AF 447 Thread No. 5

[HazelNuts39]

Events around AP disconnect

With the DFDR data released in Interim Report no.3, it is now time to add a few observations to my july 10 post #30 on page 2 of this thread. While the earlier post was for still air, the new element is the role that movements of the air masses traversed by the airplane played around 02:10.

In the preceding minute the level of turbulence had been increasing from an amplitude peak-to-peak of about 0.2 g at 02:09:30 to about 0.5 g when the AP disengaged at 02:10:05. At that time the vertical speed was passing through zero, decreasing at the rate of 400 fpm per second, or 0.2 g. The normal acceleration was thus 0.8 g at that point. The airplane’s pitch attitude was then increasing through about zero, i.e. much less than the 2.5 degrees it would have been in steady, unbanked level flight. In the two seconds after AP disconnect until the first movement of the side stick, pitch increased to about 1.3 degrees. The pitch attitude continued to increase progressively from 2:10:08.4 and the vertical speed became positive at about 02:10:10. The engine N1’s which were close to 100% up to 02:10:00, then dropped to about 83% 6 seconds later (the commanded N1 dropped to 75% in about 3 seconds). When the pilot grabbed the SS, he was looking at pitch 1.3 degrees NU, 7 degrees right bank, and –400 fpm vertical speed.

My tentative explanation of these events is that the airplane was traversing the fringe of an area with significant updraft velocity (1024 fpm fits the data). The AP pitched the nose down to maintain altitude, and the A/THR reduced engine thrust to maintain airspeed. After disconnect of AP and A/THR, the FCS increased pitch to regain 1 g normal acceleration, and thrust was locked at significantly less than required to maintain airspeed in level flight in still air.

More analysis to be done by BEA (and perhaps me). Why did AP and A/THR disconnect 2 seconds before the speeds dropped?