Definition
An FAA Advisory Circular that provides guidance on the design, certification, and operational use of aircraft ice protection systems. It describes acceptable means of meeting the icing-related requirements of the federal aviation regulations and addresses how ice forms on aircraft, the effects of ice on aircraft performance, and the methods used to detect, prevent, and remove ice from critical surfaces.
Plain English
An FAA guidance document that explains how aircraft ice protection systems should be designed and used. It is the FAA's reference paper on dealing with ice on aircraft.
Context Anchor
You may see AC 20-73 cited in FAA training material when the subject is aircraft icing, ice protection systems, or instrument flight in freezing conditions.
Derivation
AC' stands for Advisory Circular, a series of non-regulatory FAA publications offering guidance on specific topics. The number '20-73' identifies the subject area (20 = general aviation matters) and the specific circular within that series.
Why Pilots Care
Helps pilots confirm that an aircraft's ice protection system meets FAA standards before operating in potential icing environments.
Intuition Check
Do not read “AC” here as electrical alternating current or aircraft control. In this context, AC means an FAA Advisory Circular, which is a guidance document.
Example Sentence 1
When studying how the pitot heat system protects against ice blockage, the instructor pointed to AC 20-73 as the FAA's primary guidance on aircraft ice protection.
Example Sentence 2
AC 20-73 outlines the testing needed to approve ice protection systems on general aviation aircraft.