Originally Posted by ”PJ2”
That's exactly what the BEA is trying to convey: level aircraft attitude, vertical trajectory with possible slight forward and perhaps even aft, trajectory, (possible explanation of the condition of the one spoiler found - torn out by reversed airflow, as someone posited a thousand posts ago) - not a straight line down, in other words.
As usual your post is relevant and well thought out. I don’t argue with your hypotheses but let’s take this a little farther if I may. For starters, I will assume that the BEA report is accurate, i.e., the aircraft struck the ocean in a ‘level attitude” with a near vertical trajectory. Why?
I would suggest that a ‘near vertical trajectory’ (descent) – in a level or flat attitude - is not possible unless both wings are fully stalled – the aircraft isn’t flying, it is falling and there is very little ‘forward motion’. I would postulate further that in such a prolonged “stall” condition a spin of some type, would almost certainly develop. If there was a “spin” there would also be rotation. If there was a conventional spin, the nose would be down – unless the ‘spin’ was a “flat spin” – in which the tail could be down and the nose up with reference to the horizon. The BEA made no mention of “rotation” on impact.
I do not understand how an aircraft can descend with a vertical trajectory if it is not spinning. I do not understand how it can “spin” without rotation. Even if the stick was held full nose up after the stall and throughout the descent, I do not see what wound keep one wing from dropping off at some point and producing a conventional nosed down spin. However, a rearward cg or some other factor could induce a "flat spin" - and that could fit the scenario implied by the BEA.
There has been considerable debate as to whether an airliner can enter a ‘flat spin”. The answer is: YES. How do I know this - Historical record from prior accident(s). [See BOAC 911 and Vladivostok Air 352]
We all know that a “spin” by definition, requires at least two elements: 1) the wing must stall [and remain stalled]; 2) there must be rotation (about the aircraft’s cg.), [and a near vertical descent will follow].
According to NASA, there are four (4) categories (modes) of “spin” classification:
NASA Spin Mode Classification
Spin Mode .................. Angle-of-attack range, degrees
Steep .................................20 – 30
Moderately steep ................. 30 – 45
Moderately flat .................... 45 – 65
Flat .....................................65 - 90
By definition, a “Flat Spin” = A spin in which the tail of the aeroplane drops and the machine rotates in an almost horizontal pane. The nose may be slightly below or above the horizon. A rearward CG will augment the tendency to flat spin. The following video illustrates an unrecoverable flat spin in a fighter aircraft.
http://www.youtube.com/watch?v=z7VS9_Ce0sg
I am not aware of any transport category airplane that can recover from a fully developed spin - especially a flat spin, or retain its structural integrity within one, given the forces that are experienced.
I do not see how, as inferred by the BEA report, a near vertical trajectory of descent, implying alpha of 65 to 90 degrees, could occur in anything other than a flat spin. Perhaps you or someone else could enlighten me if that is wrong.
In the BOAC 911 accident (a 707-463), it was determined that the aircraft entered an area of extreme (clear air) turbulence. The vertical fin failed and was torn from the aircraft taking other parts of the empennage with it. Subsequently the severe yaw tore the engines from their mounts and the remainder of the structure entered a flat spin until impact. [You have previously posted a photo of that aircraft in descent]. A military fighter sent to search for the wreckage also experienced extreme turbulence – with recorded 9g positive and 4g negative - and temporary loss of control. Its pilot was able to recover but that was a fighter aircraft. IMHO, Airliners cannot survive such forces intact.
In the Vladivostok Air accident (TU-154), the stall was induced by pilot error during the approach and the aircraft entered a flat spin, in which it remained for 22 turns until impact. Unlike the Boeing, this is a “T-tail” aircraft with engines mounted on the rear fuselage (which would likely negate their separation), plus T-tails are susceptible to deep stalls. Also, that aircraft was much closer to the surface when the stall occurred.
IF AF447 entered extreme turbulence, it would have experienced very high g forces, both positive and negative, in rapid succession and most probably stalled. It is likely there would have been severe skidding and slipping as well – all induced by the turbulence. Its pilots would not have been able to prevent this – once the area of turbulence had already been entered. A spin could have resulted – which might have caused the VS to separate – thus creating a “flat spin”. The fin didn’t have to separate by itself – the entire aft tail structure could have separated – somewhere behind the pressure bulkhead. The centrifugal forces would most probably immobilize the crew at least initially.
If there was a flat spin, it is probable that not only the VS could have separated but, the engines would not only have flamed out (as you correctly surmise) but might likely (one or both) have been torn from their mounts – by extreme yaw when the fin/tail separated, or by centrifugal force of the flat spin. [In the AA587 accident, if I recall correctly, at least one engine separated from its mounts due to the yaw.]
Just as extreme turbulence or a complete stall could disrupt airflow to the engines, could it not also disrupt airflow to the pitot tubes and thus generate the loss of dynamic pressure and the associated errors/warnings (unreliable airspeed, etc.,) [as opposed to icing]? Could not extreme turbulence independently exceed the limitations of the Autopilot and cause it to disconnect - absent other factors? Other components, such as the spoiler to which you refer, may also have been partially deployed and separated.
If either pilot was holding the side stick when the extreme turbulence occurred – could it not have raised his arm violently (negative g) and induced an unintended control input? Anyone who has had an inadvertent encounter with severe turbulence should be able to readily imagine what might happen in extreme turbulence.
At FL350 a CB with tops at FL500 or 550 is quite likely to be capable of producing
extreme turbulence. In severe turbulence you might have a change; in extreme turbulence that is improbable.
Do you as an experienced Airbus captain believe that the built in computer ‘protections’ could prevent the aircraft from exceeding either end of its envelope in extreme turbulence? I don’t have the knowledge to guess but I don't believe so. Those things just aren't designed for conditions that are never expected to occur - neither is any other part of the aircraft.
This [flat stall/spin] could then permit the remainder of the structure (fuselage and wings) to descend ‘in a near vertical trajectory’ and strike the ocean in a right side up ‘level aircraft attitude’ – mostly intact – thus explaining the observed ‘compression’ of some recovered parts.
For this to happen, it would seem that the aircraft would also have to exit the area of turbulence – otherwise there is only a 50/50 chance of the falling structure to have remained ‘upright’ during the descent.
The plethora of ACARS messages may have been related to effects triggered by the turbulence – not to pitot icing - and not causal of the incident. I respectfully submit that there is a difference between severe turbulence and extreme turbulence. In the latter the aircraft is not controllable – with or without computers IMO.
We do not know if the aircraft encountered either severe or extreme turbulence, but it is just as probable as any of the other hypotheses. There was certainly weather in the vicinity that could have produced either. IF this did occur, we could well have a logical (though not factual) explanation of everything else.
This is NOT intended to disparage the crew. We do not know if they deviated, if they never detected the wx, or why. There could have been a variety of reasons not related to any failure on their part. I am not willing to judge them without certain knowledge. I am also convinced that they did not intentionally fly into a CB - no matter who was on the flight deck at the time. They were not that inexperienced.
If the ultimate loss of the hull did occur as a consequence of an encounter with extreme turbulence, that in no way reflects negatively on the design or structural integrity of the A-330. No airliner is designed to operate in extreme turbulence. That is why the possibility of such an encounter must be avoided at all cost.
The BEA’s assumption that compression damage observed in the wreckage indicates that the aircraft was “intact on impact” may only be partially correct. Some major parts of the fuselage may well have been intact and the wings still attached at impact. That however, does not preclude separation of other parts in flight. To bluntly state that there was “no break-up in flight” appears to be premature at best. The terminology is highly ambiguous.
Obviously, this is conjecture – but so is everything else that we have explored to this point, including the Preliminary Report