## 4 International Runway Skid Resistance Standards and Regulations
### Wet Friction Measurement Results
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Equipment** & **Test Speed, km/h** & \begin{tabular}{c} **Design Objective** \\ **for New Surface** \\ \end{tabular} & \
\begin{tabular}{c} **Maintenance** \\ **Planning Level** \\ \end{tabular} &
\begin{tabular}{c} **Minimum Friction** \\ **Level** \\ \end{tabular} \\ \hline Mu-meter Trailer & 65 & 0.72 & 0.52 & 0.42 \\  & 95 & 0.66 & 0.38 & 0.26 \\ Skiddometer Trailer & 65 & 0.82 & 0.60 & 0.50 \\  & 95 & 0.74 & 0.47 & 0.34 \\  & 65 & 0.82 & 0.60 & 0.50 \\  & 95 & 0.74 & 0.47 & 0.34 \\  & 65 & 0.82 & 0.60 & 0.50 \\  & 95 & 0.74 & 0.54 & 0.41 \\  & 65 & 0.76 & 0.57 & 0.48 \\  & 95 & 0.67 & 0.52 & 0.42 \\  & 65 & 0.69 & 0.52 & 0.45 \\  & 95 & 0.63 & 0.42 & 0.32 \\  & 65 & 0.74 & 0.53 & 0.43 \\ GRIPTESTER Trailer & 95 & 0.64 & 0.36 & 0.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Runway surface condition levels [34]. The reference friction measurement device for all CFME, dating from the 1970s, is the Mu-meter. ICAO Circular 355 states that the minimum friction levels from the table reflect historic levels for the individual friction measuring devices identified and have not adjusted to more recent comparisons of the devices. That is, the values in Table 3 have not been updated and reflect levels no longer considered unconditionally valid by ICAO [68]. Furthermore, Doc 9137 Part 2 states that the correlation between different friction measurement devices is not reliable under naturally wet conditions, and it is recommended to perform additional friction measurements under natural rain conditions, where friction characteristics can be reduced because of poor drainage due to inadequate slopes or depression [34]. It has also been stated by various researchers that those levels do not reflect the actual correlation between these devices [73]. The design objective levels listed in Table 3 are for information and reference only and are not for regulatory compliance of runways. These are meant to indicate the approximate values for a grooved surface, immediately after pavement construction and after resurfacing [34]. It has been demonstrated that the friction level of a newly constructed surface usually changes significantly within a relatively short period of time [74]. Hence, the only actual threshold for the friction level during the construction and early life of a pavement is the maintenance planning level, which provides a buffer to the actually important minimum value. Friction levels measured by CFME (Table 3) are primarily intended for maintenance and resurfacing planning purposes [68]. They are not directly relatable to aircraft braking performance, which occurs at higher speeds, with different tire pressures and wheel loads, and in any prevailing condition of natural wetness. Consequently, CFME measurements should not be routinely reported to pilots for operational decision making. Rather, RCR should be used for operational management. The philosophy of RCR is that the aerodrome operator assesses the runway surface conditions whenever water, snow, slush, ice, or frost are present on an operational runway. This format of operational or real-time reporting was adopted by ICAO in 2021. Runway surface conditions are assessed and reported as the RWYCC. The RWYCC assessment and braking performance of an airplane are presented in Table 4. RWYCC 6 corresponds to normal braking in dry conditions; hence, no description of braking action is required. The assessment is performed for each third of the runway [66]. After an initial assessment, based on visual observation for contaminants on the runway surface, the RWYCC can be upgraded or downgraded based on observations, local knowledge and experience [68]. However, the process of downgrading and upgrading the RWYCC in wet conditions is not clearly explained, especially in the case of RWYCC 3, 4, and 5.