‘Human – Technical’ thoughts
Some ‘Human – Technical’ thoughts … … and questions, … mainly questions.
It is widely assumed that the event was triggered by ice particle blockage of the pitot tubes. With all three blocked it is also likely that the TAT probes suffered blockage, probably simultaneously; ref previous events and see VH-EBA (not simultaneous), #692 graphic and report link.
If the ‘sudden failure’ of airspeed (an erroneous low value) was accompanied by an erroneous low altitude (as with VH-EBA), then the indicated altitude deviation could also attract the crew’s attention. Would the altitude alerter sound?
Thus with apparent simultaneous failure of all air data and logical auto-flight disengagement, the crew’s initial thoughts might have been of a system failure – a technical issue.
The PF took the controls, but with the perception above, may have concentrated on altitude. In every day operation there is significant focus on altitude – ‘beware altitude bust’, TCAS, interview, loss of license, etc; these could easily be hidden (subconscious) operational and social pressures which might have influence the initial action; pitch up to recover the assigned altitude, back stick and follow-up trim.
I assume that the trim datum is still attitude (flt path?) and zero control input. Would the control law be attitude or flight-path stable with these system errors. If flight-path, how is flight-path computed, inertial or with air-data mix, and if air-data and the air-data is erroneous … … ?
Would the FD still be active, if so, in which modes and what command might be shown?
We should recall the many discussions of how modern crews depend on the FD; a general low experience level of basic instrument flying. Was an erroneous FD command followed resulting in an inappropriate manoeuvre?
Would an abnormal TAT (rising towards zero, based on previous events, A/B and other aircraft types) affect any other instrument display systems of flight guidance computation?
If airspeed was erroneous, and the altitude is also affected by this and also possibly from ‘abnormal’ temperature, what about Mach, and any residual protections (MMO pitch-up bias), i.e difference between computed Mach (and rate) vs computed MMO.
From what has been describes so far it may be impossible to take any of the quoted air-data values as representing the true condition of the aircraft; except perhaps the vane derived AOA. What source of information does the FDR record, is this always the same as what the crew will see?
Roll deviation might be considered a separate but confusing issue. If the YD ‘froze’ (fail-safe) with air data ‘failure’ (as designed?), then an offset rudder would induce roll and a permanent roll bias, but its effect varying with actual airspeed – need to use rudder. But who uses rudder in normal (symmetric power) flight?
With these distractions and concerns of system failure, then any more speculation about crew thought and behavior would be unjustified without further data.
However, the trim position is a dominant issue, and without awareness of this offset, basic flying could be difficult and probably added further confusion.
At a late stage, nose down control action did effect a pitch change and an apparent speed increase (a semblance of stall recover but with the confusion and aerodynamic complications of mis-set trim). The speed increase was sufficient to reinstate the previously inhibited AOA driven stall warning; probably further confusing the mental picture – nose down pitch apparently caused a ‘stall’ ??? … what next; undo what you have just done, back stick? Perhaps this is normal (to be expected) human behavior.
Also, think about how the industry discusses stall – some thread discussion, education, and training. Virtually every aspect is speed related – stall ‘speed’, warning ‘speed’, stall displayed on a ‘speed’ tape, the importance of gaining / maintaining ‘speed’; even if a crew had been taught that the stall warning was AOA driven and that the aerodynamics of a stall involve AOA, is this information recallable, or would ‘speed’ dominate our thoughts.
There is much more to come, much to be learnt, and probably all intermixed in technical, human, and social system (operational/organizational) interactions.
At this early stage, we might consider how crews are educated / trained, and what is currently expected of them. How might everyday operation influence encounters with a very rare and unusual events? Are modern norms based on a remarkably benign (safe) operation so that it is now unrealistic to expect extremes of experienced-based innovative behavior? Has the industry overlooked ‘change’, and retains old, inappropriate assumptions?
To reassure any PAX Ppruners (and the media), the industry remains very safe. Incidents such as this are very rare and the assumed precursor (high confidence) – ice particle blocking is generally understood. Safety action is in place – changing the type of pitot tube, manufacturers have refreshed crew dills for loss of airspeed and stall recovery, and operators are applying these training aspects. The industry is not (must not be) complacent; a thorough and tenacious investigation is underway from which we expect ‘lessons to be learnt’; but it’s still up to us to learn.