getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(50)

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It is evident that the braking performance is strongly affected by a slippery runway, however, one should also consider the loss in acceleration performance and in aircraft lateral controllability.
Once the different aspects of the impact of a contaminated runway are explained, it is quite necessary to review the operational information provided to the pilots. This information mainly contains some penalties (e.g. weight penalty or maximum crosswind reduction) but as well some indications on the runway condition provided as a .friction coefficient..
All this information should be readily understood so as to jeopardize neither airline safety nor airline economics.

C1 WHAT IS A CONTAMINATED RUNWAY?

A runway is considered contaminated when more than 25% of the surface is covered with
a contaminant.
Contaminants are: water, slush, snow and ice.

The SPECIAL OPERATIONS chapter of the FCOM, provides the following definitions:

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If the layer of contaminant on the runway is thin enough, the runway is not considered
contaminated, but only wet.
FLEX takeoff is not allowed from a contaminated runway.

As far as performance determination is concerned, the following guidelines should be
considered, as mentioned in the FCOM:

C2 AIRCRAFT BRAKING MEANS

Aircraft braking performance, in other words, aircraft .stopping capability., depends on many parameters. Aircraft deceleration is obtained by means of: Wheel brakes, aerodynamic drag, air brakes (spoilers) and thrust reversers.
C2.1 Wheel brakes
Brakes are the primary means of stopping an aircraft, particularly on a dry runway.
Deceleration of the aircraft is obtained via the creation of a friction force between the runway and the tire. This friction appears at the area of contact tire/runway. By applying the brakes, the wheel is slowed down and, therefore creates a force opposite to the aircraft motion. This force depends on the wheel speed and the load applied on the wheel.
● Wheel load:
A load must be placed on the wheel to increase the contact surface between the tire and the runway, and to create a braking/friction force.
There is no optimum on the load to be placed on the wheels. The greater the load, the higher the friction, the better the braking action.
The FRICTION COEFFICIENT is defined as the ratio of the maximum available tire friction force and the vertical load acting on a tire. This coefficient is named MU or μ.

Friction force
μ = Load
Friction force

LOAD = % of aircraft gross weight on one tire
Figure C1
● Wheel speed:
The area of tire/runway contact has its own speed, which can vary between two extremes:
.
 Free rolling speed, which is equal to the aircraft speed.

.
 Lock-up speed, which is zero.

Any intermediate speed causes the tire to slip over the runway surface with a speed equal to: Aircraft speed - Speed of tire at point of contact. The slipping is often expressed in terms of percentage to the aircraft speed. Refer to Figure C2.
　
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