Term Paper: Ice and Rain Affect Normal

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[. . .] But using each of them can be time consuming. Thompson et al. (1997a) states: "in the real atmosphere, icing cannot exist in sub-saturated environments (RH<100%), except within precipitation or a decaying cloud." This was followed by another study, which had the purpose of locating regions of cloud using NOVA-AVHRR satellite data. (Thompson; Bruintjes, Brown and Hage, Thompson, G., R.T. Bruintjes, B.G. Brown, and F. Hage, 1997).

Then a comparative study on the accuracy of the data on different regions as to which of them matched with predictions and actual conditions was conducted. And it is found that the satellite imagery enables a better isolation of probable icing area, except in the cloudless regions. The method of elimination would not be able to determine the icing takes place in cloudless regions. But in regions where cloud does not exist, it is unlikely that icing would occur in those regions. But it is possible that if an aircraft moves from an area with water drops present to a cloudless area where the temperature is below freezing point, then some icing occurs. For aircraft icing to occur there should be the presence of at least SLW, if not cloud.

Even though it is a rare possibility, it is possible for icing to occur at areas outside the ones with predicted icing conditions, when the scheme developed by Thompson et al. (1997) is used. Many findings of these algorithms may give near accurate predictions on the probable icing regions; but the area specified by these findings could be very wide. The icing predictions are necessary to isolate or limit the area of icing probabilities and to give wider area for the aircraft to fly, and this is the main aim of such predictions. But another study by Brown observed the fact that by algorithms using temperature and relative damp conditions it is difficult to arrive at the task. As on the factors leading to correct icing predictions there is much dispute among researchers today. Whatever are those factors responsible, it is an established fact that ice is "the most difficult systematic threat to flight safety today." (Sand; Cooper; Politovich and Veal, 1984)

We shall now analyze the aircraft accidents, which occurred as a result of icing. Crash of Ozark Airline Flight 982, which was a Douglas DC-9-15, occurred on take off from the Sioux City Airport, Sioux City, Iowa on 27th Dec.1968. The cause of the accident was related to a stall near the upper limits of ground effect, and thereby losing control, as a result of aerodynamics and change in weight due to airfoil icing. The aircraft was not de-iced before take off. Another crash occurred on take off from Newark International Airport, Newark, New Jersey, on 27th Nov. 1978, of Trans World Airways Flight 505, which was a Douglas DC-9-10. At an altitude of about 65 feet above ground level, and speed of 154 knots, it lost control shortly after take off. Here also non-de-icing before take off and airframe icing were attributed to be the cause for the accident. Yet another crash took place on 5th Feb.1985, of an Airborne Express Douglas DC-9-15 while take off from Philadelphia International Airport, Philadelphia, Pennsylvania. Here too the cause found was due to non-de-icing before take off and airfoil icing. There are some common factors in each of these accidents, viz. 1.Below normal angle of attack stalling of aircraft shortly after take off was there in each of the crashes. 2.Freezing rain and/or snow was present. 3. De-icing of the aircraft before take off was not done.4. The aircrafts didn't have leading edge devices or de-icing devices fitted on them.

Scandinavian Airlines System (SAS) McDonnell Douglas MD-81, registering OY-KHO and departing Stockholm (ESSA) for a flight to Copenhagen (EKCH), had a forced landing in a field, immediately after take-off, as both of its engines failed. The impact of the landing was such that the aircraft broke into three, and this occurred on 27th Dec.1991. It is a known fact that if the atmospheric humidity is high or if it rains, and simultaneously if the wings also are chilled, then ice formation can take place on the upper surfaces of the wings. It is another known fact that such ice get broken off on lift off due to the swift movements of the wings. During flight from Zurich the fuel had become very chilled. There were 2550 kg of fuel in each wing tank on landing, nearly 60% of the tank volume, sufficient enough to chill the upper surfaces of the wings. Weather conditions there was such that ice formation could take place. The flight technician noticed ice formation on the wings while on inspection of the aircraft at night. The passengers noticed that indication tufts were not moving on de-icing, and on take off the ice came peeling off the wings. 2.3.2 showed that the engine damage started with "soft" objects getting sucked into them. Considering all these facts the Board of Accident Investigation reached a clear conclusion that ice peeled off the wings on take off, which then was sucked into by the engines, and caused the damage. It was known for years that this type of engines sucked ice in. In the year 1985, a DC-9-51 had similar engine damages. This risk is even greater in MD-80 series of engines because of the wing tank configuration and larger intake area for air in the engine. (Air Traffic Accident on 27 December 1991 at Gottrora)

Another aircraft occurred when Japan airlines flight 8054, a Douglas DC-8-62-F crashed on take off from Anchorage International Airport, Anchorage, Alaska. The aircraft stalled at a height of about 60 feet AGL, at or shortly after reaching V2. The airframe icing was considered to be the cause behind this accident. This aircraft also was not de-iced before take of, as in the previous crashes. As the aircraft approached land, the icing conditions were such that ice accumulated on the wings. The United States had introduced regulations banning take off, if the wings, propellers or control surfaces of the aircraft had frost, snow or ice sticking on to them. Even today these regulations are in force, as cited under Federal Aviation Regulations (FAR) 121.629,135.227, and 91.209. Known as "clean aircraft concept" these regulations, were based on the fact that in the presence of ice formation of any type, changes of aircraft flight characteristics and degraded performance of aircraft were inevitable.

Seeing the increasing number of accidents of large transport aircrafts and small general aviation aircrafts and to clear the wrong ideas prevalent on the effects of slight surface roughness by ice accumulations on the flight characteristics and aircraft performance, and the good results of ground de-icing fluids, the United States FAA published Advisory Circular (AC) 20-117, in Dec, 1982. It was meant mainly to emphasize the 'clean aircraft concept' following ground operations proper to aircraft icing and to inform on help and guidance available for going in accordance. AC20-117 stated the wide- ranging effects ice formation on the aircraft due to differing nature of effects on the different designs of aircraft, and of their unpredictable nature of the formation of ice. It also stated that reduction of wing lift of up to 30% and increase in drag up to 40% can be the result when ice, frost, or snow with a thickness and surface roughness as that of a medium or coarse sand paper, accumulated on to the leading edge and upper surface of its wings, as indicated by wind tunnel and flight tests. Such changes in lift and drag, reduce control, increase stall speed and also change flight characteristics of aircraft. (Lankford, 2000)

AC -20-117 has given first place to surface roughness in the increase in drag and decrease in lift, and advices not try to take-off until it is confirmed that there is no snow, frost or other ice formation accumulating on to any of the major parts of the aircraft. Further AC -20-117 warns that the ice, frost or snow developed on ground, changes its effect on the flight characteristics unlike the ones developed during flight, and that any aircraft that can go through the icing conditions in forward movement get a certification for flight in icing conditions, ignoring the ice formed on ground. It also called for a close observation before take off as this only can clearly confirm that the aircraft is free of such ice accumulations. There are many factors influencing the accumulation of ice frost or snow. If moisture is splashed over, blown into, or sublimated onto the aircraft surface, it can cause surface roughness, as in the case of rain, snow or similar conditions. AC-20-117 vested the ultimate responsibility on the pilot-in-command to see that the clean wing concept is adhered to. (Lankford, 2000)

It is seen that some pilots expect a number of… [END OF PREVIEW]

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