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AC 120-112 - Use of Liquid Water Equivalent System to Determine Holdover Times or Check Times for Anti-Icing Fluids

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120-112 - Use of Liquid Water Equivalent System to Determine Holdover Times or Check Times for Anti-Icing Fluids

Date Issued
July 14, 2015
Responsible Office
AFS-200
Description

This advisory circular (AC) provides guidance for the Federal Aviation Administration’s (FAA) standard for a Liquid Water Equivalent System (LWES). This AC applies to anyone proposing to design, procure, construct, install, activate, or maintain an LWES. An LWES is an automated weather measurement system that determines the Liquid Water Equivalent (LWE) rate in conditions of frozen or freezing precipitation. The LWE rate is used by the system with the appropriate endurance time (ET) regression equations and regression coefficients specified in an FAA-approved current database at http://66.46.192.186/RegressionInformation.html to determine the holdover time (HOT) or check time (CT) for an aircraft’s applied anti-icing fluid (Society of Automotive Engineer (SAE) Types I, II, III, and IV). Thus, the LWES incorporates a Holdover Time Determination System (HOTDS) or Check Time Determination System (CTDS). The HOT is used to determine how long a fluid would provide protection assuming that the current conditions do not change. The CT is used to determine the fluid’s current protection capability, while incorporating varying weather conditions.