NTSB says design must be modified and that airline's training also contributed

American Airlines flight AA587 crashed in 2001 following the separation of the fin because of excessive rudder input by the first officer, with contributory factors being the sensitivity of the Airbus A300-600R rudder system, and "elements" of the airline's advanced aircraft manoeuvring programme (AAMP), says the US National Transportation Safety Board draft accident report.

The order in which the contributory factors - rudder sensitivity and AAMP - are now stated was the result of a reversal of the two by the board at the last moment. Among the recommendations are calls to create a new certification standard for safe aircraft handling qualities in the yaw axis "including limits for rudder pedal sensitivity", and for modification of the A300-600.

The 12 November event killed all 260 people on board when the fin completely separated under sideslip overload, rendering the aircraft uncontrollable. It crashed into Belle Harbor, New York. Sideslip overload was caused, the report says, by "unnecessary and excessive rudder pedal inputs" by the first officer, who was reacting to the wake turbulence from a Boeing 747. The NTSB says: "American Airlines' [AAMP] ground school training encouraged pilots to use rudder to assist with roll control during recovery from upsets including wake turbulence." It adds that AAMP simulation of wake turbulence gave pilots an "unrealistic impression" of the power of wake vortices on a large aircraft and Flight 587 never approached "upset" proportions.

American says the rudder was unexpectedly sensitive, and it had not appreciated how easily pilots could over-use it. The NTSB finds line pilots were not aware that the A300-600R's rudder pedal forces and travel reduced at higher airspeed and found no mention in the flight and training manuals.

Airbus and Boeing jointly issued a pilot training aid in 1998 warning that mishandling a large aircraft can relatively easily overstress it, and last week they reissued it with additional advice.

 Draft conclusions

* Cyclic rudder motions after the second wake vortex encounter turbulence encounter were the result of the first officer's rudder pedal inputs.

* The vertical stabiliser performed in a manner that was consistent with its design and certification. The vertical stabiliser fractured from the fuselage in overstress, exposed to aerodynamic loads that were twice the certificated limit load design envelope and were more than the certificated ultimate load design envelope.

* Certification standards are needed to ensure that future aircraft designs minimise the potential for aircraft-pilot coupling susceptibility and to better protect against high loads in the event of large rudder inputs.

* Because of its high sensitivity (that is, light pedal forces and small pedal displacements), the A300-600 rudder control system is susceptible to potentially hazardous rudder pedal inputs at higher airspeeds.

* There is a widespread misunderstanding among pilots about the degree of structural protection that exists when full or abrupt flight control inputs are made at airspeeds below the manoeuvring speed.



Source: Flight International