Friction testing on airport pavements
The surface friction of the runway is dependent on a number of factors such as tyre material, wear and temperature as well as the amount of contaminants such as rubber deposits, mud, dust, loose stones and oil spills. Although it is common knowledge that speed affects braking distance, what isn’t so common is that the mechanics of skid resistance are fundamentally different at different speeds.
At lower speeds, Surface microtexture (the skid resistance produced by individual stones in the aggregate) is dominant in skid resistance, which provides adhesive friction. Over time, the microtexture is worn down through aggregate polishing, especially in high load areas such as the touchdown zone.
At higher speeds the adhesive friction characteristics are reduced, and the total friction relies mostly on the mechanical interaction of the surface texture (larger aggregates, surface texture and grooving) known as Hysteresis. At higher speeds, the contact area between the tyre and the pavement are also radically different due to the dynamics of the tyre and its ability to disperse water. Low friction at these speeds can be caused by the build-up of rubber deposits in the aggregate and grooves.
Skid resistance can be incredibly complex and there are a lot more variables at play other than just speed. Careful monitoring of your airport pavement is essential to ensure safety for both pilots and passengers.
In practice, any well-designed and properly constructed runway should provide sufficient friction when dry, however when water is introduced, the amount of contact that the tyre has on the runway may be limited when traveling at speed. This effect - known as aquaplaning which comes in two types (Viscous and Dynamic) depending on the aircraft’s speed.
Viscous aquaplaning occurs at lower speeds in areas of little surface texture. This can happen even with the smallest water depths and is caused by water not adequately dispersing from underneath the tyre’s footprint.
Dynamic aquaplaning occurs primarily at high speeds, where the water acts more as a wedge and penetrates the tyre’s footprint, reducing the surface contact area.
In the worst cases, the effect is so great that no part of the tyre will be in contact with the runway at all. This can lead to greatly reduced wheel spin-up on landing and can also lead to having the wheels lock, greatly reducing braking performance. In the worst cases, the excessive heat generated by locking wheels can even melt the tyre, reducing its effectiveness and causing blowouts.
If the wheels lock under this form of aquaplaning, it can cause reverted rubber skidding where the heat generated from the friction vaporises the water causing a cushion of steam to further eliminate tyre contact.
To minimise the risk of aquaplaning occurring, attention must be given to the surface macro and microtexture. An ideal macrotexture can be instrumental in ensuring that water has an efficient “escape route” that allows water to be more efficiently removed from the tyre’s path. The ICAO’s minimum texture requirement on runway pavements is 1mm. One can implement artificial measures (such as runway grooving) to further increase the runway’s texture properties.
