FAA Part 107 Remote Pilot (UAG)WeatherHard
For structural icing to form on a small unmanned aircraft in flight, which combination of atmospheric conditions must be present?
- AClear skies with a temperature below freezing and low humidity
- BStrong surface winds combined with rapidly falling barometric pressure
- CHigh relative humidity combined with temperatures above 20°C (68°F)
- DVisible moisture, such as clouds or rain, combined with a temperature at or below 0°C (32°F)
Show answer & explanationAnswer & explanation
Correct answer: D. Visible moisture, such as clouds or rain, combined with a temperature at or below 0°C (32°F)
Structural icing requires two elements simultaneously: visible moisture (clouds, rain, drizzle, or fog droplets) and a temperature at or below freezing (0°C/32°F), which allows supercooled water droplets to freeze on contact with the aircraft's surfaces.
Why the other options are wrong
- A. Clear skies mean no visible moisture is present, so icing cannot form despite the freezing temperature.
- B. Wind and pressure changes alone, without visible moisture and freezing temps, do not cause icing.
- C. Temperatures above freezing prevent supercooled droplets from freezing on the airframe.
Structural Icing
Structural icing is the accumulation of ice on an aircraft's external surfaces, requiring both visible moisture and freezing or near-freezing temperatures.
- Requires visible moisture (clouds, rain, drizzle) AND temp at/below 0°C
- Supercooled water droplets freeze instantly on contact with the airframe
- Icing adds weight, disrupts airflow, and can be catastrophic for small UAS
Memory trick: 'Wet and cold, ice takes hold' — need both moisture and freezing temps.