Why does the falling plane slow down according to black box data?

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jumpjack
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Don't know if this is the right place for such a question...

I plotted these graphs based on data taken from the black box of a plane:

http://www.planetmobile.it/jumpjack/immagini/volo93-grafici.GIF

I can't fully understand these data: looks like while the plane was falling down, it was also slowing down. Is this interpreataion correct? How could it be explained?
 

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The plane seems to have entered a tail spin, which would slow down its speed dramatically.
 
How do you get a tail spin from that? I would like to see some yaw port data before jumping there.

You are looking at groundspeed, not airspeed. With a huge roll rate and pitch down it's possible that this thing is falling at a very steep rate which is shown on the altitude plot. Do you have any data on airspeed?
 
Sorry, my mistake, I plotted GROUND speed rather than AIR speed, so I guess it's correct, being the plane going down very fast.

Here you find full FDR data, you want to play with them. ;-)
http://www.crono911.net/doc0/FOIA/UA93tab.zip
 
It just looks to me like the plane was in a mild dive, then rolled over and pitched down more steeply. Ground speed slowed because of the angle of the dive - airspeed probably went up, though not necessarily.