View Single Post
Old 27th May 2011, 17:33
  #115 (permalink)  
spagiola
 
Join Date: Jun 2001
Location: US
Posts: 245
From the BEA report: about 15 sec after 2:10:51 (ie about 2:11:06), the two recorded speeds (LHS and ISIS) are once again the same, and remain the same for the remainder of the flight. To me, this sounds like the pitot probe problem cleared (ice melted or was dislodged?) at that point, and did not recur. The speeds then are noted as becoming invalid again (eg at 2:11:40 just after the captain arrives, until 2:12:02 + about 15 sec), but the report pointedly notes that they would do so whenever they fall below 60KT.

That's over three whole minutes in which they apparently had functioning speed indicators (a little after 2:11 up to impact at 2:14:28). They also had plenty of altitude, as this is about the same time they reached their maximum altitude of 38000.

By then the THS is trimmed nose up 13 degrees. Between 2:11:40 (when we are told N1 is almost 100%) and 2:12:02 (when we are told throttles are already at idle and N1 is 55%), the throttles were retarded.

At about 2:12:02+15, the PF commands pitch down, pitch is reduced, and speeds become valid again (ie climbing above 30KT).

Impact didn't occur until over whole two minutes after pitch down was commanded. Apparently not enough pitch down as the aircraft remained nose high (AOA at least 35 degrees), descending very rapidly. Did the PF stop pitching down? Or was he continuing to pitch down, but failing to drop the nose because of the up trim on the THS? The report says nothing about pitch commands after 2:12:02+15 except to note that, in general, pitch commands were "mostly up" after AP disengagement.

To my mind, it's those last three minutes, and particularly the last two, that need to be explained. There was an initial upset, and a reaction to that which led to a stall, but by about 2:11 the speed indications were apparently valid.

Was the crew confused or fixated on the wrong problem? If we knew more about what they were saying, it might be possible to figure it out. I can't imagine that nothing was said in these whole of the last two minutes beyond the brief exchange noted at 2:13:32.

Did the resumption of the stall horn once the speeds once again become valid after commanding pitch down (at 2:12:02+15) confuse the PF into thinking he was doing the wrong thing?

Perhaps it might also be worth noting that by 2:11:40, the aircraft was already dropping at -10000fpm, and that it would have almost exactly this vertical velocity when it hit, some three minutes later. This suggests there may have been little vertical acceleration during this period, perhaps lulling the pilots into thinking they were in stable flight? Yet they did note that they were dropping through FL100.

Lots of things are just not adding up.

[edited for grammar and to correct error on min speed at which speed is considered invalid; 60KT, not 30KT (see BEA report, note at bottom of p.2 of French version]

Last edited by spagiola; 27th May 2011 at 20:58.
spagiola is offline