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Ice has been known as a danger since the dawn of aviation.The primary ice problem comes from the disfiguration of the airplane, causing increased drag and less aerodynamic efficiency.As late as in January 1982, Air Florida Flight 90 crashed soon after leaving Washington National Airport.The first officer expressed concern that something was 'not right' to the captain four times during the takeoff, but the captain took no action to reject the takeoff.The aircraft accelerated at a lower-than-normal rate during takeoff, requiring 45 seconds and nearly 5,400 feet of runway, 15 seconds and nearly 2,000 feet more than normal, to reach lift-off speed.The aircraft initially achieved a climb, but failed to accelerate after lift-off. The aircraft’s stall warning stick shaker activated almost immediately after lift-off and continued until impact.The aircraft encountered stall buffet and descended to impact at a high angle of attack.The aircraft struck the heavily congested northbound span of the 14th Street Bridge and plunged into the ice-covered Potomac River.There were 74 on-board fatalities. When the aircraft struck the bridge, it struck six occupied automobiles and a boom truck before tearing away a 41-foot section of the bridge wall and 97 feet of the bridge railings. Four persons in vehicles on the bridge were killed; four were injured, one seriously.The probable cause established by the subsequent NTSB investigation was:“The flight crew's failure to use engine anti-ice during ground operation and takeoff, their decision to take off with snow/ice on the airfoil surfaces of the aircraft, and the captain’s failure to reject the takeoff during the early stage when his attention was called to anomalous engine instrument readings. Contributing to the accident were the prolonged ground delay between de-icing and the receipt of ATC takeoff clearance during which the airplane was exposed to continual precipitation, the known inherent pitch up characteristics of the B-737 aircraft when the leading edge is contaminated with even small amounts of snow or ice, and the limited experience of the flight crew in jet transport winter operations.”ASN Aircraft accident Boeing 737-222 N62AF Washington-National Airport, DC (DCA) [Potomac River]AAR8208_Air Florida Boeing 737-222 Collision with 14th Street BridgeHowever, deicing fluids had their own problems, so further tests were carried out.Following the Air Florida accident, flight and wind-tunnel investigations of the aerodynamic effects of aircraft ground deicing/anti-icing fluids on a Boeing 737-200ADV were carried out. The night tests were performed in Kuopio, Finland, and the wind-tunnel tests were performed at the NASA Lewis Research Center Icing Research Wind Tunnel (IRT). Both types of commonly used fluids, those characterized by Newtonian and non-Newtonian viscosity behavior, were evaluated. Results of the tests indicated that the fluids remain on aircraft surfaces until well after liftoff and may cause measurable lift loss and drag increase, depending on the temperature, dilution, and specific characteristics of each fluid. A secondary wave of fluid which occurred at takeoff rotation was observed. Capillary wave action within the secondary wave is considered to be the source of the fluid's adverse aerodynamic effects at high angles of attack. Wind-tunnel testing and computational fluid dynamics analysis indicated that the fluid effects are airplane configuration dependent. Results from these tests, other data, and airplane performance analyses were used to define an aerodynamic acceptance test for the fluids.Nowadays, extensive icing tests are carried out by the certifying authorities before a commercial aircraft is certified for flight into known icing conditions.Icing CertificationActivities, Courses, Seminars & WebinarsAdvisory_Circular/AC_25.1419-1A-CERTIFICATION OF TRANSPORT CATEGORY AIRPLANES FOR FLIGHT INICING CONDITIONSThen, Roselawn happened in October 1984.Roselawn, Indiana, October 31, 1994The airplane, a French/Italian-built ATR-72 turboprop, had been placed in a holding pattern over Roselawn because of weather delays at its destination, O'Hare Airport. Soon after it was cleared to descend in the holding pattern from 10,000 feet to 8,000 feet, the ATR rolled to the right, entered a steep descent and struck the ground before the crew could recover control. None of the 64 passengers or four crewmembers survived.As usual when an airliner goes down for no apparent reason, the crash got a lot of publicity, and the National Transportation Safety Board was under a good deal of pressure to explain it.When the NTSB report on the accident finally appeared, it identified aileron hinge-moment reversal due to ice accretion on the wings as the culprit, and singled out the airfoils on the ATR as especially prone to form ice ridges on the upper surface behind the deice boots.These in turn made the ailerons try to go to full deflection rather than return to center.Since the airplane has unboosted, reversible controls, the crew, confused and overpowered, failed to stabilize the airplane.The ATR’s airfoils are variants of the inextinguishable NACA 23000 series, which must be one of the longest-running shows in aviation.This family of airfoils was designed in about 1927, hit the big time on the DC-3, and continues to appear regularly on new designs in spite of mountains of subsequent airfoil research and development.Its noteworthy properties are high maximum lift without flap deflection, and very little negative pitching moment—that is, little tendency to tuck its nose downward while producing lift.It was somewhat surprising that such a widely used wing section should suddenly display an unexpected icing behavior after so many years in service.Nevertheless, the NTSB essentially attributed the Roselawn accident to the design characteristics of the ATR-72.But airplane accident investigation is a political activity, and, not surprisingly, the NTSB’s opposite number in France, the Bureau Enquites-Accidents, did not agree.It blamed the American crew instead, accusing them of inattention and failure to treat a hold in icing conditions as a “critical phase of flight.”By mid-1995 all ATR-72s had been modified with deice boots extending farther aft.https://ral.ucar.edu/sites/default/files/public/events/2015/friends-and-partners-in-aviation-weather/docs/bond.pdfIn January, 1997, however, the Roselawn accident repeated itself.This time, the airplane was an Embraer EMB-120, a twin turboprop about half the size of the ATR but equipped with similar high-aspect-ratio wings and airfoils and unboosted controls.The accident took place at Monroe, Michigan, during the descent for a planned landing at Detroit.ATC requested a speed of 150 knots.The airplane was in a shallow left turn at 4,000 feet and 156 KIAS when the autopilot, which was engaged, attempted to level the wings.Instead, the left wing dropped further.From that moment the airplane no longer responded properly to the controls.The autopilot automatically disengaged after the bank angle exceeded 45 degrees, but the human crew could not regain control of the airplane.After oscillating wildly in both pitch and roll it eventually struck the ground up-right, wings level, in a steep dive.Hell hath no fury like a bureaucracy scorned, and the NTSB, which has been trying for years to make certification requirements for flight in icing conditions more rigorous and gotten little reward for its efforts, turned on the FAA with a vengeance.The probable cause of the accident, the Board wrote, was “the FAA’s failure to establish adequate certification standards for flight in icing conditions, the FAA’s failure to ensure that a Centro Tecnico Aerospacial/FAA-approved procedure for the accident airplane’s deice system operation was implemented by US-based air carriers, and the FAA’s failure to require the establishment of adequate minimum airspeeds for icing conditions...”The NTSB’s 350-page report on the accident is unusually voluminous and thorough, and packed with technical detail.It treats all aspects of the accident, including airline administration, crew training, documentation, dissemination of information, the reciprocal agreement between the U.S. and Brazil for aircraft certification, and so on.The crux of the Board's analysis, however, is a relatively arcane technical issue having to do with the behavior of airfoils at various angles of attack and under various kinds of icing conditions.The EMB-120 uses 23000-series airfoils similar to those of the ATR-72.The baseline aerodynamic data for this family of sections were originally measured more than half a century ago.They are reported in Abbott and von Doenhoff’s Theory of Wing Sections, a classic work first published as a NACA technical report and still available today as a Dover paperback.They show the expected difference between the maximum lift coefficient available from a smooth wing section (about 1.8) and that available (about 1.2) when "standard roughness" has been added to the airfoil surface.Standard roughness consisted of .011-inch grains thinly applied to the first two inches of the upper and lower surfaces of 24-inch-chord wind tunnel models.This level of roughness, Abbott and von Doenhoff noted, "is considerably less severe than is likely to be encountered in service as a result of accumulation of ice..."Modern measurements in the NASA Lewis Research Center icing tunnel show maximum lift coefficients in the vicinity of 0.8 for lightly iced air-foils with small aileron deflections.Equally significant is the angle of attack at which loss of lift begins: around eight degrees for the iced sections, versus 18 degrees for clean ones.It must be emphasized that the amount of ice accumulation in these tests is very slight—so little that in low light conditions a crew might not even notice it, and in any case would not consider it significant.Aviation Accident Report AAR-98-04In 1989, the NTSB investigated a series of takeoff accidents involving light accumulations of frost on wings.It determined that a small buildup of sandpaper-like roughness was sufficient to compromise wing performance severely, and to cause stall and loss of control immediately after liftoff on airplanes without leading edge slats.As a result, pre-takeoff deicing is now a common experience for air carrier aircraft, and takeoff upsets due to icing have ceased.That, the mandatory requirement for keeping ice-free surfaces on the wings before takeoff, and avoidance of certain kinds of weather are the only defence against icing.Also, the limited period of icing protection provided by the deicing fluid is well recognized; if the takeoff is delayed for some reason more than the “holdover” time (HOT) of the deicing fluid, the aircraft has to return to the gate for another shot of deicing.HOTs are determined based on the results of endurance time testing of anti-icing fluids in natural or simulated icing conditions.Fluid failure occurs when the fluid is no longer able to effectively absorb the incoming precipitation.The endurance times are determined by applying failure criteria using visual cues for fluid failure, which vary depending on the icing conditions (snow, freezing drizzle, freezing rain).https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120016397.pdfThe Roselawn and Monroe accidents, and a number of similar incidents from which crews were able to recover, highlight the importance of surface roughness in cruising flight as well.The Monroe accident could probably have been avoided if the EMB-120 crew had declined the controller's request that they slow to 150 knots.In particular, the Board thought that hand-flying in icing conditions would allow crews to sense any change in handling qualities, and that autopilots should not be used when ice is present.The operator of the EMB-120 had not incorporated into its operations manual an April, 1996 flight manual revision from Embraer concerning operations in icing conditions, because the FAA had not considered it sufficiently significant.The NTSB speculated that had the crew been aware of that revision, the accident, whose prevention may have hinged on a few knots of airspeed or the early use of deice boots, might have been prevented.In the aftermath of Roselawn, the NTSB took the position that the airfoils of the ATR-72 and its unboosted, reversible controls, in combination, made the airplane particularly susceptible to upsets caused by icing.The Board seems, in the present case, to have backed away from blaming a particular design feature.Instead, it is now taking the view that any amount of icing can cause bizarre, unpredictable behavior, and that the standards currently in use for icing certification are inadequate.Until the FAA alters the requirements and requires recertification of types now in service, the advice to pilots—at least to pilots of turboprop equipment with 23000-series and related airfoils and pneumatic deicers—is to keep the speed up, the autopilot off, and the deicers on.And take every icing encounter, however minor, very seriously.▲Deicing of an aircraft’s wings being carried out an an airport before takeoff.Since 1990, there have been at least 100 jet engine power-loss events on both commuter and large transport airplanes, mostly at altitudes higher than 22,000 feet, the highest altitude where airframe icing is expected to exist.▲Source: Boeing Aero Magazine, Q04–07High-altitude ice crystals in convective weather are now recognized as a cause of engine damage and engine power loss that affects multiple models of commercial airplanes and engines.These events typically have occurred in conditions that appear benign to pilots, including an absence of airframe icing and only light turbulence.The engines in all events have recovered to normal thrust response quickly. Research is being conducted to further understand these events.Normal thunderstorm avoidance procedures may help pilots avoid regions of high ice crystal content.The battle against icing never ends.In 1948, Air Facts magazine had an article entitled "Ice Isn't Nice." Ben Robin was the author. The title is a true statement. 70 years later, neither the FAA, nor anyone else, has added much to what was known then. In fact, in chilling words which show that something akin to the ATR problem must have existed for some airplanes 70 years ago, Ben Robin wrote, in relation to ice, "It spoils the even air flow over the wings and tail surfaces and causes burbles which, if they become severe enough, can cause loss of all control."As several general aviation accidents every year show, icing is a killer.