Definition
AC 91-74 is an FAA Advisory Circular titled 'Pilot Guide: Flight in Icing Conditions.' It provides guidance to pilots on recognizing, avoiding, and responding to airframe icing, including the hazards of supercooled large droplet (SLD) icing, the limitations of certified ice protection systems, and recommended pre-flight and in-flight decision-making when icing is forecast or encountered.
Plain English
AC 91-74 is an FAA advice document that tells pilots how to deal with ice forming on an aircraft in flight — what to look for, what to avoid, and what to do if it happens.
Context Anchor
Seen in FAA training material when the handbook points pilots to additional guidance on aircraft icing and cold-weather flight hazards.
Derivation
AC' stands for Advisory Circular, the FAA's standard format for non-regulatory guidance. The number '91-74' identifies the document: the '91' refers to the part of the Federal Aviation Regulations it relates to (Part 91, General Operating and Flight Rules), and '74' is the sequence number within that series.
Why Pilots Care
Icing can rapidly degrade aircraft performance and handling; this circular supplies practical steps that help pilots avoid or escape icing encounters safely.
Intuition Check
Do not read “AC” here as air conditioning or electrical alternating current. In this FAA context, “AC” means Advisory Circular, a guidance document for pilots and operators.
Example Sentence 1
Before her cross-country flight through forecast icing, she reviewed AC 91-74 to refresh her decision-making for in-flight ice encounters.
Example Sentence 2
During preflight planning the pilot consulted AC 91-74 to confirm the latest procedures for operating in known icing conditions.