Julian Moxon/PARIS Ramon Lopez/WASHINGTON DC

ATR IS PROPOSING a modification, which would double the size of the wing de-icing boots on all ATR 42s and ATR 72s.

The idea follows research at Edwards AFB, California, into the causes of the crash of an ATR 72 at Chicago in October 1994. ATR says that, while the tests left no doubt that the type complies with all current icing-certification regulations, there are concerns that an abnormal icing condition may have occurred involving unusually large water droplets.

This could have contributed to the asymmetric wing icing which is believed to have led to loss of control of the aircraft.

The US Federal Aviation Administration is expected shortly to lift the ban on ATR aircraft flying in actual or forecast icing, which was imposed after the crash. It will issue a new airworthiness directive at the same time setting out new cockpit procedures for the crew to follow if certain icing phenomena are observed.

ATR says that the de-icing boots would be increased in size so that they would extend over 14% of the chord instead of 7%. This would involve removing the composite leading edge, which extends back to the wing spar, and fitting larger rubber boots. "It could be done in a matter of hours," says ATR.

ATR's proposal to the FAA also calls for interim procedural changes for despatch and operation of the aircraft into known or forecast icing conditions and additional training, for pilots and despatchers.

In the Edwards tests, a Boeing KC-135 water-spray tanker was flown ahead of the ATR 72, releasing two sizes of water droplet measuring 40-70 microns and 180 microns in diameter. The latter is not covered by any certification requirement, but is representative of the droplets thought to have been present at the time of the accident.

The tests revealed that a recognisable pattern of ice builds up on the cockpit side-window when the aircraft flies into the larger droplets. This and other signals such as, abnormal aerodynamic behaviour, could be used by pilots to anticipate ice build up. "We're facing something that has not been seen before," says an ATR source. "It is like the wind-shear problem that suddenly appeared in the 1970s and 1980s".

There is opposition to the ATR plan, however. The Association of Flight Attendants (AFA) wants the FAA to keep the existing AD in place until a satisfactory design change is found to eliminate ATR control problems during icing conditions. It claims that the Edwards AFB tests confirm problems with the aircraft's design (which it does not specify).

The US Airline Pilots Association agrees. Its head, Randy Babbitt, calls the ATR plan "unacceptable" saying that "...we don't believe sufficient test data has been generated to substantiate their position".

He rejects the side-window detection procedure, because it would require an encounter with icing conditions before the conditions can be detected. "That's what we're trying to avoid - its like touching a burner to see if the stove is on," Babbitt says.

Gilbert Defer, the ATR 72 pilot for the Edwards AFB flights, says: "Never before have so many people who were on the ground, thousands of miles away, claimed to know so much about what happened in an aircraft that I was flying...I can assure you that the aircraft responded well in every test environment...the plane remained under full control at all times, in all types of severe icing conditions, including freezing rain."

Source: Flight International