These were confirmed by the changes in the longitudinal (0.20 to 0.14 G), lateral (0.05 G. To the left) and normal load factors (1.0 to 0.6 G). [NTSB ]
The data also showed that between the critical time of 0915:52 and 0915:58.5 five separate rudder movements (1.7 inches right to 1.7 inches left, 1.7 inches right, 2.0 inches right, 2.4 inches left, and 1.3 inches right) were noticeable. The FDR data also showed extreme and alternating control wheel movements during this time (64° to the right at 0915:51.5, 78° (full) to the left at 0915:53.5, 64° to the right at 0915:55.5, and 78° to the left at 0915:56.5.). [NTSB] The enormous aerodynamic stress due to these extreme control operations of the rudder ultimately resulted in the breaking away of the vertical stabilizer and the crash of the aircraft. It was found that the vertical stabilizer exceeded the design loads by 100%. [Lori Ranson, 2010] In its report the NTSB stated, "that the probable cause of this accident was the in-flight separation of the vertical stabilizer as a result of the loads beyond ultimate design that were created by the first officer's unnecessary and excessive rudder pedal inputs." [NTSB, 2004, pg 174]
Rudder Sensitivity (The ignored Danger?)
While the NTSB investigation attributed pilot error as one of the contributing factors for the disaster, the investigation also revealed some technical aspects connected with rudder sensitivity of the airbus A300-605R that could be potential dangerous. The airbus A300-600 model has a variable stop design, which includes a rudder travel limiter system. This means that the rudder sensitivity varies according to the airspeed, with the rudder being more sensitive at higher speed. It also has one of the lightest pedal forces among the different aircraft models. This increasing rudder sensitivity also increases the 'pilot coupling susceptibility'. In fact, couple of other aviation incidents have since been reported that directly relate to the rudder problem. For instance, the March 6, 2005 incident in which the rudder of a Canadian airline 'Air Transat' flight A310 detached and flew off from the tail during the flight from Cuba to Quebec. [Bob Cox, (2006)]
More recently in 2008, Air Canada Airbus A319 experienced wake turbulence due to a Boeing 747 flying ahead of it. At 36,000ft, the flight experienced severe rolls with a maximum roll of up to 55e_SDgr. The FDR also showed severe vertical load factor oscillations similar to the 2001 American airline 587. The three alternating rudder inputs reported in this incident increased the vertical stabilizer load to exceed the maximum permissible limit by 29%. [Lori Ranson, 2010] In a report to the European Aviation Safety Agency in connection to the incident, the NTSB stated, "The similarities between the Air Canada Flight 190 and the Flight 587 crew's responses to wake encounters indicate that the A320 family is also susceptible to potentially hazardous rudder pedal inputs at higher speeds." [Lori Ranson, 2010]
What has been Done?
The NTSB has made mandatory regular rudder inspections. Prior to the accident, airline pilot training programs did not include the rudder sensitivity topic. In fact, in its report to the NTSB, American airlines maintained that Airbus did not disclose the special rudder sensitivity issues in the specific models. Marion Blakey the former chairman of the NTSB issued a serious warning to all pilots "Certain rudder inputs by pilots made during certain stages of a flight can cause catastrophic failure of an airplane's vertical stabilizer," [PBS] In particular, the NTSB issued clear guidelines strongly recommending the necessity of standardizing training programs with particular focus on the dangers of alternate rudder inputs and that such rudder controls are not necessary for controlling transport category planes. The NTSB has also pressed the EASA to change its design certification requirements and to consider variable-ratio rudder travel limiter in lieu of the variable-stop systems that are in use in the airbus 300-605 models.
The American airline 587 involved in the fatal crash in Nov 2001 was an eye opener in many ways. The sad fact is that the lack of proper communication between Airbus and American airlines was a contributing factor to the disaster. Had airbus specifically warned American airlines of the rudder sensitivity issue it would have been incorporated into the pilot training programs. The investigation, which originally focused on a possible terror attack, changed focus into the possible defects of the Carbon composite material used in the tail joints. The recovery of the FDR and analysis of the data further changed the course of the investigation from the machine to the pilot who was controlling the machine. First officer Sten Molin's lack of awareness of the rudder sensitivity and the potentially catastrophic dangers of alternate rudder inputs at high speeds lead to the disaster. Some changes such as corrected training programs for pilots, frequent inspections of the rudder and a general sense of awareness of the problem have happened. However, the NTSB's recommendation to the EASA to make changes in the certification standards relating to the rudder design is still under study and no changes have happened in this regard. A more stringent testing and approval from the FAA is urgently required to avert such dangers in future.
1) National Geographic Channel, ' Seconds Before Disaster, Plane Crash In Queens', retrieved Oct 30th 2010, from, http://www.youtube.com/watch?v=AFmVUok05E4
2) Ellen Engleman Conners, (Oct 29, 2004), 'Message from the Chairman', retrieved Oct 30th 2010, from, http://www.ntsb.gov/Speeches/engleman/cc/cc20041029.pdf
3) NTSB, (Oct 26th 2004), 'In Flight Separation of Vertical Stabilizer, American Airlines Flight 587, Airbus Industry A300-605R, N14053, Nov 12, 2001' Aircraft Accident report, retrieved Oct 30th 2010, from, http://www.ntsb.gov/publictn/2004/AAR0404.pdf
5) Lori Ranson, (Aug 2010), 'NTSB Flags concern about Airbus Rudder Sensitivity to EASA', retrieved Oct 30th 2010, from, http://www.flightglobal.com/articles/2010/08/11/346014/ntsb-flags-concern-about-airbus-rudder-sensitivity-to.html