THE US FEDERAL Aviation Administration will allow ATR 42s and 72s to be flown in icing conditions as long as pilots, despatchers and air traffic controllers follow new flight-safety and training procedures. The conditions remain in effect until an improved de-icing boot is certificated for the aircraft.

Flights in known icing conditions in North America had been banned as a result of the investigation into the fatal crash of an American Eagle ATR 72 in Roselawn, Indiana, on 31 October, 1994. The FAA action disrupted regional-airline operations in the USA and Canada, forcing operators to shift their ATR aircraft to warmer climates.

Critical to the easing of the ATR restrictions is the FAA's decision to bar the use of flaps when aircraft are in a holding pattern in icing conditions and to prohibit a change of flap position, should icing be encountered. The flap-use restriction results from testing at Edwards AFB, California, and in Toulouse, France.

The Simmons Airlines ATR 72 which crashed had been holding for 37min in icing conditions. The autopilot was engaged and flaps set at 15¡. On descent, the flaps were retracted, the autopilot disconnected and the aircraft rolled severely.

Windtunnel tests and high-speed taxi tests conducted at Toulouse revealed that ATR aircraft could suffer from the effects of icing-induced aerodynamic aileron-lock under certain conditions. It was suspected that a specific type of ice build-up behind the de-icing boots on the upper surface of the right outer wing could produce the problem encountered by the crashed aircraft. This could have contributed to the asymmetric wing icing believed to have led to loss of aircraft control.

Flight testing at Edwards AFB, California, left no doubt that the ATR 42 and ATR 72 comply with icing-certification regulations. An abnormal icing condition not covered by any certification requirement may have occurred, however, involving unusually large water droplets, measuring 180 microns in diameter.

When the severe weather conditions over Roselawn were simulated at Edwards AFB, it was demonstrated that an ATR aircraft with a 15¡ flap-down setting suffered ragged-ice accretion behind the de-icing boots. It was shown that, when the flaps are retracted in asymmetric wing-icing conditions, aileron lock can occur.

FAA certification chief Anthony Broderick says: "The tests show that you can build up ice on unprotected wing areas that produce roll forces consistent with what occurred in the Roselawn accident." He adds that aileron-induced roll conditions can be avoided, provided that the flap setting is not altered during severe icing conditions.

The short-term fix requires pilots to undergo re-training on aircraft operations and weather procedures before they may fly ATR aircraft in icing conditions. Despatchers and air-traffic controllers must also be briefed on the new procedures.

"We are confident that the measures we have put in place address any icing condition that would give us concern," adds Broderick. The long-term solution is a larger de-icing boot. Broderick says that the FAA will mandate installation of the hardware modification once it is proven in flight tests. He says that some ATR operators could comply with the new rules in as little as seven days.

Some Simmons Airlines pilots refused to fly the ATR 72 in the wake of the Roselawn accident, and the US Air Line Pilots Association (ALPA) had opposed lifting the ATR flight restrictions. ALPA boss Randy Babbitt now backs the latest FAA action.

ATR says that the FAA move vindicates the aircraft, although it acknowledges that "...the tests showed that some characteristics of freezing drizzle pose a potential hazard to all aircraft". ATR believes that the fixes have established "...a new industry standard that should quickly be incorporated into airworthiness-certification requirements for all commercial aircraft".

Source: Flight International