Understanding Friction Data

Mu numbers (friction values) measured by Continuous Friction Measuring Equipment (CFME) can be used as guidelines for evaluating the surface friction deterioration of runway pavements and for identifying appropriate corrective actions required for safe aircraft operations. Table I depicts the friction values for three classification levels for FAA qualified CFME operated at 40 and 60 mph test speeds. This table was developed from qualification and correlation tests conducted at NASA’s Wallops Flight Facility and is re-evaluated annually. In accordance with AC 150/5320-12C, the friction tests began from the runway end at 40 miles-per-hour (mph), recording the data to the opposite runway end, then repeating the test, returning to where the procedure began. The test requires 500 feet from both runway ends for acceleration and deceleration. After the 40 mph test is complete, the friction test was repeated at 60 mph in the same manner as the 40 mph test; however, using 1,000 feet for acceleration and deceleration. 40mph testing determines the overall microtexture. (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. Microtexture provides frictional properties for aircraft operating at low speeds.) 60mph testing is used to determine the overall macrotexture. (Macrotexture refers to visible roughness of the pavement surface as a whole. Macrotexture provides frictional properties for aircraft operating at high speeds.)

TABLE 1: FRICTION LEVEL CLASSIFICATION FOR RUNWAY PAVEMENT SURFACES

		40 mph			60 mph
	Minimum	Maintenance	New Design/	Minimum	Maintenance	New Design/
Continuous Friction Measuring Equipment		Planning	Construction		Planning	Construction

Mu Numbers	0.50	0.60	0.82	0.34	0.47	0.74