Recent data indicates that applying a 15 percent safety margin to calculate wet runway stopping distance, as recommended by previous guidance, may be inadequate in certain conditions to prevent a runway excursion, according to a new safety alert for pilots (SAFO 19003). This new alert replaces the guidance in previous SAFO 15009.
“Several recent runway-landing incidents/accidents have raised concerns with wet runway stopping performance assumptions,” according to this new alert. “Analysis of the stopping data from these incidents/accidents indicates the braking coefficient of friction in each case was significantly lower than expected for a wet runway.” These mishaps occurred on both grooved and un-grooved runways.
Takeoff and Landing Performance Assessment (Talpa) procedures implemented by the FAA on Oct. 1, 2016, added new insight as to how flight crews can evaluate runway braking performance before landing. Talpa defines a “wet runway” as damp to 1/8-inch depth or less of water, while a “contaminated runway” is a surface covered with “greater than 1/8-inch of water.”
The FAA recommends that airports report “wet” conditions, but that is not required. Further, an airport may not be able to generate an accurate report from sudden rain showers that result in water on the runway more than 1/8 of an inch in depth ("contaminated"). The alert concludes that because “Rainfall intensity may be the only indication available to the pilot that the water depth present on the runway may be excessive, it is recommended that pilots use landing performance data associated with medium to poor braking.” However, the FAA notes that using all available data to prevent a contaminated runway excursion is moot when the landings involve delayed touchdowns, improper application of deceleration devices and landing with a tailwind.