AC 91-74B

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.

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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.

Study This Document in One Loop

What is the Study Loop?

A 30-60 minute scenario-first session that replaces hours of passive reading.

Stage 1

Scenario

IFR climb-out of a 4,200 ft airport, night, OAT +1°C. You enter the bases of a stratus layer at 3,800 AGL. OAT shows -3°C. Your ammeter is fine, your windshield looks clean. You check visually — the wings show a thin, clear, almost invisible coating on the leading edge. The cloud layer is 2,500 ft thick above you.

Stage 2

Decision

Climb, descend, turn around, or declare? Your airplane is not FIKI. What does AC 91-74 say about "known icing" and when does trace become a problem?

Write your answer before you open the handbook. That exposes the gap.

Stage 3

Targeted Learning

Open only these sections of the AC 91-74B:

Chapter on Types of Ice (clear, rime, mixed — where each forms)
Chapter on Effects of Ice (stall speed increase, weight, lift loss, thrust loss)
Chapter on Supercooled Large Droplets (SLD behavior, ridge ice past protected surfaces)
Chapter on Exit Strategy (climb vs. descend vs. 180 — pick one fast)

Stage 4

Debrief

Compare your Decision to what the handbook says:

?Clear ice at near-freezing temps is the killer. Why is it worse than rime?
?Non-FIKI airplane in forecast/observed icing = violating §91.9 (operating limits) and §91.13 (careless/reckless). Is that where you are?
?Escape decision criteria: climb only if you're confident tops are close. Descend to warmer air. 180° if both are uncertain.
?Stall speed increase with even a thin layer of ice: 20-30% is not unusual. Does your approach speed still clear the new stall speed?

Stage 5

Reinforcement

Turn your biggest miss into fast-recall rules:

Non-FIKI + known icing = regulatory violation AND performance hazard. Both matter.
Three exit options: climb out, descend out, 180° out. Pick fast, execute fully.
Add ice = add approach speed. 10-20 kt is a common correction for known or suspected ice.

What Order to Read the AC 91-74B

Don't read by chapter number. Work the four phases. Start with whichever you're weakest in.

Survival Thinking

“What can hurt me?”

•Exit strategies (climb, descend, 180°) and when to pick which
•Effects of ice on performance (stall, climb, approach speed)

Interpretation

“What am I looking at?”

•Types of ice (clear, rime, mixed) and visual recognition
•SLD conditions (the reason FIKI aircraft still can't fly in all ice)

Prediction

“What will happen?”

•Temperature and cloud conditions that produce icing (0°C to -15°C, liquid clouds)

Checkride Mode

“Can I explain it under pressure?”

•"Known icing conditions" definition per legal interpretation — know what triggers FAR violation
•Exit procedure if you encounter ice: verbalize the decision logic

Chapter-by-Chapter Guide

What each section covers and the key topics to study

Types and Effects of Icing

Structural icing types, their formation conditions, and effects on aircraft performance.

Key Topics

Clear ice (most dangerous, temperatures near freezing)Rime ice (milky, rough, colder temperatures)Mixed iceEffects on lift, drag, stall speed, and control

Icing Avoidance and Escape

Recognizing icing conditions, forecasting tools, and escape strategies when icing is encountered.

Key Topics

PIREPs for icing reportsFreezing level chartsEscape strategies (climb, descend, or turn)SLD (supercooled large droplet) recognition

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

PrivateInstrumentCommercialCFIATP

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