After lots of speculations, most based statements of so called ´experts´ we returned to a more technical point of view.
Nice to speculate on thrust reverser deployment, eventually caused by sabotage, but the very first NTSB report stated ´No evidence was found of an uncontained engine failure, loss of blades, bird strike or in-flight fire. The thrust reversers were in the stowed position.´ so let´s forget about this theory.
What we have are the pictures of the fin, the rudder and the fuselage attachment brackets on
<a href="http://www.ntsb.gov/Events/2001/AA587/tailcomp.htm" target="_blank">NTSB-Report page</a>and <a href="http://www.aviationnow.com/content/publication/awst/20011203/avi_air.htm" target="_blank">Aviation Now</a>and our knowledge about loads, structures and aerodynamics.
This tells us, that the 6 main attachment points failed in at least 3 different ways (maybe 4). The two front one failed in net tension of the eye, the mid right and aft left failed in the unidirectional carbon fibre skin of the fin itself, the mid left failed in tension at the uppermost rivet row of the repair patch. We don´t know about the rigt aft in detail, but it is for shure an eye failure, maybe in shear because no remainders are left on the fuselage bracket.
If there is a design flaw in the composite attachment points, why did they fail in different ways ? This points very strong to an overload above ultimate strength of an otherwise good fin.
The rudder and fin are seperated, they were found in a certain distance in Jamaica Bay. This tells us, they were already seperated when hitting the water. The (much heavier) fin is nearly undamaged, the (much lighter and therefor much slower impacting) rudder is broken. It is very unlikely that the rudder broke on impact, so it must have broken while connected to the fin or while seperating.
If the complete fin and rudder assemby broke of the plane, would they seperate in free fall ? Very unlikely, the forces that could act on such a free falling part are small compared to the attached condition because they are compensated by inertia forces (Sir Isaac Newton tells us so) and not taken by the attach points. So the rudder must have seperated or at least became strongly damaged while the fin was still attached to the fuselage.
The rudder is broken in two pieces, a large area of debonding is visible on the sidefaces, the trailing edge failed in an area of about one rudder chord, the lower piece is also damaged at its lower end, maybe due to contact with the fuselage. The failure in the rudder´s front spar is clearly visible right below the uppermost lightening hole, thats between the upper and the mid rudder actuator.
Some members already discussed about previous incidents and accidents where tails ripped of, what about cases where the rudder ripped of ?
There are 4 very interesting incidents on the Concorde. (remember, the Concorde was in some respect the predecessor of the Airbus, a lot of engineers were the same in both projects) for example <a href="http://www.aaib.dtlr.gov.uk/bulletin/dec00/gboac2.htm" target="_blank">AAIB-Report 1</a>or <a href="http://www.aaib.dtlr.gov.uk/bulletin/dec00/gboac1.htm" target="_blank">AAIB-Report 2</a>.
The pictures don´t look to different from the failure of AA587 rudder, the failure started at the trailing edge and resulted in delamination of the outer skins. There are no ribs at the actuator attachment points in an A300 rudder, this means all the loads are introduced into the sidepannels (in tension and compression) and distributed to the upper and lower part in shear. If the outer skin is debonded or the trailing edge fails in the area of the actator attachment, the sidepannels are no more able to carry compression loads, resulting in much softer attachment chracteristics and reduced ultimate loads.
We already discussed a lot about system failures (actualy its the topic´s title), so could a softer rudder caused by trailing edge damage due to wake encounter loads affect the stability of the control system, especially the yaw damper ?
Systems specialist, please think about this.
One last interesting point the pictures tell us : All 3 attachment brackets on the fin broke of the rudder´s front spar, they all look the same, they are all alligned in the center position. Does anybody know more about Airbus hydraulic control system ? Does the yaw damper act on all of the hydraulic actoators, or only one one ? Are there springs in the system that center the actuators in case of hydraulic pressure lost ? Does the actuator position tell us, that the control system was working normally ? Is it possible that the upper and mid actuator worked against each other (this would explain the location of the rudder front spar failure) are they connected to separate control systems ?
At last a link to three nice pictures that show how a wake vortex of an B747 can look like
<a href="http://www.aviationpics.com/pretty/m-smoke1.jpg" target="_blank">Picture 1</a>
<a href="http://www.aviationpics.com/pretty/m-smoke2.jpg" target="_blank">Picture 2</a>
<a href="http://www.aviationpics.com/pretty/m-smoke3.jpg" target="_blank">Picture 3</a>
have a nice christmas folks