Pilot Guide: Flight in Icing Conditions (AC 91-74B)
Advisory Circular 91-74B provides guidance for pilots on recognizing and avoiding icing conditions. It covers types of structural icing (clear, rime, mixed), effects of ice on aircraft performance, supercooled large droplet (SLD) conditions, and decision-making strategies for icing encounters.
Why This Document Matters
Aircraft icing is one of the leading causes of weather-related accidents, and this AC provides comprehensive guidance on the subject. It covers the meteorological conditions that produce icing, the types of ice that form on aircraft surfaces, the aerodynamic effects of icing, and what to do if you encounter icing in flight. For instrument pilots, understanding icing is critical because IFR flight often involves flying in clouds where icing conditions may exist. Even VFR pilots need to understand icing for winter operations and for the knowledge test.
Chapter-by-Chapter Guide
What each section covers and the key topics to study
1Types and Effects of Icing
Structural icing types, their formation conditions, and effects on aircraft performance.
Types and Effects of Icing
Structural icing types, their formation conditions, and effects on aircraft performance.
Key Topics
2Icing Avoidance and Escape
Recognizing icing conditions, forecasting tools, and escape strategies when icing is encountered.
Icing Avoidance and Escape
Recognizing icing conditions, forecasting tools, and escape strategies when icing is encountered.
Key Topics
Study Tips
- Know the two main types: rime ice (milky, rough, forms in colder temps with small droplets) and clear ice (smooth, hard, forms near freezing with large droplets — more dangerous because it is heavier and harder to remove).
- Remember that ice increases drag, increases stall speed, decreases lift, and adds weight — all of which degrade performance. Even a small amount of ice can increase stall speed significantly.
- For the knowledge test, understand that the most dangerous icing conditions occur in clouds with temperatures between 0C and -20C, with the worst icing near 0C where supercooled large water droplets exist.
Frequently Asked Questions
Can I fly into known icing conditions?
Only if your aircraft is certificated for flight in known icing conditions (FIKI) and equipped with functioning anti-ice/de-ice systems. Most general aviation aircraft are NOT approved for known icing. Flying a non-FIKI aircraft into known icing conditions violates 14 CFR 91.9 (operating limitations) and 14 CFR 91.13 (careless or reckless operation).
Which type of ice is most dangerous?
Clear ice is generally considered most dangerous. It forms in temperatures near freezing with large water droplets, resulting in a smooth, heavy layer that is difficult to see and remove. It adds significant weight and can dramatically alter the airfoil shape, increasing stall speed.
Quick Facts
- Document ID
- AC 91-74B
- Last Updated
- 2015
- Cost
- Free
- Publisher
- FAA
Applies To
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Pilot Guide: Flight in Icing Conditions (AC 91-74B) is an official FAA publication available at FAA.gov
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