Predicting an Icy Future

With its years of accumulated research about all aspects of icing—i. e., weather conditions that produce it, types of ice that form under vari­ous conditions, de-icing and anti-icing measures and when to employ them—NASA’s data would be useless unless they were somehow pack­aged and made available to the aviation community in a convenient manner so that safety could be improved on a daily basis. And so with desktop computers becoming more affordable, available, and increas­ingly powerful enough to crunch fairly complex datasets, in 1983, NASA researchers at what was still named the Lewis Research Center began developing a computer program that would at first aid NASA’s in-house researchers, but would grow to become a tool that would aid pilots, air traffic controllers, and any other interested party in the flight plan­ning process through potential areas of icing. The software was dubbed LEWICE, and version 0.1 originated in 1983 as a research code for in­house use only. As of the beginning of 2010, version 2.0 is the official cur­rent version, although a version 3.2.2 is in development, as is the first 0.1 version of GlennICE, which is intended to accurately predict ice growth under any weather conditions for any aircraft surface.[1243]

LEWICE, which spelled out is the Lewis Ice Accretion Program, is a freely available desktop software program used by hundreds of people in the aviation community for purposes of predicting the amount, type, and shape of ice an aircraft might experience given a particular weather forecast, as well as what kind of anti-icing heat requirements may be necessary to prevent any buildup of ice from beginning. The software

runs on a desktop PC and provides its analysis of the input data within minutes, fast enough that the user can try out some different numbers to get a range of possible icing experiences in flight. All of the predic­tions are based on extensive research and real-life observations of icing collected through the years both in flight and in icing wind tunnel tests.[1244]

At its heart, LEWICE attempts to predict how ice will grow on an aircraft surface by evaluating the thermodynamics of the freezing pro­cess that occurs when supercooled droplets of moisture strike an air­craft in flight. Variables considered include the atmospheric parameters of temperature, pressure, and velocity, while meteorological parame­ters of liquid water content, droplet diameter, and relative humidity are used to determine the shape of the ice accretion. Meanwhile, the aircraft surface geometry is defined by segments joining a set of dis­crete body coordinates. All of that data are crunched by the software in four major modules that result in a flow field calculation, a parti­cle trajectory and impingement calculation, a thermodynamic and ice growth calculation, and an allowance for changes in the aircraft geom­etry because of the ice growth. In processing the data, LEWICE applies a time-stepping procedure that runs through the calculations repeat­edly to "grow” the ice. Initially, the flow field and droplet impingement characteristics are determined for the bare aircraft surface. Then the rate of ice growth on each surface segment is determined by applying the thermodynamic model. Depending on the desired time increment, the resulting ice growth is calculated, and the shape of the aircraft sur­face is adjusted accordingly. Then the process repeats and continues to predict the total ice expected based on the time the aircraft is flying through icing conditions.[1245]

The basic functions of LEWICE essentially account for the capa­bilities of the software up through version 1.6. Version 2.0 was the next release, and although it did not change the fundamental process or mod­els involved in calculating ice accretion, it vastly improved the robust­ness and accuracy of the software. The current version was extensively tested on different computer platforms to ensure identical results and also incorporated the very latest and complete datasets based on the most
recent research available, while also having its prediction results ver­ified in controlled laboratory tests using the Glenn IRT. Version 3.2— not yet released to date—will add the ability to account for the presence and use of anti-icing and de-icing systems in determining the amount, shape, and potential hazard of ice accretion in flight. Previously these variables could be calculated by reading LEWICE output files into other software such as ANTICE 1.0 or LEWICE/Thermal 1.6.[1246]

According to Jaiwon Shin, the current NASA Associate Administrator for the Aeronautics Research Mission Directorate, the LEWICE software is the most significant contribution NASA has made and continues to make to the aviation industry in terms of the topic of icing accretion. Shin said LEWICE continues to be used by the aviation community to improve safety, has helped save lives, and is an incredibly useful tool in the classroom to help teach future pilots, aeronautical engineers, traf­fic controllers, and even meteorologists about the icing phenomenon.[1247]