getting to grips with aircraft performance 如何掌握飞机性能 1(34)

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For “grooved” or “porous friction course”2 wet runways, specific friction coefficients wet (between μdry and μwet) can be used, if provided in the Aircraft Flight Manual. The resulting ASD improvement can sometimes result in higher takeoff weights than on smooth wet runways.  Nevertheless, Airbus AFMs don’t provide any specific data for these runway types.
“JAR-OPS 1.480
(2) Contaminated runway: A runway is considered to be contaminated when more than 25% of the runway surface area within the required length and width being used is covered by the following:”
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Standing water: Caused by heavy rainfall and/or insufficient runway drainage with a depth of more than 3 mm (0.125 in).

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Slush: Water saturated with snow, which spatters when stepping firmly on it. It is encountered at temperature around 5q C, and its density is approximately 0.85 kg/liter ( 7.1 lb / US GAL).

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Wet snow: If compacted by hand, snow will stick together and tend to form a snowball. Its density is approximately 0.4 kg/liter ( 3.35 lb / US GAL).

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Dry snow: Snow can be blown if loose, or if compacted by hand, will fall apart again upon release. Its density is approximately 0.2 kg/liter ( 1.7 lb / US GAL).

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Compacted snow: Snow has been compressed (a typical friction coefficient is 0.2).

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Ice : The friction coefficient is 0.05 or below.

1 μ = friction coefficient = ratio of maximum available tire friction force and vertical load acting on a tire. 2 Runways specially prepared and treated with a porous friction course (PFC) overlay

5.5.2. Effect on Performance
There is a clear distinction of the effect of contaminants on aircraft performance. Contaminants can be divided  into hard and fluid contaminants.
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Hard contaminants are : Compacted snow and ice. They reduce friction forces.

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Fluid contaminants are : Water, slush, and loose snow. They reduce friction forces, and cause precipitation drag and aquaplaning.

5.5.2.1. Reduction of Friction Forces
The friction forces on a dry runway vary with aircraft speed. Flight tests help to establish the direct relation between the aircraft’s friction coefficient (μ) and the ground speed (Figure C26).

Until recently, regulations stated that, for a wet runway and for a runway covered with standing water or slush, the aircraft’s friction coefficient could be deduced from the one obtained on a dry runway, as follows:
μwet = μdry/2 (limited to 0.4)
μconta = μdry/4
This concerns A300, A300-600, A310, A320 (except A320-233), A321-100 (JAA certification only), A330-300 (JAA certification only) and A340 basic versions.

As of today, a new method, known as ESDU, has been developed and introduced by post-amendment 42 in JAR/FAR 25.109. The proposed calculation method of the μwet accounts for the tire pressure, the tire wear state, the type of runway and the anti-skid efficiency demonstrated through flight tests. The μconta (water and slush) results from an amendment based on a flight test campaign. The ESDU model concerns all aircraft types which are not mentioned above.
　
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