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As seen about 53 seconds into this video of the Qantas Dreamliner landing, the engine housings seem to extend upon landing. What specifically is the purpose of this? To balance? To help with air flow?

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  • $\begingroup$ Note that you can include a timestamp in your video link by right-clicking on the timeline "cursor" in the youtube video and selecting "copy url including timestamp" or something like that. $\endgroup$ – AnoE Mar 16 '18 at 16:12
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Those are the thrust reversers, which cause the engines to produce thrust "backwards" to help slow the aircraft during the landing.

You can see a close-up video of 787 thrust reversers in operation, including a look at the panels at the back of the engine which block the normal flow of air.

We also have a question How do thrust reversers work on a jet engine, and when should they be used?, which describes their operation in general.

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    $\begingroup$ And they're not unique to the 787. Most if not all commercial airliners have them. $\endgroup$ – jamesqf Mar 16 '18 at 4:11
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    $\begingroup$ And the loud roar you hear just after touchdown every time is exactly that! $\endgroup$ – Hanky Panky Mar 16 '18 at 8:52
  • $\begingroup$ @HankyPanky The spoilers also make a lot of noise, even when reversers aren't used. $\endgroup$ – David Richerby Mar 16 '18 at 11:27
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those are the thrust reversers. when they open up, the fan air flow is diverted and discharged forward instead of aft. this helps slow down the plane and lets the wheel brakes last longer.

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    $\begingroup$ Most of the time the aircraft uses the aircraft brakes, they provide far more breaking power than the reversers. Most airports have noise restrictions and thus only idle reverse thrust is used. Modern carbon brakes last a set of applications where brakes force is no factor. Older steel brakes wear out earlier if there is larger force applied. $\endgroup$ – Brilsmurfffje Mar 16 '18 at 8:18

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