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When watching landing videos of 787's that are in service I've been noticing a fan that is located under the right rear section of the fuselage. I've now seen it on 4 different 787's from 4 different airlines.

Is this an emergency fan that's deploying, or is it just something that's designed to recharge some of the batteries on board? Some auxiliary type of system, perhaps?

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    $\begingroup$ do you have a photo? $\endgroup$ – Keegan Dec 10 '14 at 2:15
  • $\begingroup$ I'll see if can find one, or take a capture from the video. $\endgroup$ – AllenM Dec 10 '14 at 3:04
  • $\begingroup$ (Going off of the answer below) Maybe it's just used to add more drag? Probably used on the 787 to help charge the batteries while descending... But that's just a guess. Don't take my word for it. $\endgroup$ – Keegan Dec 11 '14 at 4:40
  • $\begingroup$ Any 787 pilots around here? $\endgroup$ – Keegan Dec 11 '14 at 4:40
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If you are speaking of the small fan behind the gear in the photo below (click to magnify) 787 RAT

Image from here.

you are speaking of a Ram Air Turbine, usually used in an emergency, not in normal operations.

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    $\begingroup$ Note that the above photo is taken in Settle at Boeing Field (As can be seen from the distinctive hangar in the background), indicating a test flight: the majority of the videos you see will be at the same airport, as most airlines do a few test flights with Boeing on their first delivered aircraft. Plane-spotters will often hang out here to get videos/photos of the new planes on their test flights, so you'll see an unusual number of videos of test flights (plus it's easier to get close to the runway here than at many other airports). $\endgroup$ – Jon Story Dec 10 '14 at 10:53
  • $\begingroup$ Why would it be used in so many landings? Is that a standard test flight landing procedure? $\endgroup$ – Xen2050 Dec 10 '14 at 13:12
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    $\begingroup$ @Xen2050 they need to test the RAT itsefl to see there are no chewed on cables that may bring the thing down. $\endgroup$ – ratchet freak Dec 10 '14 at 14:15
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    $\begingroup$ @Xen2050 - it isn't necessarily being used in the landing. Once the RAT has been extended on a 787, it can't be retracted again in flight. Many test flights will test the RAT, or it will be extended as a precaution during other tests. I'm unsure whether it's standard procedure to keep it extended on test flights, but it would make some sense: the nature of testing means it's nice to have backups :) $\endgroup$ – Jon Story Dec 17 '14 at 19:06
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    $\begingroup$ I'm late to the party, but @JonStory - that's Paine Field (KPAE) in Everett, WA, where Boeing manufactures the 787. $\endgroup$ – egid Nov 17 '16 at 20:17

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