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In multiple Hollywood movies and TV shows I've seen scenes where a plane enters turbulences and for a moment the people in the plane seem to move upwards relative to the plane.

As far as I know, as long as the plane doesn't accelerate downward with more than gravity's acceleration, the people on the plane should only feel less of their weight but they shouldn't jump up and hit the ceiling.

I know that certain planes like fighter jets can reach such negative G values, but I'm talking about passenger planes.

Now I'm wondering if it's really possible that for an instant a plane would accelerate downward with more than the acceleration of gravity due to the turbulences maybe pushing it down? Or are scenes where people hit the ceiling of the plane just pure Hollywood fantasy?

One example of such a scene is here at 02:00 https://www.youtube.com/watch?v=xuYbd15n5t8

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    $\begingroup$ Why do you think that an airplane can not accelerate downward at more than 1 G? If you have never experienced a wind shear event, consider yourself lucky, but large masses of air related to thunderheads certainly can accelerate an airplane down at much greater than one G. $\endgroup$ – Jon Custer Feb 8 '16 at 17:20
  • $\begingroup$ Guess why they ask you to wear a seat belt while the plane is cruising. It's not going to save your life in case of a mid-air collision: It's to keep you (and all of the other passengers) from flying around the cabin in case of a negative G event. $\endgroup$ – Solomon Slow Feb 8 '16 at 21:52
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Yes, airplanes can experience strong vertical acceleration simply by flying across the boundary between an updraft and a downdraft.

Example: flying from an updraft of 5m/s to a downdraft of 5m/s, in one second, gives a downward acceleration of about 1g. It can easily be more. Such turbulence is quite common. Planes are built strongly to handle it. The Cessna 172 is stressed for +4g and -2g, in other words normal gravity +/- 3g. Aerobatic and military fighters are built for +/- 10g which, if they are going fast enough, they can readily achieve.

That's why passengers are told to wear their seatbelts.

It depends on the airplane's speed. Every airplane has a speed called "maneuvering speed" or $V_A$. If the plane is going slower than that, violent control actions cannot break it. When entering turbulent air, it is recommended to reduce to that speed, or below.

P.S. Look up vomit comet.

P.P.S. Trying to figure out why you asked this question, it occurred to me that if you believed the traditional wrong explanation of how wings work (assuming that air parcels had to re-unite at the trailing edge of the wing) then you could wonder how negative-G was possible. If instead you understood that lift was roughly proportional to angle of attack (AOA), then you could see that negative G occurs when AOA is negative. When entering a downdraft, the air comes from above, so AOA is negative, temporarily.

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It's happened. For instance,UA Flight 826 in 1997 had a passenger die after she hit the ceiling. Peak negative g of 0.824.

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    $\begingroup$ Is there a specific reason for the downvote? It certainly answers the question "I'm wondering if it's really possible that for an instant a plane would accelerate downward with more than the acceleration of gravity ". If it happened it must be possible. $\endgroup$ – WhatRoughBeast Feb 8 '16 at 21:02

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