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“We Freeze to Please” On 8 November 1994, the National Transportation Safety Board (NTSB) contacted the icing branch at Lewis and asked for technical support in its investigation of the Roselawn tragedy. The NTSB was especially interested in obtaining information on ice accretion, ice-protection systems, and data on iced airplane stability and control. NASA responded to assigning researchers Jaiwon Shin and Thomas P. Ratvasky to the NTSB’s Airplane Performance Group.3 Shin and Ratvasky traveled with the group to Toulouse, France, on 12 November. For a week, the group met with the ATR engineering staff to review icing certification documents and test procedures, and to discuss previous roll-control anomalies due to icing. Shortly after Shin and Ratvasky returned to the United States in early December, the NTSB asked NASA to conduct tests in the IRT of an MS-317 airfoil, which had a similar cross-section profile to the outboard portion of an ATR 72 wing. The tests, con- ducted on 8–10 and 12–14 December, showed that large super-cooled water droplets, which had been detected in the vicinity of the Roselawn accident, could accrete beyond the area that was protected by pneumatic boots. Even before the tests were completed, the FAA had sufficient concern about the problem to prohibit ATR 42/72s from flying in “known or forecast” icing conditions. While NASA developed a program to look more carefully in the phenomenon of large droplet icing, an FAA special certification review team arranged with the U.S. Air Force to use their KC-135 spray tanker for flight tests with an ATR 72. Again, the Board requested the technical assistance of the icing section. This time, Richard Ranaudo, an experienced Twin Otter research pilot, accompanied Ratvasky to Edwards Air Force Base, California, the site of the tests. The tanker flights began on 14 December. Ranaudo’s experience soon proved valu- able to the test team. For example, he suggested using tufts on the upper surface of the ATR 72 wing to help detect flow separation. At the same time, Ratvasky held discussions with Air Force personnel about icing cloud characteristics and measuring techniques. The tanker tests demonstrated that in large droplet conditions with flaps at 15°, a ridge of ice could form at the 9 percent chord position of the ATR 72’s wings, whereas the air- craft’s pneumatic boots covered only to the 7 percent chord. This problem no doubt caused the uncommanded roll event at Roselawn. On 11 January 1995, the FAA withdrew its blanket prohibition against flight into icing conditions for the ATR 42/72, pending the fit- ting of new pneumatic boots to cover a larger wing area and new training for pilots. The Roselawn investigation marked the beginning of a major program at Lewis to study the icing conditions found in freezing drizzle and freezing rain, collectively known 3 Icing Technology Branch to the director of aeronautics, “Summary of NASA LeRC Involvement in the ATR-72 Accident in Roselawn, IN,” 5 January 1995. The author is indebted to Mr. Ratvasky for a copy of this document. 134PDF Image | History of NASA Icing Research Tunnel
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