It is most unlikely that we will determine the reasons why this crew did not detect the state of the autopilot. These ‘crew error’ incidents usually have deep underlying causes. One of which may relate to my first post, that crews should not be ‘alarmed’. I may be playing with words, but this is to make the point that in unusual circumstances crews have to control their emotions, have knowledge of what is expected to occur, and to be extra vigilant. The surprise and stress of the situation may lead to tunnel vision (over focus) on less important items. In addition, the senses are reduced, hearing is the first to degrade, and thus this is one possible reason for not hearing an autopilot disconnect warning. Another more obscure reason may be in the design in the priority of warnings; usually windshear takes precedence over GPWS, TCAS, etc. Can a 744 driver educate us as to how a 744 system works? An active PWS warning and then the autopilot disengages, – do you get an audio warning for the autopilot disconnect, if so when? Does the autopilot disconnect at stick shake?
jtr I think that you have a common misunderstanding about the operation of the PLI. The principle of most systems (I do not know the 747 in detail) is to provide an indication of the stick shake angle of attack (AOA) on the attitude indicator. Thus, the ‘pitch’ attitude shown by the PLI is actually AOA relative to the local airflow (or flight path in some systems).
In severe a windshear this ‘pitch’ angle can be very low; if a downburst causes the aircraft to descend at 12 deg (see the previous link example), the PLI is referenced to -12 deg, thus if the stick shake AOA is 17 deg, the PLI will only be at + 5 deg attitude. An upper limit (15 deg) is logical but in gusty windshear, the AOA could be very dynamic and depending on the sensor and damping there could be some over swing.
For some aircraft during windshear the stick shake may activate, this may be normal – this is ‘respecting’ the stick shake; not a failure in that it must not occur. Therefore, it is important for crews to be trained to know their system and not to over react to an alert that in specific circumstances could occur naturally.
An hypothesis in this incident may be that if the stick shake operated the crew pushed forward harshly causing a bunt; the stick shake was due to low speed, which was due to the lack of autopilot … and we are back to the beginning … why?