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I was flying on a Boeing 747-800 today and the pilot opened up flaps at high altitude. This caused sharp descend for about 5 seconds ( Causing slight panic onboard ) , the pilot leveled the flaps again after about a minute and the aircraft became much smoother. Is it normal to use flaps like these or this might be an error?

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    $\begingroup$ Do you maybe have a picture, a more detailed description of what you saw or a flight number? As flaps increase the lift-to-drag ratio, I'd suspect the aircraft would've climbed. Spoiler or speed brake deployment seems more likely. $\endgroup$ – Bram Jan 26 at 12:45
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    $\begingroup$ Flaps are the the parts that come out and down from the trailing edge. Spoilers are the things that come up from the middle of the wing. Which ones did you see? $\endgroup$ – Jan Hudec Jan 26 at 13:36
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    $\begingroup$ @Bram, flaps (and spoilers) are generally used to increase angle of descent at a given speed, as does reducing power and pitching down. You do get more lift from flaps, but at the expense of much more drag. The 747 may have been getting out of someone's way. Agree it may have been spoiler, or perhaps both (as a speed brake). May have been nice if the pilot intercomed what was going on. $\endgroup$ – Robert DiGiovanni Jan 26 at 15:26
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What you saw and felt was the normal operation of the Speed Brakes.

Speed Brakes can also be called Spoilers. They are panels on the top of the wing which open up to create drag and reduce lift. Some of the panels are also used for roll control.

They are often used in flight when ATC requires a steep descent or an immeadiate reduction in airspeed. They are also used in an Emergency Descent in case of cabin pressure loss. During landings they automatically deploy to increase wheel brake effectiveness.

enter image description here

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    $\begingroup$ I think i misunderstood the part and it may infact be speed brakes. Thank you for the picture and explanation $\endgroup$ – Atul Jan 26 at 15:27

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