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The Lost Art of Airborne Ice Accretion Testing
On Oct.31, 1994, American Eagle flight 4184 impacted the cold Indiana turf as the aircraft suffered from supercooled droplets freezing and compromising the use of the ATR 72’s ailerons. The immediate impact of this new concept of supercooled droplets would create a massive increase in the use of airborne icing tankers in the United States and simultaneously start the downfall of airborne testing facilities.
While proving an aircraft can handle icing conditions is not uncommon, the early use of icing tankers was done with the U.S. military, with the United States Army Corps of Engineers modifying a Boeing CH-47D Chinook to test how ice would form on various helicopters. The Chinook would be equipped with a spray bar system to create droplets to show how they reacted on a helicopter’s rotors.
The Chinook came equipped with one 1,200-gallon tank that allows for roughly 30 minutes of spray time. The tanker’s sprayer system is hung below the aircraft and allows even distribution across the helicopter under testing. The stray system had holes in the leading edge of the tube to allow for wind to flow and create a water-air blend and dissipate the water into the correct droplet sizes.
The aircraft under testing was monitored by a Beechcraft King Air that had been modified with icing sensors to see how severe the icing conditions get on the testing airframe. The tests would stop if the aircraft in question was showing poor prospects to avoid risking the test flight crew on board the aircraft under icing conditions.
To perform ice testing on large fixed wing aircraft, the Federal Aviation Administration would work along the United States’ 412th Test Wing in Edwards Air Force Base in California to use an older KC-135 to create a subsonic fixed-wing icing tanker. The 412th would modify two Boeing KC-135s into icing tankers that would allow the government to test various aircraft for icing issues.
The two aircraft would utilize the boom system that the KC-135 uses and instead of taking on fuel it would take on water in its tanks. On the tip of the boom, a circular web would be placed with holes to allow air to pass through, intercept the water and form small droplets that could then impact the test aircraft and freeze. Special consideration would be made to airspeed, outside air temperature and density to make sure the correct liquid water content and mean droplet diameter would be created. Data would be measured with photographs and lasers to measure accumulation over time.
The KC-135 would see mass usage after the crash of American Eagle Flight 4184 with numerous aerospace companies testing their jets alongside the FAA in the mid to late 1990s. The ATR’s crash and the new discovery of supercooled droplets reducing performance on light commercial and business jets forced the KC-135R to test a large portion of the business and regional jet aircraft to certify their airworthiness during severe icing conditions.
The tanker program would be overhauled through the late 1990s with one aircraft being retired and the other being modified to attempt to increase the spay into newer icing envelopes. Since then, the KC-135R has gone on to providing icing tests for various other aircraft, large and small as it is the required testing aircraft for large transport planes.
Some aircraft that have been tested using the KC-135R include the Boeing 777-200 and Saab 2000, which both passed their respective tests. Further enhancements have been studied as Edwards Air Force Base released a study in 2013 highlighting attempted improvements in structural flow around the icing boom.
Despite the mass utilization of the 412th Test Wing’s aircraft, some manufacturers have opted to keep ice testing in house. Both Dornier and Beechcraft use propeller-based aircraft to ice test new models. For Dornier, a Do-228 would be modified 61 spray nozzles and 210-gallon tank to allow for testing to be done for up to 30 minutes. Similarly, Beechcraft would use a King Air to serve the same purpose with the aircraft being based in Wichita.
Although most of these aircraft have been removed in the previous years as their limited icing creation hurts their validation for certification, Cessna has gone ahead and created its own icing tanker to run tests in house. The company modified a used Cessna Citation XLS+ with registration N563XP to allow for the aircraft to operate as an aircraft icing tanker.
DARCorporation would perform the structural and assembly planning and managed to avoid adding any boom to the aircraft. Instead, the 7-bar sprayer system would be mounted on the top leading edge of the tail and would have water travel up the vertical tail to reach the spray rig from the tanks in the main cabin. The Cessna is still utilized by the Wichita-based corporation and is used for icing certification on new models like the Cessna Citation Latitude and Longitude.
With only the Cessna and a single KC-135 still active, the airborne icing tanker operations are a shadow of it’s former self but in a good way. The expansion of the tests for icing envelopes that have been enacted by the Federal Aviation Administration in the past few years has seen the usage of aircraft with limited icing envelopes like the KC-135 diminish as the conditions expand more and more.
As a result, the limited proving grounds of airborne icing tankers has brought forward more successful ice mapping software such as LEWICE, CANICE, HELICE, and ICECREMO to allow for companies to view ice accretion over time. Furthermore, testing can be done using icing tunnels with the enclosed environment of a wind tunnel allowing for more human control of the surrounding then flight testing at altitude.
While aircraft icing can still play a factor in year-round airline operations, the improvements made by scientists and engineers has allowed commercial travel to become safer than the days of American Eagle Flight 4184, even if that means the loss of the airborne icing tanker along the way.
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