Graham Warwick/ATLANTA
CRUCIAL TO THE disagreement between US and French accident-investigation agencies on the cause of the October 1994 American Eagle ATR 72 crash is whether previous icing-related incidents involving ATR aircraft provided evidence which could have averted disaster.
The US National Transportation Safety Board (NTSB) says that aileron hinge-moment reversal, the root cause of the 1994 Roselawn crash, was observed in previous ATR incidents. France's Bureau Enquettes Accidents (BEA) maintains that no investigation of the previous events had identified the phenomenon which caused the American Eagle accident.
The NTSB reviewed 24 ATR roll-control incidents, all involving ATR 42s, determining that 13 were related to icing, of which five occurred in freezing drizzle and rain - the conditions in which the Roselawn accident occurred. The incidents involved:
American Eagle, at Mosinee, Wisconsin, in 1988;
Air Mauritius, over the Indian Ocean in 1991;
Ryan Air, over Ireland in 1991;
Continental Express, at Newark, New Jersey, in 1993;
Continental Express at Burlington, Massachusetts in 1994.
The NTSB investigated the Mosinee and Newark incidents, the BEA participated in the Mosinee investigation and ATR probed the Indian Ocean and Ireland events and participated in all five investigations. There is basic disagreement between the USA and the French over what was observed in these incidents - and the BEA issued a statement claiming that the factual basis of the NTSB's Roselawn accident-report is wrong.
According to the Board, ATR's analysis of the Mosinee incident concludes that ice formed on the upper surface of the wing, aft of the de-icing boots, changing the aileron hinge-moments and resulting in an auto-pilot disconnect, uncommanded aileron deflection and subsequent roll excursions. This is uncannily close to what happened at Roselawn.
The NTSB says that ATR had experienced aileron hinge-moment reversals during ATR 42 and 72 development and that, in the late 1980s, it had installed vortex generators to raise the angle of attack (AoA) at which the ailerons would became unstable and hinge-moment reversal would occur. It says that ATR also designed the stall-protection system to activate at AoAs lower than those at which the ailerons would become unstable.
After Mosinee, ATR warned operators of possible roll-control problems resulting from ice accretion, added vortex generators and an anti-icing advisory system to all aircraft and developed an icing package for ATR simulators. In 1992, ATR issued an all-weather operations brochure, providing information on freezing rain and re-iterating the warning about roll-control problems.
Despite these actions, the Continental Express incidents in 1993 and 1994 involved aileron hinge-moment reversal, the NTSB says. After these events, the Board says, the French DGAC airworthiness-authority should have recognised that the design changes and warnings were insufficient to prevent the recurrence of the phenomenon, and should have required ATR to take further action. Its failure to do so led directly to the Roselawn accident, the Board states.
The NTSB and the BEA agree that the five ATR 42 incidents, and the ATR 72 crash, occurred when the aircraft were flown in icing conditions outside the certification envelope. The BEA says that the Roselawn flightcrew - largely exonerated by the NTSB - failed to comply with flight manuals, federal rules and company policies.
Here, there is major disagreement. The NTSB argues that the accident could have been avoided had there been explicit reference in the flight manual to aileron hinge-moment reversal, and a specific warning about "-unexpected autopilot disconnects and the rapid and uncommanded aileron and control-wheel deflections to near their full travel limits with unusually high, unstable, control-wheel forces experienced by the flight crew at Mosinee".
The BEA says that ATR specifically advised operators that icing could affect roll-control forces, leading to an autopilot disconnect and resulting roll excursions, and described recovery procedures. The Roselawn flightcrew, the French agency says, "-was so oblivious to the icing conditions they encountered that they ignored the multiple warnings, instructions and regulations they already had received regarding proper operations in such conditions" and, surprised by the sudden roll when the autopilot disconnected, they failed to recover the aircraft.
As to the NTSB's assertion that aileron hinge-moment reversal had been observed in previous ice-induced roll-control incidents, the BEA says that flight-recorder data from the Mosinee, Ryan Air, Air Mauritius and Burlington accidents show that "-they were all stall departures after ice accumulations which resulted from flightcrew failures to follow basic procedures for operation in icing".
Source: Flight International
