UNCTUOUS comments on BBC scenario
All,
I am not a pilot, nor an aerospace engineer per se. But I do have an MSME degree, have spent most of my life in computer programming, and have an elementary grasp of statistics and probability. I've also followed these AF447 threads with great interest, since the beginning a year ago. I have some comments regarding UNCTUOUS' very interesting theory, as expressed in comments to the BBC script.
- The BBC obviously did not do much research on previous incidents of IAS inconsistencies. As you said, most of these seem to have occurred in contexts that suggest ice crystals, rather than supercooled water, as the primary cause.
- I had understood the possibility, before, that engine thrust at AT disconnect might have been different than the pilots may have intuitively expected, due to intervention by the flight management system in response to turbulence settings or other phenomena. I had not known, though, that selection of the "fuel monitoring" screen would further obscure the true current engine thrust settings.
- Without doing any in-depth analysis, your theory of relatively gradual degradation in speed readings leading to mach-effect overspeed phenomena, while plausible, does not seem very likely. The pitot tube is a very simple analog device, whose basic design predates the dawn of aviation. It operates primarily on pressure difference, rather than flow, per se - very little air actually flows through the tube (some air does exit through drain holes, etc, but that is by design a small amount, or the tube won't serve its intended purpose). This in turn means that the tube will need to be almost fully blocked before significant pressure and airspeed indication loss is observed. And that, in turn, means that airspeed indication loss due to icing will very, very likely be 1). rapid, 2). unstable, and 3). inconsistent between redundant tubes. This is consistent with the summaries of known IAS anomalies previous to AF447. While I have no knowledge of the airspeed validation algorithms in the Airbus flight management systems, it seems likely that they would be programmed to reject airspeed data inconsistent with the physical capabilities of the aircraft and known limits of windspeed variation, which would further limit the kind of airspeed variation which could lead to your scenario. It is certainly possible that high-altitude upset could have occurred due to overspeed because of pilot input in a confusing environment with no accurate feedback, but I don't see that overspeed due to AT correction is especially likely.
- The issue of training for high-altitude upset (stall, mach-tuck, etc) is interesting. The apparent inability of most commercial simulators to simulate stall in combinations of pitch and roll is something I'd not understood. Beyond that, there is the fact that humans are not machines, and will react differently when primed to expect surprises than when they are lulled into complacence by thousands of hours of uneventful automated flight at cruise altitude. The point made by the military pilot - that military craft are almost always manually flown, and thus the pilot is always in that "zone of awareness", is significant. And then there's the question: with all of the air miles flown today, and the fragmentation of air traveler miles into more and more flights on smaller and smaller planes, can we possibly expect all of the burgeoning ranks of pilots to have the degree of manual flight skill needed to recover from the hypothetical AF447 situations?
- It is easy to identify accidents related to failures of automated flight systems, or the misuse thereof. It is impossible to track accidents avoided by such systems. The overall statistics, which seem to show fewer and fewer accidents per air mile traveled, suggest that if automation is either a net safety benefit, or it is being overwhelmed by other safety improvements. I have no data to say one way or another.