EMBRAER'S EMB-120 Brasilia regional turboprop has been cleared for flight in super-cooled large-droplet (SLD) icing, without modification, following a series of ground and flight tests. The tests were required by the US Federal Aviation Administration following the October 1994 crash of an ATR 72, caused by SLD icing.

The FAA required manufacturers of all regional aircraft with pneumatic de-icing boots and mechanical flight-controls to test for roll-control problems caused by severe icing.

High-speed taxi tests, were required with ice shapes attached to the wing leading-edges ahead of the ailerons, but Embraer elected to flight-test, says engineering director Luis Affonso.

The test flight in October 1995 revealed aileron control-forces "slightly higher" than the 27kg limit set by the FAA, he says, although the aircraft was fully controllable and was landed safely even after the wooden ice-shape on one side was jettisoned intentionally to simulate the worst icing conditions.

Because of the control forces encountered, Embraer elected to conduct in-flight icing tests with the EMB-120, using the US Air Force's Boeing NKC-135A icing tanker. Whereas the exaggerated, 25mm-high, shapes used in the initial tests were "conservative", the icing tests would produce real shapes, Affonso says.

Normal small-droplet and super-cooled large-droplet icing tests were conducted and resulted in much less coverage of the wing span, with "very few" 25mm peaks, he says. Using these results, new ice shapes were made and flight-tested and aileron forces of 18-27kg measured. "These were still very conservative tests," Affonso says.

Tanker tests also enabled Embraer to identify visual cues to warn of SLD icing. In normal icing, Affonso explains, ice forms, on the forward part of the propeller spinner, while in SLD icing, the ice forms further back near the blade roots. "If you see ice on the spinner, then it is on the wing," he warns.

Affonso says that Embraer's EMB-110 Bandeirante was cleared for flight in severe icing after high-speed taxi tests.

Source: Flight International