Originally Posted by
sandiego89
For educational sake, the vertical tail surface is called a vertical stabilizer. The rudder is the movable control surface at the aft end of the vertical stabilizer. In the flight 587 case you share the video of the entire vertical stabilizer departed the aircraft due to overstress from improper rudder inputs.
AA587's loss had a number of factors going on, and for an event in Nov 2001, the term "improper" loses a bit of it's crispness as causation.
- The FO who was PF had experience in aircraft that were able to take aggressive control inputs.
- The Airbus manner of giving a rudder ration limiting effect does not act to reduce the sensitivity of the input, ti does the exact opposite; instead of the Boeing method of reducing the deflection for a given deflection of pedal input to reduce loads, the bus does the opposite, it reduces the pedal deflection available, to reduce the rudder deflection, and that means for a given aero load, the Airbus sensitivity of the rudder pedals becomes more sensitive.
- The certification of the load sequence of the rudder was not required to be accounted for in the loads analysis or in the TIA.
- the sequence of alternating deflections coinciding with high yaw angles resulted in very high bending loads, and the rudder deflection added alternating torsion loads, and they went wild.
- The vertical stabiliser was mounted by a series of pins running longitudinally along the perimeter of the composite stabiliser to fix the primary load paths. The secondary load path was by an internal yoke system.... the load required to shear the attachment lugs results in a lever arm for the structure that exceeds the failure loads for the secondary structure, as the lug/pin failure results in a lever arm acting on the secondary structure.
- AI and TBC both had guidance for jet upset that had crew pre primed (a Gary Klein sort of concept) to wiggle them pedals.
- The awareness of the structure sensitivity to alternating torsion-bnnding was not recognised in Part 25 before the accident.
We know a lot more after that event than we did beforehand. Amazingly, we were recording QAR/DFDR data of cyclical rudder imputs after that disaster, with the penny not dropped.