FAA-G-8082-22, ch. 3b and FAA-H-8083-28B, ch. 20

Structural icing and frost

Ice and frost both ruin lift. The exam asks what conditions create them and where icing is worst.

Part 107, explained › Weather · Last reviewed October 25, 2026 against the eCFR and faa.gov. UAV Campus is not affiliated with the FAA.

What is structural icing, and when does frost form?

Structural icing is ice that sticks to the outside of an aircraft when supercooled water droplets strike it and freeze. It needs visible moisture, such as rain or cloud droplets, and a temperature of 0°C or colder where the moisture strikes. Frost forms when a surface cools below the dew point of the air and the temperature is below freezing.

Structural icing comes in rime, clear (glaze) and mixed types. A favored location for severe clear icing is freezing rain or freezing drizzle below a front, because the large supercooled drops freeze on impact. Aerodynamic cooling can lower the temperature of an airfoil to 0°C even though the surrounding air is a few degrees warmer.

Frost poses a definite flight safety hazard: it disrupts the flow of air over the wing, can drastically reduce lift and increases drag. A small UA must be thoroughly cleaned and free of frost before a flight. Dew, in contrast, poses no threat.

Sources: Remote Pilot Study Guide, p. 33 · Remote Pilot Study Guide, p. 34 · FAA-H-8083-28B, p. 244 · FAA-H-8083-28B, p. 247 · FAA-H-8083-25C (PHAK), p. 299

What the FAA says

Remote Pilot Study Guide, p. 34

Two conditions are necessary for structural icing in flight: 1. The aircraft must be flying through visible water such as rain or cloud droplets 2. The temperature at the point where the moisture strikes the aircraft must be 0° C or colder.

FAA-H-8083-28B, p. 244

Structural icing is the ice that sticks to the outside of the airplane. It occurs when supercooled water droplets strike the airframe and freeze.

FAA-H-8083-28B, p. 247

A favored location for severe clear icing is freezing rain and/or freezing drizzle below a front.

FAA-H-8083-25C (PHAK), p. 299

On cool, clear, calm nights, the temperature of the ground and objects on the surface can cause temperatures of the surrounding air to drop below the dew point.

Sources: Remote Pilot Study Guide, p. 34 · FAA-H-8083-28B, p. 244 · FAA-H-8083-28B, p. 247 · FAA-H-8083-25C (PHAK), p. 299

Exam traps

  • Structural icing needs both visible moisture and a temperature of 0°C or colder at the point where the moisture strikes the aircraft.
  • Freezing rain or freezing drizzle below a front is a favored location for severe clear icing, because the large supercooled drops freeze on impact.
  • Frost forms when the surface is at or below the dew point of the adjacent air and the temperature is below freezing; if not below freezing, the moisture is dew.
  • Dew poses no threat, but frost disrupts airflow, drastically reduces lift and increases drag.

Quick quiz: 3 questions

Similar to the FAA test, not the actual test questions. Each answer shows its FAA source.

Question 1 of 3

Which conditions result in the formation of frost?

  1. A The temperature of the surrounding air is at or below freezing when small drops of moisture fall on the collecting surface.
  2. B The temperature of the collecting surface is at or below freezing when small droplets of moisture fall on the surface.
  3. C The temperature of the collecting surface is at or below the dewpoint of the adjacent air and the dew point is below freezing.
Show the answer
Answer: C. The temperature of the collecting surface is at or below the dewpoint of the adjacent air and the dew point is below freezing.

Frost forms when a surface cools below the dew point of the air next to it and that dew point is below freezing, so the moisture deposits directly as ice crystals (sublimation). No visible moisture is needed (FAA-H-8083-25C ch. 12 and Glossary). The other two options describe moisture falling on a freezing surface, which is how ice from freezing drizzle or rain forms, not frost.

FAA source: FAA-H-8083-25C ch. 12, Dew and Frost (p. 12-15); Glossary, Frost

Question 2 of 3

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

  1. A Cumulus clouds with below freezing temperatures.
  2. B Freezing rain.
  3. C Freezing drizzle.
Show the answer
Answer: B. Freezing rain.

Freezing rain has a tendency to freeze on impact on the aircraft and will have the highest rate of accumulation.

FAA source: FAA-H-8083-28B ch. 20, Icing with Fronts

Question 3 of 3

One in-flight condition necessary for structural icing to form is

  1. A visible moisture.
  2. B stratiform clouds.
  3. C small temperature/dewpoint spread.
Show the answer
Answer: A. visible moisture.

Icing occurs when two conditions are met: visible moisture and temperature below freezing.

FAA source: FAA-G-8082-22 ch. 3b, Structural Icing

Can a drone ice up when the air is slightly above freezing?

Possibly. Aerodynamic cooling can lower the temperature of an airfoil to 0°C even though the ambient temperature is a few degrees warmer.

Source: Remote Pilot Study Guide, p. 34

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