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Normal weather evolution calls for precipitation, which vary against time due to temperature change. Snow often hides underneath ice, etc...
In all cases, a wing must have been cleaned prior to takeoff, regardless of the kind of contamination. (See Chapter B - Aircraft De-icing / anti-icing on the ground)
To further complete the picture, taking off, landing and taxiing in slush may lead to projection of large amounts of wet snow which may freeze upon impact on sensitive parts of the airframe: flaps, slats and landing gear.
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A2 ICING CERTIFICATION
A2.1 Aerodynamics at a glance
Aircraft designers do their best to ensure airframes have smooth surfaces to ease the surrounding airflow. This rule is applied with special care to the wing leading edge and upper surface, because smoothness in these areas produces the best lift force. Any type of ice accretion is an obstacle to smooth airflow. Any obstacle will slow the airflow down and introduce turbulence. That will degrade the lifting performance of the wing. Figure A3 gives the lift coefficient of a clean wing, and that of a wing spoiled by ice.
LIFT COEFFICIENT
Figure A3
Both the maximum lift and the maximum achievable angle of attack have been decreased. The mechanism by which lift is affected has to do with the evolution of the boundary layer along the wing chord. Figure A4
shows what happens at relatively high angle of attack.
This set of sketches gives comparative explanation of the impact of ice accretion and how these flight conditions are certified.
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Sketch #1 is a reference: clean wing with normal boundary layer
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Sketch #2 is an iced wing in configuration zero. The ice accretion on the leading edge is bigger than to scale. Aircraft is certified in those conditions because, although the boundary layer is thicker, the aerodynamic .circulation. around the wing is not severely affected. Lift is not highly affected, only flow separation, therefore stall, occurs at a little lower angle of attack. Aircraft minimum operational speeds take that maximum lift loss into account.
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Sketch #3 shows the same wing at landing conditions. In spite of the .pollution. of the slat, the slat slot restores a .normal. boundary layer on the wing box. Again, the .circulation. around the full wing is not severely affected and aircraft is certified to land in those conditions.
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Sketch #4 shows the result of morning frost after an overnight stay in clear sky conditions. Even a very thin layer of velvet ice will destroy the boundary layer all on the overwing. Result is a large decrease of .circulation.. Lift loss may be large and is not predictable. This is why these conditions are not certified.
Figure A4
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