## 4 International Runway Skid Resistance Standards and Regulations
### Wet Friction Measurement Results  
According to the Manual on Certification of Aerodromes [67], airports should provide airfield pavement maintenance equipment, including friction measurement devices. Provision of sufficient friction is recommended to be controlled using continuous friction measurement devices (CFME) with a fixed slip ratio between 10% and 20%, measured immediately behind a 1 mm thick film of water applied to the surface. CFME is used during the construction of new pavements and periodically thereafter, to determine any deterioration in the friction level, either by surface ageing, surface erosion or surface contamination [68]. According to Annex 14 [7], the frequency of the runway friction survey should be sufficient to determine the trend of the surface friction characteristics of the runway. Doc 9137 P2 [34] suggests friction survey frequencies based on the operational tempo of the runway, as detailed in Table 2, and most ICAO member states recommend these same frequencies. However, no testing frequency is provided for less busy runways, nor for runways with no jet aircraft operation. Some guidance is required for those less busy, but airports and their runway friction are still more important. During CFME surveys, it is important to establish friction change dynamics for each runway, in order to plan maintenance (Figure 14). To avoid uncertainties in the measured values, it is important to identify anomalies, evaluate them, and reduce them where possible. Uncertainties can be grouped into the following categories [68]:  
* Operator: anyone involved in the measurement process;
* Methods: specific requirements for performing the measurement;
* Means: any means (computer, software, device, and parts) used to perform the measurement;
* Materials: raw materials, such as tires, used to produce the final results;
* Environment: environmental conditions, such as temperature, location, and weather. \begin{table}
\begin{tabular}{c c} \hline \hline
**Number of Daily Minimum Turbojet** & **Minimum Friction Survey Frequency** \\
**Aircraft Landings per Runway End** & **Minimum Friction Survey Frequency** \\ \hline Less than 15 & 1 year \\
16 to 30 & 6 months \\
31 to 90 & 3 months \\
91 to 150 & 1 month \\
151 to 210 & 2 weeks \\ Greater than 210 & 1 week \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Friction survey frequency based on level of turbojet airplane operations for each runway end [34]. Reliable and repeatable operation of the friction measuring device can be improved through measurements of reference surfaces and constant control of results. Specific recommendations to increase the reliability of testing devices are provided. However, these do not include environmental conditions, such as temperature and humidity [68]. These factors could lead to errors and some researchers have recommended these factors be considered in runway friction measurement management [69; 70]. There are a number of devices for friction assessment that can be used for conducting friction surveys. The list of devices and friction levels recommended by ICAO is presented in Table 3. Table 3 is based on the outcome of extensive trials performed at NASA's Wallops Flight Facility in Virginia, USA in 1989. The various friction limits and targets are also reflected in many member state regulations, such as New Zealand [71], Australia [72] and the USA [30]. Figure 14: Trend monitoring scheme according to ICAO [68].