getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(5)

Hard contaminants (compacted snow and ice) only affect the braking performance of the aircraft by a reduction of the friction coefficient.

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Airbus Industrie publishes the takeoff and landing performance according to the type of contaminant, and to the depth of fluid contaminants.

AIRCRAFT DIRECTIONAL CONTROL
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When the wheel is yawed, a side-friction force appears. The total friction force is then divided into the braking force (component opposite to the aircraft motion) and the cornering force (side-friction). The maximum cornering force (i.e. directional control) is obtained when the braking force is nil, while a maximum braking force means no cornering.

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The sharing between cornering and braking is dependent on the slip ratio, that is, on

the anti-skid system.

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Cornering capability is usually not a problem on a dry runway, nevertheless when the total friction force is significantly reduced by the presence of a contaminant on the runway, in crosswind conditions, the pilot may have to choose between braking or controlling the aircraft.

CROSSWIND
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Airbus Industrie provides a maximum demonstrated crosswind for dry and wet runways. This value is not a limitation. This shows the maximum crosswind obtained during the flight test campaign at which the aircraft was actually landed. Operators have to use this information in order to establish their own limitation.

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The maximum crosswind for automatic landing is a limitation.

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Airbus Industrie provides as well some recommendations concerning maximum crosswind for contaminated runways. These conservative values have been established from calculations and operational experience.

PERFORMANCE OPTIMIZATION AND DETERMINATION
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The presence of a contaminant on the runway leads to an increased accelerate-stop distance, as well as an increased accelerate-go distance (due to the precipitation drag). This results in a lower takeoff weight which can be significantly impacted when the runway is short.

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To minimize the loss, flap setting and takeoff speeds should be optimized. Increasing the flap and slats extension results in better runway performance. Both the accelerate-stop and accelerate-go distances are reduced. A short and contaminated runway naturally calls for a high flap setting. Nevertheless, one should bear in mind that the presence of an obstacle in the takeoff flight path could still require a lower flap setting as it provides better climb performance. An optimum should be determined. This optimum is usually found manually by a quick comparison of the different takeoff charts. The Airbus LPC (Less Paper in the Cockpit) enables an automatic computerized selection of the optimum flap.

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The takeoff speeds, namely V1, VR and V2 also have a significant impact the takeoff performance. High speeds generate good climb performance. The price to pay for obtaining high speeds is to spend a long time on the runway. Consequently, takeoff distances are increased and the runway performance is degraded. Thus, a contaminated runway calls for lower speeds. Once again, the presence of an obstacle may limit the speed reduction and the right balance must be found. Airbus performance programs, used to generate takeoff charts, take advantage of the so-
　
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