Why do we measure at 65km/h and 95km/h when we do a Runway Friction Measurement? Are there differences?
A range of speeds were tested at NASA's Wallops Right Facility: 20, 40, 60, and 80 MPH.
20 was deemed too slow to cover a long runway in an acceptable time, and 80 was deemed too fast to achieve (accelerate to) in a short distance -- the test vehicle needs to reach the test speed by the touchdown zone (arriving from the other end also leaves no room for stopping).
So 40 and 60 MPH were chosen, which are 65 and 95 km/h. The two speeds measure different properties. The slow one is for the macrotexture of the runway, and the fast one for the microtexture.
A complete survey should include tests at both speeds.
Microtexture refers to the fine scale roughness contributed by small individual aggregate particles on pavement surfaces which are not readily discernible to the eye but are apparent to the touch, i.e., the feel of fine sandpaper. Macrotexture refers to visible roughness of the pavement surface as a whole. Microtexture provides frictional properties for aircraft operating at low speeds and macrotexture provides frictional properties for aircraft operating at high speeds. Together they provide adequate frictional properties for aircraft throughout their landing/takeoff speed range.
A good macrotexture result means the tires are able to drain the water and remain in contact and not aquaplane. A good microtexture is desired for airplane speeds below 100 MPH.
I haven't mixed macro/slow and micro/fast. Testing those properties has nothing to do with the actual airplane speeds. For example, if 20 MPH wasn't deemed too slow, it would have been the better speed for the macro testing for the aquaplaning at fast speeds.
The lower speed determines the overall macrotexture/contaminant/drainage condition of the pavement surface. The higher speed provides an indication of the condition of the surface's microtexture.
At speeds over 100 mph, the hysteresis component of friction governs. This component is the effect of damping or reacting elastic pressure of rubber when deformed around aggregate particles. The deformation is produced best by good macrotextured surfaces.
- ICAO Doc 9137 Part 2 Pavement Surface Conditions
- FAA Advisory Circular 150/5320-12C