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Aircraft icing that results from super cooled drizzle drops (SCDD) can be extremely hazardous to general aviation and many commuter aircraft Encounters with these drizzle drops (40 400 um diameter) by the Wyoming King Air B200T research aircraft have forced the aircraft to descend to deice after as little as 5 minutes. These drizzle drops freeze as moderate feathers and glaze nodules at or just beyond the deicing equipment. These feathers tend to form on small imperfections on the wing and fuselage such as seams and rivets. The accreted ice results in a substantial increase in drag, a substantial increase in stall speed, and a moderate decrease in lift The possibility that SCDD contributed to the ATR-72 crash near Roselawn, Indiana the night of 31 October 94 has raised the awareness in the aviation community of the hazards of aircraft icing by SCDD. With this awareness will, undoubtedly, come increased research and evaluations corresponding to the drizzle conditions. One method of aircraft evaluation in the SCDD conditions is artificial icing utilizing the USAF Water Spray Tanker at Edwards Air Force Base. The artificial cloud distributions produced by the tanker have been compared to natural cloud distributions obtained by the Wyoming King Air. This paper presents some of the Wyoming data on SCDD, the USAF Water Spray Tanker background and description, and the favorable results of the droplet mass distribution comparisons to the natural environment and to the FAA FAR, Part 25, Appendix C Envelopes. BACKGROUND Governing Regulation for Aircraft Certification Aircraft anti/de-icing systems are typically designed for a droplet range specified by the Federal Aviation Administration (FAA) and other certifying agencies. The current FAA Federal Aviation Regulations, Part 25, Appendix C Icing Envelopes in Figure 1 show that the maximum evaluated droplet Median Volumetric Diameter (MVD) is 50 microns. The Appendix C envelopes are used as certification standards and are referenced by FAR parts 23.1419 and 25.1419. These envelopes (developed in the late 1940s) represent a majority of the icing clouds encountered by aircraft; however, a low percentage of icing clouds contain drizzle and rain drop sizes. These conditions are rare, but they can be extremely hazardous for aircraft Drizzle Research at Wyoming Aircraft icing that results from super cooled drizzle drops (SCDD) can be extremely hazardous to general aviation and many commuter aircraft. Encounters with these drizzle drops (40 400 pm diameter by Byers 1974; Marwitz et al. 1995) by the Wyoming King Air B200T have forced the aircraft to descend to deice after as little as 5 minutes. The Wyoming King Air is instrumented to measure drop size distributions, liquid water content, ambient temperature, engine torque and other aircraft parameters to allow for the determination of aircraft performance in a measured icing condition. The Wyoming King Air has encountered roughly 30 super cooled drizzle and rain size drop conditions since 1982. These drizzle drops freeze as moderate feathers and glaze nodules at or just beyond the deicing equipment. The feathers and nodules are typically 1 to 15 mm in length. They tend to form on small imperfections on the wing and fuselage such as seams and rivets. The accreted ice results in a substantial increase in drag, a substantial increase in stall speed, and a moderate decrease in lift (Cooper et al. 1984; Sand et al. 1984). Aircraft ice accretion is dependent on many variables such as liquid water content, drop diameter, ambient' temperature, airspeed, angle of attack, shape of the object (airfoil), and the time spent in the icing condition. Many of these variables are interdependent, that is, the airfoil collection efficiency is dependent on the size of the drops and the shape of the airfoil. The size of the drops also determines the impingement limits, that is, how far back from the leading edge the drops hit the object such as a propeller spinner or airfoil. This impingement limit increases in percent chord from the leading edge as the relative sizes of the drops * Graduate Student at UW, Former USAF/AFFTC Aircraft Icing Engineer t Prafcuor, Univenity of Wyoming 1 Copyright© 1996 by the American Institute of Aeronautics and A»tronautics, Inc. All rights reter/ed. increase. The bigger the drops, the farther back on the object the ice will accrete. This effect can best be seen on cockpit side windows, prop-spinners, and wing leading edges. Visual cues for drizzle drop induced icing can be determined during artificial aircraft icing evaluations. Flight crews can then be trained to recognize SCDD conditions and take appropriate evasive action. Utilizing the USAF Water Spray Tanker is one proven method of evaluating aircraft in artificial SCDD conditions where these visual cues can be readily obtained. In addition, evaluating aircraft in artificial SCDD conditions is recommended due to the risks involved with evaluating aircraft in natural SCDD conditions. These risks are due to full aircraft icing exposure and the difficulty in evading the natural conditions. ATR-72 Artificial Icing Evaluation An artificial icing evaluation of the ATR-72 was completed in December 94 at Edwards Air Force Base utilizing the water spray tanker. The program was conducted as part of a Special Certification Review by the FAA and the DGAC (French equivalent of the FAA) and the investigation into the crash of American Eagle Flight 4184 near Roselawn, Indiana the night of 31 October 94. The tests were designed to examine both the upper limits of the certification envelope at temperatures just below freezing and the icing characteristics of SCDD not currently covered by the FAA FAR, Part 25-C. These tests required the USAF water spray tanker to produce conditions consistent with SCDD conditions. Based on recent research at Wyoming (Pobanz et al. 1994) it was argued by a group of scientists (Marwitz et al. 1995) that SCDD were probably present in the ATR-72 holding pattern prior to the crash. In order to simulate the natural SCDD conditions, the USAF tanker test team referred to the SCDD distributions obtained from the Wyoming King Air. The spray tanker system was then modified to provide an artificial SCDD cloud consistent with those measured in the natural environment. USAF WATER SPRAY TANKER DESCRIPTION General The USAF has used a modified NKC-135A tanker aircraft since the late 1950's for aerial icing and rain testing. This testing is conducted to assure that Department of Defense weapon systems have the capability to operate to design specifications in icing and rain atmospheric conditions. The NKC-135A S/N 55-3128 tanker aircraft (Figure 2), is modified for aerial refueling as well as water spray for icing and rain tests. Water spray or aerial refueling •missions are ground selectable, and the tanker capabilities in the water spray configuration are 150 to 335 KIAS and up to 40K Ft pressure altitude. Artificial Icing and Rain Test History The US Air Force modified NKC-13SA has been used to test an extensive selection of aircraft and missiles. These include: B-58, AV-8, F-lll, A-7D, Concorde, DC-8, T-38, C-130, C-5, B-52, F-14, EA6B, F-15, F-4, A-10, E-3A, F-16, F-18, Canadair Cl601, Boeing 737 and 757, KC-135R, Piaggio Avanti P180, B-1B, HU-25A, AGM-109, AGM-129, C27 A, C-17, B-2, V-22, MU-2B-60, and the ATR-72. |
