The recent spate of runway excursions at Indian airports may have multiple contributory causes including but not limited to :
The pilots are required to calculate the landing distance of the aircraft based on the aircraft condition , environmental factors and runway conditions. After the calculation there is a 15% safety margin added to cater for various inaccuracies.
FAA has issued a safety alert for operators SAFO19003. As per FAA several recent runway-landing incidents/accidents have raised concerns with wet runway stopping performance assumptions. 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 incidents/accidents occurred on both grooved and un-grooved runways. The data indicates that applying a 15% safety margin to wet runway time-of-arrival advisory data, as recommended earlier.
These overruns have occurred on grooved and smooth runways during periods of moderate to heavy rain. Analysis of these incidents/accidents indicates that the braking coefficient of friction in each case was significantly lower than expected, and that 30 to 40 percent of additional stopping distance may be required if the runway transitions from wet to contaminated based on the rainfall intensity or reported water contamination (greater than 1/8-inch depth). For the operational in-flight landing assessment, determining whether the runway is wet or potentially contaminated is the pilot’s responsibility.
Airports like Mumbai , Kochi, Kozhikode where there is heavy rainfall during monsoons and accompanied with tail winds, the runway becomes contaminated. In the absence of runway condition reporting process and the possibility of sudden heavy shower, the crew needs to calculate landing distance performance for contaminated runway with margins more than 15% as currently used. Rubber deposits further increase the risk of poor braking.
From 2020 ICAO will change the definition of damp runway to be considered as wet. As an immediate risk mitigation action, operators must asses the risk as per the SMS program and consider increasing the safety margin for wet runway beyond the current 15%.