18 years ago it was the last time i was in an aerodynamic course, so forgive me if i get it totally wrong, and feel free to correct my thoughts.
In nearly all contributions on this thread and on the one in R+N stall and the associated AOA is viewed in relation to the whole airframe. Basically there are two major lifting devices, the wings (upward lift) and the THS (downward lift). The efficiency of those two devices is influenced by airspeed and aoa and some other points like CG, configuration, bank angle, and so on. In straight and level unaccellerated flight the AOA of the THS with itīs downforce balances the weight and the lift of the airframe with the wings. To get the nose up without changing any other parameter there is the need to increase the lift of the THS by increasing the THS AOA thus forcing the tail down and the nose up, and vice versa. This is done by changing the elevators in the short term and by changing the THS trim (autotrim) in the long term.
Letīs just concentrate on the wings and the THS with regard to AF447.
When AP +AThr disengaged with speed of 0.8 mach, and the initial roll correction and NU input of the PF the airframe reached an Pitch of 10° and increasing, and a climb rate up to 7.000 f/min. The PF made a ND input with the SS and reduced the AOA to 4° and the climbrate to 700 f/min.
BEA doesnīt say what kind of pitch it finally reached. The AOA reduction does not necessarily cause an immidiate gross reduction in pitch, the nose might still have been a lot higher above the horizon than desired. The reduction in climb rate might as well not have been the direct result of the decrease of AOA and pitch, but the result of rapidly decaying forward speed thus reducing the initial peek climb speed. In other words, the energy state of the aircraft could already have been that of a stalled one, only the unloading had prevented the stall until right now.
At 2:10:51 the stall warning came on, the AOA was increasing above 6°.
Lets see in what kind of AOA the lifting devices wing and THS had been at that point. For me as a non engineer its easy to get those figures simplified and wrong, so feel free to correct me. The whole frame had 6° AOA, so i put the wings to the same value. The THS has the AOA of the fuselage, the AOA of the THS (3° nose up trim as later stated) plus the full nose down input of the elevators (full deflection 15° down, i take the half of it as AOA change) My sum here -6+3-7= -10 AOA. If the trend of my thinking is not totally wrong, the THS at that low speed might have been close to a negativ tail stall.
At that point PF applies TO/GA power and replaces his ND SS input by a NU SS input. The pitchup moment of the engines plus the pitchup NU SS input (which moves the THS trim from 3° to 13°) settles the airframe in a solid state of stall with an AOA of 16° and an increasing sinkrate up to 10.000 f/min. Applying my above right or wrong formula the AOA of the wing would equal 16°, the AOA of the THS would be -16+13+ the elevator NU input (max 38°, producing an THS AOA change of estimated 15°). That somes up to an THS AOA of 0 to 12 AOA, due to low speed a full aft stick probably produced a possitive Tail stall.
What does full ND SS input produce at that point?
-16 + 13 -7 brings us to - 10 again.
My values might be totally off, but i cant get rid of the feeling that with the low speed there was only one THS position favorable for recovery, but that the THS was stallable in the positive and in the negative THS AOA region. The crew had no means available to find out what that exact point would have been and how long it would have taken to get the nose below the horizon. Again, an old style AOA gauge might have helped to find the point, where the input produced a positive effect in total AOA.
Thats why our problem solving in the f-4 stall recovery was getting the nose down:
F-4 hard memory item for "out of control":
Stick - forward (it was not full forward!!)
Ailerons and rudder - neutral
If aircraft not recovered - maintain full forward stick and deploy drag chute
With the stick forward we tried to reach the ideal position of the stabilator (full forward might have caused the stabilator stall on the opposite side).
If we missed that point the drag of the chute would kick our tail up and force the nose down.
Edited for latest version of BEA statement, sorry i missed it first. Thanks for the heads up to Conf and Bear
By the way, BEA has it still wrong in the german version.