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The boundary layer is thicker and more turbulent along the wing chord, and therefore, flow separation will occur at a lower angle of attack. Stall speed will be increased. Note how insidious that effect is, because at a moderate angle of attack, lift is about the same, as seen in figure A3.
As it is not possible to take into account the whole possible variety of ice shapes, Airbus Industrie has defined procedures based on the worst possible ice shapes, as tested in flight with artificial ice shapes. As a consequence, in case of icing conditions, minimum speeds are defined allowing keeping adequate margins in terms of maneuverability relative to the actual stall with ice accretions. For example, when landing in configuration FULL with ice shapes, speed must be above VREF+5 kt. However, for the FBW system, the settings of the alpha protection system have been adjusted with ice shapes. This means that the aircraft remains protected in case of ice accretions. In turn, this means also that there is an increased margin relative to the stall in the normal clean wing status.
In the case of ground icing, a similar result will be reached because the boundary layer will thicken more rapidly along the chord. Earlier separation will occur, resulting in lower max angle of attack and max lift. As a relatively high angle of attack is normally reached during the takeoff rotation, it is easy to understand that wings must be cleaned prior to takeoff.
Even the very thin layer of velvet morning frost must be cleared. Thickness may be very
If the lower airframe structure has been small, but it covers 100% of the upperwing extensively slushed during taxi time, it surface and the rate of thickening of the might be advisable not to takeoff. The slush boundary layer along the wing chord is still would freeze in flight, and an incident on
landing gear retraction might occur. Upon
considerable. That is a threat for takeoff, as
return to the gate, braking should be
nothing tells the pilot that he might not have
cautious and slats and flaps should not be
the desirable lift for lift-off.
retracted prior to cleaning.
This also applies to the tailplane. Ice deposits must be cleared off tailplane before takeoff to provide the expected rotation efficiency.
A2.2 The history of icing certification
In order to better understand today’s status of icing certification, it is necessary to give a historical view of how the applicable rule was developed.
Icing became a problem on the dawn of air transportation. It also became a big concern for bombers during World War II. Then, statistical data on ice accretion and icing cloud characteristics was gathered. This remarkable piece of work gave birth to an icing database which is still very valid today. Given the variety of ice shapes encountered at the time, it was already clear that it would have been impossible to require flight testing of all
35
cases and to certify them individually. The idea of defining a .worst case. was born,
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