## 6 Surface Characteristics Assessment
### Interaction Between Surface Texture, Runway Friction and Aircraft Skid Resistance  
Surface texture plays a significant role in friction [150]. In runway design practice, most of the methods of friction improvement are based on texture improvement, which can be achieved by using a high macrotexture pavement surface layer or grooving [148; 151; 152]. This is because microtexture and the overall contribution of a runway surface to aircraft skid resistance cannot be directly measured or controlled. It is also well known that microtexture affects low-speed friction and macrotexture affects high-speed friction [153]. However, most aircraft skid resistance issues are associated with wet runways, with the dry surface having more friction and macrotexture than necessary. Therefore, macrotexture is a key focus for tire-surface contact area drainage. In 1995, PIARC made an attempt to unify friction measurement practice by using texture parameters and the Penn State friction model [154]. This led to the development of the unified International Friction Index [104], as detailed in Equation (7). \[F60=A+B\times FRS\times\exp\left[\frac{-(60-S)}{(a+b\times Tx)}\right]+C\times T \tag{7}\]  
where \(FRS\) is a friction testing result at speed \(S\) and \(Tx\) is a texture measurement result. \(A\), \(B\), \(C\), and \(a\), \(b\) are coefficients for particular friction and texture testing methods. Device-specific coefficients were obtained with good correlation coefficients for each device [104]. Later, IFI calculation methods for new and different devices were standardized [130]. Despite the fact that some studies have found coefficients obtained by PIARC to be incorrect [105; 106], the concept of IFI, as well as the dependence between friction testing results and texture measurements, have been proven to be valid [155; 156]. A similar attempt was made by the Forum of European National Highway Research laboratories in 2006 as a further update of the International Friction Index method [157]. This method, however, has not become popular due to the same problems. The second problem of the International Friction Index and further similar studies is a lack of microtexture measurement techniques [73]. Besides the PIARC study, there have been various attempts to relate friction and texture parameters (Table 8). \begin{table}
\begin{tabular}{c c c c c} \hline
**No.** & **Model** & **R2** & **Friction Assessment** & **Texture Assessment** & **Reference** \\ \hline \multirow{4}{*}{1} & \multirow{4}{*}{\(\Delta BPN=6.2\ N_{B^{0}}\left(\frac{Tp}{s}\right)^{-2.2}\times 10^{-2}\)} & & Difference between & \multirow{4}{*}{Macrotexture depth} & \multirow{4}{*}{[158]} \\  & & BPN value of flat & & & \\  & & surface and surface & & & \\  & & with macrotexture & & & \\  & & & Skid Number & & \\
2 & \(c_{0}=-31+1.38\ BPN\) & & & & \\  & \(c_{1}=-0.041\ MTD^{-0.47}\) & & & & \\  & & & Skid Number & & \\
3 & \(SN_{64}^{R}=0.766\ BPN+4.72\ MTD-9.7\) & & & & \\  & \(SN_{64}^{R}=0.628\ BPN+17.3\ MTD-19.5\) & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \end{tabular}
\end{table} 
Table 8: Correlation between friction and texture parameters in different studies. Most of these studies appear to allow for the evaluation of friction using only texture parameters, without requiring friction to be measured at any reference or zero speed. Rather, to estimate the CFME value, various combinations of microtexture and macrotexture are used, with some studies using the BPN as an estimate of the microtexture contribution. Other studies [137; 165; 21] use theoretical models for friction calculations. These studies are generally based on a hysteresis friction model [22]. This model also allows for the inclusion of contamination, such as water film thickness, using the same textural parameters, assuming that contamination acts as a filler for the texture, reducing the height of the asperities. These models also showed good correlation with CFME data.