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Monday, July 24, 2006

New Ice Prediction System Offered by BAE

BAE Systems has developed a new ice protection modeling technology, predicting how ice will accumulate on aircraft, which could be a significant breakthrough for regional turboprops which have experienced major icing problems since the mid 1990s. (RAN, July 17, p. 1) The technology is a new computer model that predicts patterns of ice accumulation on complex, three-dimensional surfaces such as wings, engine intakes, propellers and rotor blades. Developed at the company's Advanced Technology Centre (ATC) in Bristol, the new technology which differs from traditional two-dimensional analysis, combines a model of ice accretion with computational fluid dynamics codes, already used in aerodynamic design, to predict ice build up in three-dimensional airflows.

The calculations, said the company, determine the trajectories of water droplets in the airflow and can include the effect of droplet splashing and the spectrum of droplet sizes typically encountered in a natural cloud. The unsteady development of the water flow across surfaces is simulated together with the rate, type and location of ice. Heat input from thermal ice protection systems together with the effect of the underlying aircraft structure can also be included in the analysis.

Six years in development, the system was funded by DTI CARAD, with partners Airbus UK, Rolls-Royce, Dunlop Aerospace, Westland Helicopters, Qinetiq and Cranfield University. Already used on the Nimrod MRA4 and tornado support programs, BAE said code findings enabled one project to avoid the requirement for an experimental icing trial that could have cost up to ?1 million. "Increasing the use of simulation both reduces costs and enables assessment of designs in conditions which cannot be regularly found in natural icing trials," said Andrew Press, an ATC aerodynamicist. "This modeling gives us a step change in the complexity of configurations we are able to simulate."

In the mid-90s, the regional turboprop fleet was subjected to icing trials as the result of an ATR accident at Roselawn, Ind. in 1994. Recently, the National Transportation Safety Board issued recommendations calling for an upward revision in the safe speeds in icing conditions for the Saab 340 after four roll-upset in icing conditions. (RAN, July 17, p.1)

After Roselawn, the Federal Aviation Association tested all regional turboprops for roll-control difficulties if a ridge of ice forms aft of the deicing boots and forward of the ailerons. The SAAB 340 had acceptable roll control forces when high-speed taxi tests were done with a one-inch quarter round (simulating the ice accretion on the Roselawn aircraft) located aft of the deicing boots and forward of the aileron. Of the airplanes tested, the EMB-120 and SAAB 2000 had higher than acceptable roll control forces using the one-inch quarter round. Consequently, they were flown behind the Air Force icing tanker to develop realistic ice accretions. The SAAB 2000 and EMB-120 were tested with the realistic ice shapes and found to have acceptable roll control forces. Its findings resulted in the issuance of an Airworthiness Directive (AD 96-09-21) applicable to SAAB SF340A, 340B, and SAAB 2000, requiring the revision of the Airplane Flight Manual to provide the flight crew with recognition cues for, and procedures for exiting from, severe icing conditions, and to limit or prohibit the use of various flight control devices. The AD does not prohibit the airplane from operating in icing conditions. In addition, similar ADs were issued on most of the regional turboprops.

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