Large versus small droplet icing

Ice accumulation on aircraft surfaces depends on many factors, one of which is supercooled liquid water droplet size. Aircraft collect more supercooled water when the droplet size is larger. However, the aircraft ice accumulation with large droplets often is smooth so the resultant performance loss often is minimal. Research has suggested that droplets in the drizzle size range (50-500 micron) are the most hazardous.

The National Aeronautics and Space Administration (NASA) John H. Glenn Research Center has developed software that evaluates the thermodynamics of supercooled droplets as they impinge on a body given aerodynamic and meteorological inputs, including droplet size. LEWICE 2.2.2 provides an ideal way to test the range of droplet size on ice accumulation. Once computed, the change in lift and drag coefficients provides a measure of aircraft performance loss.

The results on a NACA 23012 airfoil were surprising. For a given liquid water content, at all temperatures between -2C and -20C the aircraft performance loss after 5 minutes accumulation at 20 micron droplet size typically was at least as large as any other size. This result remains true even for longer duration accumulation. Experimenting with different scenarios on the data for the 1995 Roselawn ATR-72 icing accident shows that a possible encounter with 20 micron droplets was more hazardous than with a 200 micron droplet encounter primarily because more cloud liquid water was available at the lower droplet size. Interestingly, a possible encounter with both sizes was no more hazardous than encountering 200 micron droplets alone, even after accounting for a larger cloud liquid water amount.

The results suggest that large droplet ice accumulation may be overemphasized. This is good news for forecasters because it is one less variable to consider.