|CASB Majority Report|
AERODYNAMIC EFFECTS OF ICING
The most significant effect of snow or ice on the wing surface is its influence on the smooth flow of air over the surface contour. Changes in the contour shape and roughness of the surface will cause the airflow to begin to separate from the wing at a lower angle of attack than normal and cause a reduction in the lift which will normally be developed by a wing at a given angle of attack and a given airspeed (see figure below). Both the maximum lift which can be developed and the angle of attack at which it will be developed will be reduced significantly. Stall buffet and stall will be encountered at higher than normal airspeeds.
Lift and Drag Effects of Wing Contamination
Ice contamination of an aircraft wing also has a significant detrimental effect on the aircraft's total drag, that is, the force which resists the aircraft's forward motion through the air. The total drag has two components, parasite drag and induced drag. Induced drag is that drag which is produced by the generation of lift. Induced drag increases as the angle of attack increases. Therefore, since a contaminated wing must fly at a higher angle of attack at a given airspeed to produce the required lift, the induced drag generated at that airspeed will be higher than the induced drag of an uncontaminated wing. Furthermore, since ice contamination causes the airflow to separate earlier from the upper surface of the wing, it results in a higher induced drag value at any angle of attack. The increase in parasite drag as a result of ice contamination is small in comparison to the increase in induced drag.
On a wing contaminated by surface roughness, the normal stall progression of a swept wing is altered. The normal nose-down pitching moment in the direction of stall recovery which accompanies a stall is reduced when the wing is contaminated. The effects of the degraded pitching moment characteristics can range from an out-of-trim condition that can have a different than expected response to control column inputs. to a severe pitch-up as the angle of attack is increased.
The leading edge portion of the wing is most sensitive to ice contamination. The effects of the contamination decrease as the forward most extent of the contamination moves farther aft of the leading edge.
Glaze ice accretions which occur at temperatures just below freezing provide the largest aerodynamic penalty.
Ice accumulation, in particular, the detrimental effects on lift and drag associated with wing surface roughness has been identified as a causal factor in a number of take-off accidents involving jet transport aircraft.
On 27 December 1968, Ozark Airline Flight 982, a Douglas DC-9-15, crashed while taking off from the Sioux City Airport, Sioux City, Iowa. The NTSB determined that the probable cause of the accident was a stall near the upper limits of ground effect, with subsequent loss of control as a result of the aerodynamic and weight penalties of airfoil icing. The crew had not de-iced before the attempted take-off.
On 27 November 1978, Trans World Airways Flight 505, a Douglas DC-9-10, crashed while taking off from Newark International Airport, Newark, New Jersey. Aircraft control was lost shortly after lift-off at an airspeed of 154 knots and at an altitude of about 65 feet agl. The NTSB identified airframe icing and a failure to de-ice before take-off as causal factors.
On 05 February 1985, an Airborne Express Douglas DC-9-15 crashed while taking off from Philadelphia International Airport, Philadelphia, Pennsylvania. The NTSB determined that airfoil icing and failure to de-ice before take-off were cause factors in the accident.
All three of the above accidents contained several common elements:
1. Each aircraft stalled at a lower than normal angle of attack shortly after take-off;
2. Precipitation was present in the form of freezing rain and/or snow;
3. The aircraft were not de-iced before take-off;
4. None of the aircraft was equipped with leading edge devices.
On 13 January 1977, Japan Airlines Flight 8054, a Douglas DC-8-62-F, crashed while taking off from Anchorage International Airport, Anchorage, Alaska. The aircraft stalled at, or shortly after reaching, V2 at an altitude of about 60 feet above ground level. The NTSB determined that airframe icing was a contributing factor in the accident. As in the other three cases, the aircraft was not de-iced prior to take-off. Conditions during the approach to land were conducive to the accretion of ice on the wings of the aircraft.
In 1950, the United States established regulations which prohibited take-off of aircraft when frost, snow, or ice was adhering to the wings, propellers, or control surfaces of an aircraft. These regulations remain in effect today as cited under Federal Aviation Regulations (FAR) 121.629, 135.227, and 91.209. These regulations are commonly known as the "clean aircraft concept" and were based on the known degradation of aircraft performance and changes of aircraft flight characteristics when ice formations of any type are present.
In December 1982, in response to a number of accidents involving large transport and small general aviation aircraft resulting from what it believed to be misconceptions that existed regarding the effects of slight surface roughness caused by ice accumulations on aircraft performance and flight characteristics and the effectiveness of ground de-icing fluids, the United States FAA published Advisory Circular (AC) 20-117. Its purpose was to emphasize the clean aircraft concept following ground operations conducive to aircraft icing and to provide information to assist in compliance.
AC 20-117 identifies that the effects of ice formation on an aircraft are wide ranging, unpredictable, and dependent upon individual aircraft design. It states that wind tunnel and flight tests indicate that when ice, frost, or snow, having a thickness and surface roughness similar to medium or coarse sandpaper, accumulates on the leading edge and upper surface of a wing, wing lift can be reduced by as much as 30 per cent and drag can be increased by 40 per cent.
These changes in lift and drag will significantly increase stall speed, reduce controllability, and alter aircraft flight characteristics. It identifies surface roughness as the primary influence in the decrease in lift and increase in drag and emphasizes that take-off not be attempted unless it has been ascertained that all critical components of the aircraft are free of adhering snow, frost, or other ice formations.
AC-20-117 cautions that aircraft certified for flight in icing conditions have only demonstrated the capability of penetrating icing conditions in forward flight regime and that ice, frost, or snow formed on aircraft surfaces on the ground can have a totally different effect on aircraft flight characteristics than ice formed in flight.
AC-20-117 states that the only method currently known of positively ascertaining whether an aircraft is clean prior to take-off is by close inspection. Many factors are identified which influence the accumulation of ice, frost, or snow. Surface roughness results under conditions of precipitation or when moisture is splashed, blown, or sublimated onto aircraft surfaces. The circular states that the pilot-in-command is ultimately responsible for ensuring that the clean wing concept is followed.