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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?

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    $\begingroup$ It was used to great effect in the Gimli glider incident. $\endgroup$
    – isanae
    Dec 1, 2016 at 19:09
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    $\begingroup$ how cool is that ?! $\endgroup$
    – Fattie
    Dec 2, 2016 at 21:37

3 Answers 3

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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)
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    $\begingroup$ The picture of the Lufthansa(?) technician really needs to be a caption competition. $\endgroup$
    – David Richerby
    Dec 1, 2016 at 16:33
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    $\begingroup$ I like the idea of it being an emergency propeller to fly the plane with. $\endgroup$
    – TonyK
    Dec 1, 2016 at 19:53
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    $\begingroup$ 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? $\endgroup$
    – Wossname
    Dec 1, 2016 at 21:05
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    $\begingroup$ @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. $\endgroup$
    – aeroalias
    Dec 2, 2016 at 13:17
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    $\begingroup$ @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). $\endgroup$
    – mins
    Dec 10, 2016 at 15:59
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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.

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    $\begingroup$ In fact, it increases drag and speeds (slightly) the descent of the crippled aircraft, while supplying essential electrical energy... $\endgroup$
    – DJohnM
    Dec 1, 2016 at 19:26
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    $\begingroup$ I know this is pedantic, but a propeller is used to propel things. This would be a turbine. $\endgroup$
    – J W
    Dec 2, 2016 at 5:55
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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."

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