Definition
AC 91-51 is an FAA Advisory Circular titled 'Effect of Icing on Aircraft Control and Airplane Deice and Anti-Ice Systems.' It provides guidance to pilots on how airframe icing affects aircraft handling, stall behavior, and control, and on the proper use of deicing and anti-icing equipment.
Plain English
It is an FAA guidance document that explains how ice on the aircraft changes the way it flies and how to use the systems that remove or prevent that ice.
Context Anchor
Seen as a reference in instrument flying study material, especially when learning about weather hazards and aircraft icing.
Derivation
Advisory Circular' means an FAA-issued document offering guidance rather than a binding regulation. The number '91-51' identifies it within the AC series tied to Part 91 (general operating and flight rules), with 51 being its sequential identifier in that group.
Why Pilots Care
Pilots must follow the manuals described in this circular to meet legal and safety standards during instrument flight.
Intuition Check
Do not read AC 91-51 as a regulation number. It is an FAA guidance document; it supports safe decisions but is not, by itself, a rule.
Example Sentence 1
Before her first winter IFR trip, she reread AC 91-51 to refresh her understanding of how ice affects stall speed and control response.
Example Sentence 2
Before the checkride the student pilot confirmed their POH met the standards outlined in AC 91-51.