What is Skid Resistance?

Skid Resistance

Skid resistance is the friction that a pavement surface gives a tyre, and it depends on the surface texture. FAA AC 150/5380-6C defines it as the ability of a pavement to give the desired friction characteristics under all weather conditions. ICAO Annex 14 and the EASA rules use the term “surface friction characteristics” for a runway.

Texture

Two scales of texture set the friction. ICAO Annex 14, Attachment A, describes them this way:

• Microtexture is the texture of the individual stones. It is hardly detectable by the eye. It is a primary part of skid resistance at slow speeds.
• Macrotexture is the texture among the individual stones. It comes from the size of the aggregate or from a surface treatment. It is the major factor for drainage at high speeds.

Annex 14 recommends an average texture depth of at least 1.0 mm for a new runway surface (3.1.26). Grooving a runway makes the drainage path shorter and so speeds up drainage.

What reduces skid resistance

EASA says the operator should monitor the trend that three causes produce: rubber deposits, surface polishing and poor drainage. FAA AC 150/5380-6C adds oil, fuel and dust, and lists polished aggregate as a pavement distress. Rubber deposits in the touchdown area can mask microtexture without reducing macrotexture. Microtexture can change in a short time without being easy to detect (Annex 14, Attachment A, 7.3.6).

Pools of water as shallow as about 3 mm can cause aquaplaning where landing aeroplanes meet them at high speed (EASA GM1 ADR.OPS.C.010(b)(2)). Rubber in grooves makes the grooves less effective and increases the likelihood of hydroplaning (FAA AC 150/5380-6C).

Measurement and limits

Annex 14 requires the operator to keep a paved runway at or above a minimum friction level that the State specifies (10.2.3). The operator measures the friction periodically with a self-wetting continuous friction measuring device and documents it. The frequency must be sufficient to show the trend (10.2.4).

EASA gives levels for each device. The Mu Meter has a minimum of 0.42 and a maintenance planning level of 0.52 at 65 km/h. At 95 km/h, the same device has 0.26 and 0.38. The GripTester has 0.43 and 0.53 at 65 km/h, and 0.24 and 0.36 at 95 km/h. A reading at or below the maintenance planning level triggers a survey. The survey covers the texture, the contaminants and the drainage of that runway third (AMC1 ADR.OPS.C.010(b)(3)).

In the United States, the FAA asks for friction measurements on a periodic basis (AC 150/5380-7B) and refers to AC 150/5320-12 for the method. AC 150/5370-10H uses the same two test speeds, 65 and 95 km/h. 49 USC 47101(f)(2) recommends grooving or a friction treatment for primary and secondary runways at commercial service airports (AC 150/5320-6G, 1.7).

Reporting

The operator makes information available when a runway or part of it is slippery wet (Annex 14, 2.9.9). The operator notifies users when the friction level is below the minimum (2.9.10). Under EU rules the operator does not report friction measurements to pilots (ADR.OPS.A.065).

Skid resistance and drone inspection

A drone does not measure friction. FAA AC 150/5380-6C states that visual observations made during a pavement inspection are an inadequate predictor of skid resistance. A friction device on the surface gives the measurement.