8.2 Correlation between reported μ and braking performance
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Airports release a friction coefficient derived from a measuring vehicle. This friction coefficient is termed as “reported μ”.
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The actual friction coefficient, termed as “effective μ” is the result of the interaction between tire and the runway and depends on the tire pressure, tire wear, aircraft speed, aircraft weight and anti-skid system efficiency.
8.2.1 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.
8.2.2 It is then very difficult to link the published performance on a contaminated runway to a “reported μ” only.
Issue II 02.07.2009 Rev. 0
8.2.3 The presence of fluid contaminants (water, slush and loose snow) on the runway surface reduces the friction coefficient, and may lead to aquaplaning (also called hydroplaning) and creation of 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 adverse impact on the acceleration performance leads to a limitation in the depth of the contaminant for takeoff.
8.2.4 Hard contaminants (compacted snow and ice) only affect the braking performance of the aircraft by a reduction of the friction coefficient.
8.2.5 The Manufacturer publishes the takeoff and landing performance according to the type and depth of the contaminant.
8.3 Aircraft Directional Control
8.3.1 A rolling aircraft wheel on the ground is subjected to a friction force. The total friction force can be divided into two components -the braking force (component opposite to the aircraft motion) and the cornering force (side-friction).
8.3.2 The maximum cornering force (i.e. directional control) is obtained when the braking force is nil, while a maximum braking force means no cornering. There must therefore be some compromise between braking and directional control.
8.3.3 The sharing between cornering and braking is dependent on the slip ratio, that is, on the anti-skid system.
8.3.4 Cornering capability (directional control) 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 and directional control of the aircraft.
8.3.5 Reduction in braking force has serious implications on landing distances. While the manufacturers provide tables to be used to calculate landing field length requirements on contaminated runways, the data is advisory and field conditions may be far different than those assumed in the tables. Additional information in the form of Runway Friction Index tables could be used to further refine landing distances required on contaminated runways. Tables published by Transport Canada depicting recommended landing distances under different runway contamination conditions with and without Reverse Thrust are placed at Appendix – ‘A’
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