Took the CSV file FR24 published on their site and graphed the altitude as reported by the stations.
The results are below:
KGL9268 Altitude data - Album on Imgur
Whatever happened started some 255 seconds from the start of the published log, upto that point the aircraft was climbing normally and all stations were reporting consistent data. After that it gets fuzzy. The album also contains 4 more images - one is all altitude data after the 250. second and the rest is altitude data after the 250. second separated by station ID.
No interpretation from my side, thought it might come handy to someone.
A few points to help with interpretation.
* We don't have enough data. The aircraft normally transmits information twice a second. But even the station with most records (2593) only has 8 data points between 04:13:00 and 04:13:21. Clearly, many packets were lost.
* Velocity and heading are transmitted in one packet. Latitude, longitude and altitude are transmitted in a different packet. Which means that, e.g., if a station gets a position packet but not a velocity packet, you'll see a row with current altitude but outdated velocity, and vice versa.
* There's no timestamp in the packet so entries with slightly different times received by different stations may correspond to the same packet.
* Vertical velocity seems to be extremely noisy and seems to be derived from altitude anyway, so I'd disregard it altogether.
I tried to parse the data provided by FR24 with all this in mind, and here's the picture I'm getting.
* Prior to 04:12:58, all is normal.
* Around 04:13:00, there's a couple of erratic altitude readings. (There are three packets within three seconds reporting altitude 29750, 30975 and 33275, and clearly all of them can't be right. The aircraft was at 30750 just prior to this time.) The aircraft begins slowing and turning to the right.
* At some point between 04:12:58 and 04:13:05 (the exact time is hard to pinpoint because of the erratic readings), it begins descending.
* At 04:13:10, it's slowed down below 250 knots, turned 25 degrees to the right, and it's descending at the rate of at least 7000 fpm.
* Last received position packet, at 04:13:19.5, gives its altitude as 27925'.
* Last received velocity packet, at 04:13:22, gives its horizontal airspeed as 62 knots.