FBW and unreliable IAS
On the A330 FBW, with the IAS indicating excessively low speed, the pilot's correct action of pushing the nose down and pushing the thrust to max may, in my opinion, cause the auto trim to trim "backwards" causing a pitch up. In a normally indicating IAS, the pitch trim should trim forwards, but because of the false low speed indication, the forward application of the side stick will be interpreted by the FBW as a pitch forward and therefore countered with the pitch trim going backwards.
If I am not mistaken, a similar situation developed in a non-FBW A300 doing a botched-up go-around in Nagoya, Japan, many years ago. In that incident, the auto pilot interpreted the pitch forward command of the pilot through the control column as a situation that required the auto trim to trim backwards, causing the aircraft to stall, and eventually crash.
In FBW, with the auto pilot off, the pitch trim still acts on its own, as if the auto pilot is on (in a conventional aircraft). You must remember that there is no trim switch on the control column wheel on the A330 for the pilots to trim forwards or backwards as Normal and Alternate Laws do all the trimming for you. In fact, most FBW pilots will not notice the position of the pitch trim at any given moment, so it would not surprise me if the pilots on the ill fated AF447 had not noticed the extreme pitch up position the auto trim had placed the aircraft in... and so they would naturally react to the stall situation by pushing the side stick forward more, further aggravating the situation.
As I see it, with hindsight, the correct action to recover from the stall in this case would be to release the side stick, and manually roll the pitch trim forwards until it is out of the extreme pitch up position the aircraft was in.
Last edited by Yipoyan; 28th May 2011 at 02:43.