getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(58)

C6 PERFORMANCE OPTIMIZATION AND DETERMINATION

C6.1 Performance Optimization
A contaminated runway impacts runway-related performance. The accelerate-go distance is increased due to the precipitation drag, and the accelerate-stop distance is increased due to the reduction in the friction forces.
The natural loss of payload, resulting from lower takeoff weight, can be minimized by different means. Optimization of flap setting, takeoff speeds and derated takeoff thrust are the main ways of limiting a loss in takeoff weight.
C6.1.1 Flap setting
Three different flap settings are proposed for takeoff. If the name of these flap/slat position used to be related to the actual deflection in degrees for the A300 and A310 models, it has been standardized to 1+F, 2 and 3 for the A320 family and A330/A340 models. Of course, the actual flap/slat deflection differs for each aircraft.
The influence of the flap setting on the takeoff performance is well-known. Low flap settings (e.g. Conf1+F) provide good climb performance (good lift to drag ratio) while the takeoff distance is longer (in other words bad runway performance). A higher flap setting (e.g. Conf3) helps reduce the takeoff distance (improvement of the runway performance) at the expense of the climb performance (degradation of the lift to drag ratio).
Most of the time, a contaminated runway calls for higher flap setting. The accelerate-go and the accelerate-stop distances are then reduced. Yet, the presence of an obstacle may still require a minimum climb gradient calling for a lower flap setting. The right balance must be found.
The choice of the optimum flap setting is usually done manually. A quick comparison of the performance for the three different flap settings reveals which one is best.
The LPC (Less Paper in the Cockpit) allows for an automatic selection of the optimum configuration.
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C6.1.2 Takeoff speeds
An improvement of runway performance can be achieved by reducing the takeoff speeds. A reduced V2 generates a reduced accelerate-go distance, while a reduced V1 generates a reduced accelerate-stop distance.
Regulations are helpful in this matter, as the 35ft screen-height is reduced to 15 ft should the runway be contaminated. For a given set of conditions, the V2 speed is lower at 15 ft than at 35 ft. In other words, the screen-height reduction allows for lower speeds.
Remark: Just like the effect of thrust reverse (See C2.3),
 the screen-height reduction may lead to a performance-limited weight on a wet or contaminated runway being greater than the performance-limited weight on a dry runway. It is compulsory to restrict the performance-limited weight on a wet/contaminated runway to that of a dry runway. Nevertheless, the determination of takeoff speeds must always account for the runway condition.
Moreover, Airbus takeoff charts take advantage of the so-called .optimized speeds.. That is, speeds are increased, to meet a climb limitation or reduced to meet a runway limitation, accordingly. In the case of a short and contaminated runway, the optimization process automatically selects lower speeds - yet, speeds are high enough to clear an obstacle, if any.
　
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