A joint University of Illinois - NASA Glenn flight test program has been conducted as part of the Smart Icing System program (being developed by researchers at the University of Illinois and the Ohio State University). The objective of this test was to develop and evaluate the identification and characterization methods used in the smart icing system using flight data from clear air and in natural icing conditions. The first phase of the flight test was conducted in February and March 2001 using the NASA Twin Otter research aircraft to provide flight test data. A linear regression fit of the flight data to acquire stability and control derivatives showed many promising results. Significant changes in the aircraft trim values and stability and control derivates were observed due to ice accretion. The longitudinal derivatives were very sensitive to ice accretion and showed promise as a means to characterize the ice accretion effects. The effect of selectively deicing the aircraft component by component was also observed through the changes in the aircraft trim and stability and control derivatives. Atmospheric turbulence, as measured by aircraft z-accelerations, were seen to have a detrimental effect on the identification.