## 6 Surface Characteristics Assessment
### Fixed and Braking Test Methods  
CFMEs, also known as continuous friction measuring devices (CFMDs), measure the friction coefficient continuously while the testing device is moving along the pavement surface. Various devices are generally either towed trailers or modified cars. These devices have been used since the 1950s in road and highway friction evaluation and were introduced to the runway skid resistance assessment in the 1960s [91]. The first CFMEs were used widely in the USA, for example, such as the Penn State Road Friction Tester [92; 93]. In 1990, NASA and FAA returned their attention to the CFME assessment of runway skid resistance and carried out a comparative study of different CFMEs during the FAA/NASA Runway Friction Program [94]. Tests were performed in different conditions, including dry runway, wet runway, ice contamination, and snow contamination, with six ground-testing vehicles and two instrumented aircraft. According to the results, ground vehicles correlated well to aircraft braking friction. However, the correlation did not allow the use of a CFME-measured friction value to be used for aircraft braking performance evaluation. All ground vehicle tests reported a friction coefficient that was significantly higher than the actual aircraft-measured friction coefficient [94]. Some authors have reported that the repeatability of CFME can be low [95; 96; 97; 98]. For example, a comparison of the same CFME models showed 10-20% variability between different testing machines of the same type [97]. This means that the variation in results can be higher than the difference between the acceptable and minimum levels of friction on the runway. According to a similar study, the variation of the Grip Tester results depending on the tire actual diameter can be calculated using Equation (4) [99]. Based on this, it is clear that as the tire wears, the friction value goes up. Due to the allowed wear level of a testing machine tire, the value can increase by 0.02-0.05. \[F=GN\frac{(SD-CD)}{(MD-SD)} \tag{4}\]  
where \(F\) is a friction value, \(GN\) is a Grip Tester number, \(SD\) is a standard tire diameter (260 mm), \(CD\) is a chain cog effective diameter (130 mm), and \(MD\) is a measured tire diameter. Moreover, as part of the research project sponsored by EASA in 2008, it was found that the braking slip, tire pressure, tire design, tire trad materials, derivation of friction coefficient, and self-wetting system all significantly affect the friction reading from different devices [73]. This part of the project was the harmonization of friction measurement techniques; as a result, basing the harmonization efforts on relationships to aircraft performance was considered to be the best option, which resulted in the development of the Global Reporting Format [100]. Friction measuring device harmonization, however, was not carried out due to the inconsistency of correlation models among devices. At the same time, it was recommended to use a Dynamic Friction Tester (DFT) as a reference device for the friction testing and a Circular Track (CT) meter as a reference device for the macrotexture measurement. These recommendations, however, were not incorporated into the ICAO standards [100]. According to NASA Annual Tire/Friction Workshop results, CFME reliability and repeatability can be increased by the following [101]:* Performing the test during naturally dry surface conditions. * Ensuring the self-wetting system accurately provides a reliable 1 mm water film immediately in front of the test tire. * Performing testing at three test speeds, when assessing aquaplaning potential. * Omitting the first two runs because these are often outliers to the rest of the results. Some of the above-mentioned recommendations were presented in national standards, such as in Australia, New Zealand, the USA, etc. [71; 102; 103].