Ramon Lopez/WASHINGTON DC
THE US FEDERAL Aviation Administration is introducing new flight-operations rules in icing conditions for a range of turboprop regional airliners. The agency has not, however, demanded any design changes similar to those already required for the ATR 42 and 72.
The aviation agency evaluated 17 different turboprop types following the crash of an American Eagle ATR 72 on descent to Chicago O'Hare International Airport in severe icing conditions on 31 October, 1994, killing 68 people.
The US National Transportation Safety Board (NTSB) has yet to issue its findings, but extensive testing by the FAA and the manufacturer shows that ATR 72could suffer from the effects of icing-induced aerodynamic aileron-lock if subjected to unusually severe freezing rain or freezing drizzle. Such abnormally severe icing was not covered by any certification requirement.
Although US carriers flying ATR 42/72s have installed larger wing- leading-edge de-icing boots, which nearly double the area of coverage, the FAA believes that new procedures and restrictions will provide an additional margin of safety for flight in severe icing conditions.
The proposed airworthiness directive (AD) for ATR 42/72s prohibits flight in freezing rain, a meteorological condition, which could produce problems for any aircraft. It also limits the use of the flaps and prohibits use of the autopilot when ice forms, aft of the de-icing boots. The AD also requires that all icing-detection lights be operative, before entering icing conditions.
The FAA estimates that 158 ATR 42/72s are affected by the proposed AD. Those aimed at other affected turboprops will outline similar flight-manual revisions to that for the ATR aircraft. Flight-crews must be provided with visual cues for recognising severe icing, and procedures for dealing with unexpected icing conditions.
Aircraft types covered by the proposed Ads, include the ATR 42/72; Beech 99/1900; British Aerospace HS.748; Cessna 208; CASA C.212 and CN-235; de Havilland DHC6 Twin Otter, Dash 7/8; Dornier 228/328; Embraer EMB-110 Bandeirante and EMB-120 Brasilia; Fairchild SA226/227; Fokker F27 and Fokker 50; Jetstream 31/32/41 and ATP; Saab SF340 and 2000; and the Shorts SD3-30/3-60 and Sherpas
Source: Flight International
