Originally Posted by
crosvs
The conclusion this morning that this was an intentional act by the co-pilot angered/saddened me nearly to tears, though I can't say it surprised me. I think this is what many of us feared all along. This is devastatingly horrific news indeed.
Two things though:
(1) Do we have verification that the autopilot was indeed set to descend? It seems more like it was a manual descent: as many of you probably know, once the flight computer altitude knob is dialed to the desired altitude and pulled out, towards you, to override the flight computer, the descent begins at a constant descent rate. Here, we saw that the descent rate was fluctuating quite a bit. That said, as we know, on an Airbus, the FBW system holds the plane at the attitude you set it (to a point). Simply setting your attitude and releasing the controls should have produced a steady descent.
(2) That said, if indeed it was an auto-pilot managed descent after all, I do have one horrific theory to explain the descent rate fluctuations: we saw how shifts in the center of gravity can cause even a 747 to lose control (remember the military base incident a year ago?). It's conceivable that all passengers were told to move as close as possible to the rear of the plane in an effort to slow its descent:
Figure a fully-loaded A320 with 7,835 US gallons @ 6.8 lbs / gallon = 53,278 lbs.
This was a short flight, so figure only about a third of that was used, at most(?). So maybe 18,000 lbs or so, most of which is of course over the wings. Shifting 149 people to the rear of the plane, and assuming an average weight of just 150 lbs / person means 22,000 lbs suddenly to the rear of the plane, in a desperate attempt to slow the descent. This cloud explain the varying descent rate.
Net-net, there are only two possibilities, only one of which can be true:
(1) The descent was performed manually, thus (maybe) explaining the varying descent rates (but, the aforementioned point about FBW seems to negate this)
(2) The descent was performed by autopilot, with some circumstance(s) affecting its performance so as to produce the varying descent rates
This is incorrect. Pulling the altitude knob after selecting 100 ft will cause the aircraft to descend at idle thrust at a constant airspeed, not vertical speed or ground speed. Due to wind changes, air density and other factors those values can vary along the descent path. Doesn't mean he didn't do it manually, just that those variations you describe could have occurred with the autopilot on the whole descent.