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I once heard, or misheard, this:

There is an emergency propeller that comes down in an emergency of jet-engine aircraft. It is behind the main landing gears.

Is this true or a story somebody made up?

Ralph J
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Ian Sonic
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3 Answers3

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I think you might have heard about the Ram Air Turbine, which is deployed in case of some aircraft in case of loss of main electrical power supply. From A320 Systems briefing:

In case of total loss of all main generators, the RAT is automatically extended and drives the emergency generator via a hydraulic motor.

The location of the ram air turbine varies with different aircraft and some of them (like the popular A320) have them near the landing gear. The following image is from @mins' excellent answer to another question.

RAM air turbine

Location of Ram Air Turbines in different aircraft, image from aviation.stackexchange.com

  1. Bombardier BD700-1A11 Global 5000 (source)
  2. Airbus A320 Flight test (source)
  3. Bombardier CRJ700 (source)
  4. US Navy T-45A (source)
  5. Airbus A330 (source)
  6. Boeing B747-8 (picture by Gregor Schläger, source)
  7. Boeing B777, RAT extension switch (source)
Community
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aeroalias
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  • Yeah, i saw the picture 2 before. Thanks! – Ian Sonic Dec 01 '16 at 16:22
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    The picture of the Lufthansa(?) technician really needs to be a caption competition. – David Richerby Dec 01 '16 at 16:33
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    I like the idea of it being an emergency propeller to fly the plane with. – TonyK Dec 01 '16 at 19:53
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    How is this device automatically deployed in the loss of main system power? Is it held back by some solenoid which lets go to deploy it when power is lost? What happens when the plane is parked in a hangar? – Wossname Dec 01 '16 at 21:05
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    @Wossname I'm guessing here, but batteries are one option. It's a lot easier to bring along batteries to run systems for a brief period and then augment the RAT, than to bring along batteries to run systems for however long is required to land the plane in case of an all-out power failure. – user Dec 02 '16 at 13:06
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    @Wossname You're right that the system is held back by a solenoid that deploys when the power is lost (or commanded by the pilot). Though they can be deployed manually when parked on hangar (automatic deployment is prevented when airspeed is less than a value), it is not recommended. You can see the linked question for further details. – aeroalias Dec 02 '16 at 13:17
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    @aeroalias: Wossname is asking why the system doesn't deploy automatically and unwantedly, when your plane is in the hangar powered down. Presumably the system is mechanically "armed" on take off then disarmed after landing before you disengage the electrical system. – Lightness Races in Orbit Dec 02 '16 at 19:32
  • @DavidRicherby: That is definitely a safety vest with a Lufthansa logo. – Martin Ueding Dec 03 '16 at 13:28
  • @mins Don't tell me, tell Wossname! – Lightness Races in Orbit Dec 10 '16 at 15:43
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    @Wossname: There are conditions for the RAT to be deployed, see Is there a maximum airspeed to deploy a ram air turbine? (there is a minimum airspeed and the logic on Airbus aircraft is this one). – mins Dec 10 '16 at 15:59
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    @mins, +1 for the info. I like the drawing style for that logic diagram, It's very clearly laid out (as it needs to be in a safety critical system!). Good stuff. – Wossname Dec 10 '16 at 16:15
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    @DavidRicherby: Caption for the photo: "What do you mean it flies? Like, in the air? I might be the new guy but I'm not falling for that one!" – Wossname Dec 10 '16 at 16:21
  • @aCVn: Especially since aircraft already have emergency batteries (for that matter, some aircraft only have batteries, and no RAT). – Vikki Dec 05 '18 at 22:48
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There is a rotor, but not to provide thrust.

The rotor spins in the air stream to generate electrical energy, which is used to power avionics and hydraulic pumps so that the crew can continue to navigate and communicate.

abelenky
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    In fact, it increases drag and speeds (slightly) the descent of the crippled aircraft, while supplying essential electrical energy... – DJohnM Dec 01 '16 at 19:26
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    I know this is pedantic, but a propeller is used to propel things. This would be a turbine. – J W Dec 02 '16 at 05:55
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The DC-10 had an under-belly propeller, driven by air during flight, as a backup power source in case of total failure of the plane's hydraulic system(s). The upgrade was shown on a Discovery Channel episode of "Mayday: Air Disaster."

David Springer
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