In which environment is aircraft structural ice most likely to accumulate the fastest?

• A. Freezing rain
• B. Slight drizzle in calm air
• C. Light snow mixed with drizzle
• D. Cold air with no moisture

Answer: (A)

Ice buildup rate is driven by how quickly liquid water droplets strike and freeze on the aircraft’s surface. Freezing rain consists of supercooled droplets that remain liquid as they fall but freeze instantly on contact with any surface at or near 0°C, creating a rapid, thick glaze of ice. That immediate freezing adds ice with each impact, so the accumulation happens much faster than with other precipitation types.

In contrast, a slight drizzle in calm air involves tiny droplets that may not all be supercooled or may not freeze immediately, so the accreted ice forms more slowly. Light snow mixed with drizzle adds frozen particles, which accumulates but not as rapidly as glaze ice. Cold air with no moisture has no liquid droplets to freeze, so ice doesn’t form.