Once they got into the coffin corner of not understanding what was really happening, they ended up in a state of cognitive tunnel vision and were powerless to do anything about it. Short of being in an airplane that would either tell them exactly what to do or take over and do the recovery itself, I doubt that anything would have changed it.
There is some aeronautical engineering data from an extremely good source that indicates that, although requiring attitudes which line crews would not be comfortable with at low altitudes, the aircraft could have been recovered as late as 6000'.
Some explanation of the graph is in order:
The graph plots CAS against Flight Path Altitude. There is no time element in the graph. The bottom (x) axis is CAS, the side, (y) axis is Altitude in ft.
The yellow line is the flight path CAS derived from an exhaustive re-analysis of the DFDR data in the BEA Third Interim Report.
The purple line is CAS. The CAS starts out at roughly 275kts, slightly increases then rapidly drops to about 120kts (purple line moving to the left, still at FL350). We can also see the rapid, initial climb against the loss of airspeed. Times are indicated in blue and begin at 02:10:00, the time of the UAS event.
The purple line then shows increases/decreases in the actual CAS as the aircraft descends.
The black solid line is the Vs1g stall CAS.
The black dashed line is the Vs2.5g.
The black dotted lines show Mach Number.
The group of lines extending to the right, off the actual CAS (purple) line, represent increases in the CAS in 3 recovery scenarios which involve pitching the aircraft down to reduce the AoA and unstall the wing.
The first set of three lines extends to the right at FL350, the next group at FL200 and the last line at about 6000ft.
Each of the three lines indicates potential recovery (unstalling the wing) at various rates-of-change of the AoA and pitch attitude.
The first set at FL350 indicates the nature of the recovery (in terms of altitude lost as speed is gained and AoA is reduced), when the aircraft is pitched down to 10deg and the AoA reduction (from a nominal 35+ deg), of 3, 2, and 1 degree per second.
The second set at FL200 indicates the nature of the recovery at 20deg ND pitch, 10deg ND pitch and 5deg ND pitch, with a reduction in AoA of 2deg/sec.
The third, I am assured, is doable, but the author of this data does not expect that a line crew would pitch the aircraft to 10deg ND at 6000' with a descent rate of 10,000+ fpm down. But, he says, it can be done.
The extended lines have three components - red, yellow and blue.
The red portion of each line indicates that the wing is still stalled even as CAS increases.
The yellow portion is the recovery portion as AoA reduces from a stalled to an unstalled wing.
The blue portion indicates the "run-out"...the continued descent and increased speed as the aircraft is leveled off while avoiding the secondary stall.Off-topic?
This is off-topic in one way, but very much on-topic in another way. It indicates that perhaps not enough is known and understood or even taught in ground school and recurrent training, regarding high altitude, high Mach No., swept-wing aerodynamics and in particular, the dynamics of the stall of these kinds of aircraft.
The argument for teaching/learning more about these areas is in the trended data where over the past eight years or so, almost a dozen transport category aircraft accidents, many fatal, have occurred when crews have stalled and lost control of their aircraft.
My understanding of jet transport flight came almost exclusively from D.P. Davies and my own self-directed study after I joined the airline. I thought jet transports stalled at AoAs of 12 to 15deg. They do, but Davies only discussed the approach case, when all the high-lift devices were out, (1972).
I did not know, until AF447 and detailed discussions with two aeronautical engineers regarding a deeper understanding of Mach effect that our transport aircraft are always about 1 to 3 degrees above the stall AoA in cruise. The important corollary to this fact is, of course, that changing AoAs at very high Mach numbers is very difficult. The rapid loss of energy/airspeed as a result of the instant pull-up in the AF447 accident brought the Mach down to the point where the Mach number stall AoA and the actual AoA met, and the aircraft stalled...within about 30 seconds of the initial event.
I think these are important understandings about one's aircraft. Has anyone here ever been exposed to these notions in ground school or recurrent sim training? Heck, I never once had a UAS abnormal in 35 years of recurrent training or even a discussion about it.
Edited by Don Hudson, 15 May 2012 - 10:08 AM.