C. PERFORMANCE ON CONTAMINATED RUNWAYS
AIRCRAFT BRAKING MEANS
●  There are three ways of decelerating an aircraft: The primary way is with the wheel brakes. Wheel brakes stopping performance depends on the load applied on the wheels and on the slip ratio. The efficiency of the brakes can be improved by increasing the load on the wheels and by maintaining the slip ratio at its optimum (anti-skid system).
Secondary, ground spoiler decelerates the aircraft by increasing the drag and, most importantly, improves the brake efficiency by adding load on the wheels.
Thirdly, thrust reversers decelerate the aircraft by creating a force opposite to the aircraft’s motion regardless of the runway’s condition. The use of thrust reversers is indispensable on contaminated runways.
BRAKING PERFORMANCE
●  
The presence of contaminants on the runway affects the performance by:

1.
A reduction of the friction forces (μ) between the tire and the runway surface,

2.
An additional drag due to contaminant spray impingement and contaminant  
displacement drag,

3.
Aquaplaning (hydroplaning) phenomenon.

●  
There is a clear distinction between the effect of fluid contaminants and hard contaminants:

-
 Hard contaminants (compacted snow and ice) reduce the friction forces.

-
 Fluid contaminants (water, slush, and loose snow) reduce the friction forces, create an additional drag and may lead to aquaplaning.

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● To develop a model of the reduced μ according to the type of contaminant is a difficult
issue. Until recently regulations stated that (wet and (cont can be derived from the μ observed on a dry runway (μdry/2 for wet runway, μdry/4 for water and slush).
Nevertheless, recent studies and tests have improved the model of μ for wet and contaminated runways, which are no longer derived from μdry. The certification of the most recent aircraft already incorporates these improvements.
CORRELATION BETWEEN REPORTED μ AND BRAKING PERFORMANCE
●  
Airports release a friction coefficient derived from a measuring vehicle. This friction coefficient is termed as “reported μ”. The actual friction coefficient, termed as “effective μ” is the result of the interaction tire/runway and depends on the tire pressure, tire wear, aircraft speed, aircraft weight and anti-skid system efficiency. To date, there is no way to establish a clear correlation between the “reported μ” and the “effective μ”. There is even a poor correlation between the “reported μ” of the different measuring vehicles.

It is then very difficult to link the published performance on a contaminated runway to a “reported μ” only.

●  
The presence of fluid contaminants (water, slush and loose snow) on the runway surface reduces the friction coefficient, may lead to aquaplaning (also called hydroplaning) and creates an additional drag. This additional drag is due to the precipitation of the contaminant onto the landing gear and the airframe, and to the displacement of the fluid from the path of the tire. Consequently, braking and accelerating performance are affected. The impact on the accelerating performance leads to a limitation in the depth of the contaminant for takeoff.
　
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