
From the VC10 to the A380
Many airliners from the Vickers VC10 of the 1960s onwards have carried a RAT. The largest in the world is fitted to the Airbus A380, at about 1.63 metres across, while around 80 centimetres is more typical. Depending on the generator, a large airliner RAT can produce roughly 5 to 70 kW.
RATs are checked on the ground and exercised during flight testing. The Emirates A380 in the video below landed at the Airbus plant at Hamburg Finkenwerder with its RAT spinning.
When the RAT saved the day
The list of incidents in which a RAT was deployed reads like a history of aviation's most famous emergencies. It includes the 1983 Gimli Glider, when an Air Canada 767 ran out of fuel, the 2001 Air Transat Flight 236 that glided to the Azores after losing both engines, and the 2009 landing of US Airways Flight 1549 on the Hudson River.
In each case, the turbine did not save the aircraft on its own. It gave a highly skilled crew the hydraulic or electrical power they needed to keep flying the aeroplane while they solved the problem. That is all a RAT is designed to do, and on those days it was enough.
Sources: Wikipedia (Ram air turbine), MiGFlug
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