## 3 Review of Friction Related Accidents on a Runway
### Australian Transport Safety Bureau  
The Australian Transport Safety Bureau (ATSB), which provides independent investigation of transport accidents and safety occurrences, reported 45 investigations of runway excursions in Australia between 2003 and 2023 [53]. Of those 45 accidents, 10 (22.2%) were related to insufficient braking performance. In four cases, insufficient braking performance and an underestimation of landing distance were the main reasons for the incident on the paved runway (Table A1). Analysis of these cases shows that one of the key factors in aircraft safety is a lack of understanding of the actual friction coefficient on the runway. In cases 1-7, friction coefficient and understanding of the actual braking capability of an aircraft were not the main contributing factors. However, it is clear that good friction and an understanding of actual runway conditions can help avoid incidents. In case 7, reverted rubber marks on the tire can be clearly seen (Figure 6). However, ATSB did not report it specifically. This condition occurred due to an anti-skid system malfunction on a wet runway.  
Cases 8-10 (Table A1), however, show that poor friction on a runway can be the main contributing factor for runway excursion. In all of these cases, the final approach and landing were normal, and the aircraft braking system worked as usual. Unfortunately, only in case 8 were runway friction testing results presented. However, in case 10, rubber build-ups and poor growing conditions were reported.  
In cases 3, 4, and 6-10 (Table A1), the runway surface was damp or wet (Figure 7). Water reduces the friction coefficient on the runway, increasing the risk of runway excursion. To avoid this risk, surface condition monitoring should be performed before and during the landing. ATBS concluded in some cases that pilots' unawareness of surface conditions also contributed to runway excursions.  
The most detailed investigation was published for the runway excursion at Newman Airport [54]. As it was reported, the runway surface at Newman Airport did not provide sufficient water drainage, which resulted in puddle formation in the aircraft wheel paths (Figure 7). This increased the risk of slipperiness-related accidents. Furthermore, CFME results were regularly obtained, and the friction was lower than the maintenance level reported in the case of 100 m averages (Figure 8). At the same time, 10 m average results show friction levels below the minimum value (Figure 9). ATBS reported that the friction values recorded later were generally higher, which indicates that friction measurements can change during the year, which also affects the awareness and management of poor surface conditions.  
Based on the above review of overrun and veer-off accidents in Australia reported by ATSB, it was concluded that there are a number of factors that can increase the risk of a runway excursion related to slipperiness:  
* Lack of awareness of actual surface conditions;
* Water contamination of the runway and rains and showers before and during the landing;
* Poor runway maintenance;
* The absence of friction testing results, or inaccurate test results.  
Figure 7: Standing water on the runway of Newman Airport [54].  
Based on the analysis of ATSB reports and recommendations provided by ATSB, recommendations for the reduction in safety risks related to skid resistance were proposed. According to investigations, it is possible to lower the risk of runway excursion related to slipperiness by providing:  
* Regular monitoring of the friction coefficient of pavement;
* Regular runway maintenance, including rubber removal, regrooving, and resurfacing;
* Constant monitoring of surface and weather conditions.  
Figure 8: The 100 m average results of CFME using the Griptester trailer on the runway of Newman Airport [54].  
Figure 9: The 10 m average results of CFME using the Griptester trailer on the runway of Newman Airport [54].