Investigation into 1997 American Airlines incident reveals flight control system allowed stabiliser to be overloaded
The US National Transportation Safety Board (NTSB) has urged design modifications to the Airbus A300-600 rudder travel limiter system in the wake of the 1997 upset of an American Airlines aircraft which overloaded the vertical stabiliser and led to its replacement.
The NTSB says its renewed interest in the data from the incident on Flight 903 "resulted from preliminary findings in its investigation of the 12 November 2001 accident involving American Airlines Flight 587". In a letter to US Federal Aviation Administration administrator Marion Blakey, it adds that, although the cause of the fatal accident in New York is still being investigated, "information to date indicates that the vertical stabiliser was subjected to large aerodynamic structural loading during the accident sequence". It also adds that the safety issues discussed were "not a factor in the Flight 587 accident".
In the 1997 incident, an in-flight upset at an altitude of 16,000ft near West Palm Beach, Florida triggered the aircraft's stall warning system. During the event, the aircraft rolled to extreme bank angles left and right, and was stalled several times during a rapid descent of more than 3,000ft. One crew member was severely injured and the aircraft damaged. However, it was not until five years later, during the Flight 587 crash investigation, that the incident aircraft's composite vertical stabiliser was inspected and found to have delamination damage.
As with other large transport aircraft, the A300-600's flight control system is designed to reduce the maximum possible displacement of control surfaces as airspeed increases. The A300-600's rudder travel limiter (RTL) allows up to a maximum of 30° up to 165kt (300km/h) and deflection with gradual reductions down to 3.5° at 395kt and above. However, investigations reveal that the RTL "cannot maintain this schedule in response to more rapid airspeed changes, such as those experienced during the Flight 903 upset", says the NTSB.
Because of this, it adds, the rudder travelled in excess of its RTL design limit for about 20s. The rudder twice exceeded its design limit by about 8¡, and twice by about 5¡.
Further investigations showed that if a pilot applies enough rudder pedal force when the pedal is at its travel limit, the effectiveness of the RTL can be affected. The NTSB says it is "concerned that such an increase in available rudder beyond the designed RTL restrictions could permit excessive rudder movements and possibly result in high loads on the vertical stabiliser".
It is therefore calling for FAA-mandated modifications by Airbus to enable the system to respond effectively to rapid airspeed changes "such as those that might be experienced during upsets and not be adversely affected by pedal forces". Airbus says it acknowledges the NTSB's recommendation and is awaiting the FAA's reaction.
GUY NORRIS / LOS ANGELES
Source: Flight International
