NTSB Shines Spotlight on Wet Runway Safety

Three new NTSB safety recommendations call for the FAA to make changes to how runway conditions are assessed during heavy rainfall, warning that pilots face a greater risk for overruns under the current assessment system.

These recommendations follow NTSB investigations of 11 runway overrun accidents and incidents from 2008 through 2022 that occurred after landings on wet runways. In nine of these overruns, a shortfall in the measure of the runway’s slipperiness—the wheel braking friction coefficient—most likely resulted from moderate to heavy rainfall intensities and the associated increased water depths on the runways. Low runway friction was cited as a causal or contributing factor in eight of the 11 overruns, the report says.

The NTSB recommended that the FAA update its runway condition assessment matrix (RCAM) to account for the progressive decrease in the wheel braking friction coefficient associated with increasing rainfall intensity. Pilots and dispatchers rely on the matrix to determine how much runway is needed to stop after landing on a wet surface.

Also among the recommendations is that the FAA add rainfall intensity descriptors for aviation weather reports “to identify rainfall intensities that can substantially exceed the current heavy rain threshold of 0.3 inches per hour.” In six of the overruns analyzed, rainfall measured between 1.3 and 20 times the rate that triggers a heavy rain designation. The NTSB’s third recommendation calls for the FAA to incorporate the new rainfall descriptors into the RCAM once they are established.

Aviation weather reports top out at “heavy rain” as a descriptor, leaving pilots without terminology to distinguish between rainfall that barely crosses the threshold and rainfall that may be 20 times that rate. The report suggested possible new descriptors, such as “heavy +” or “heavy ++,” allowing the RCAM to assign more nuanced runway condition codes as rainfall intensifies.

In one of the investigations referenced in the report, the 2019 runway overrun of a Boeing 737 in Jacksonville, Florida, the NTSB determined the probable cause was, in part, “an extreme loss of braking friction due to heavy rain and the water depth on the ungrooved runway, which resulted in viscous hydroplaning.” At the time, rainfall was measuring two to eight times the rate defined as “heavy” rain.

Another example involved a 2016 Embraer Phenom 300 landing overrun in Sugar Land, Texas, where rainfall was recorded at 4.2 to 6.0 inches per hour, between 14 and 20 times the rate that defines heavy rain.

In extreme rainfall conditions, braking capability can deteriorate so severely that landing should not be attempted, the NTSB concluded.