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VREF Landing reference speed
VS Stalling speed (=VS1g for
Airbus FBW aircraft)
WAI Wing Anti Ice
WPT Waypoint
WX Weather
WXR Weather Radar
ZFW Zero Fuel Weight
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A. AIRCRAFT CONTAMINATION IN FLIGHT
The objective of this chapter is to explain some of the difficulties encountered by flight crews in winter time or cold/wet air. Many forms of ice may deposit or accrete on the airframe, in flight or on ground, and that will affect aircraft performance. It is difficult to determine how much the performance is affected. There are cases when the amount of ice looks benign and proves to produce large performance degradation. The opposite case may also be true.
However, thorough analysis of the incident/accident record strongly suggests that enhanced pilot awareness of icing is a key factor in dominating the icing threat.
A1 ICING PRINCIPLES
A1.1 Atmospheric physics at a glance
Water is a well-known component of atmospheric air. Clear air includes water vapor in very variable proportions according to air temperature (SAT or OAT). The maximum amount of water vapor allowable in the air is about 0.5 g/m3 at - 30°C and 5 g/m3 at 0°C for moderate altitudes. These limiting conditions are called saturation.
Any amount of water in excess of the saturation conditions will show under the form of water drops or ice crystals. These form clouds.
Saturation conditions may be exceeded by two processes:
-First, is the lifting of warm air. Air lifting may be produced by meteorological instability or orography. Instability is associated with weather systems, perturbations or large amounts of clouds. Orographic effect is due to wind blowing onto a mountain, hence lifting on the exposed side.
- Second is the rapid cooling of the lower air layer during a night with clear sky.
In both of these conditions, the amount of water initially present in the air mass may become in excess of the saturation conditions at the new (lower) temperature. Excess water precipitates in the form of drops, droplets or ice crystals.
The icing phenomenon is due to the fact that water does not necessarily turn into ice just at, or below 0°C. Water at negative Celsius temperature may remain liquid; then, it is called supercooled. But supercooled drops and droplets are unstable. This means that they can freeze all of a sudden if they hit, or are hit by, an object, especially if the object is at negative temperature. That is the basic mechanism for aircraft icing.
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