Private Pilot Private Exam

In which environment is aircraft structural ice most likely to have the highest accumulation rate?

1. Freezing rain

2. Snow showers

3. Fog

4. Cloudy skies

The correct answer is: Freezing rain

The highest accumulation rate of aircraft structural ice occurs in freezing rain. This phenomenon happens when supercooled water droplets fall from the clouds and freeze upon contact with a surface, such as an aircraft, that is at or below freezing temperature. The size of these droplets contributes significantly to the rate of ice accumulation because they can be larger and, upon impact, create a rapid buildup of ice. This is especially dangerous as the ice can accumulate quickly and lead to significant increases in weight and changes in aerodynamics. In contrast, while snow showers can also contribute to ice accumulation, the snowflakes are often less dense and can not stick as readily as supercooled droplets. Fog primarily consists of tiny water droplets that are generally not in a state to freeze upon contact, leading to minimal ice accumulation. Cloudy skies do not inherently contain conditions conducive to ice accumulation; clouds need to be producing moisture that can become supercooled to create ice. Therefore, freezing rain is the most conducive environment for significant ice accumulation on aircraft structures.