Accounting for the Progressive Decrease in Runway Friction Associated with Increasing Rainfall Intensity

​​We are providing the following information to urge the Federal Aviation Administration (FAA) to take action on the safety recommendations in this report. They are derived from our investigations of 11 runway overrun accidents and incidents from a 15-year period, 2008–2022, that occurred after the airplanes involved landed on wet runways. The National Transportation Safety Board (NTSB) is issuing three safety recommendations to the FAA.

​​The wheel braking friction coefficient corresponding to a wet runway can be substantially less than that specified by runway condition assessment matrix (RCAM) runway condition code 5 at Title 14 Code of Federal Regulations 25.109(c) due to limitations in the factors considered in the regulation, and therefore, wet-runway landing distances calculated using this wheel braking friction coefficient can underestimate the actual landing distances required and increase the risk of a runway overrun.

The runway condition assessment matrix would better represent the behavior of the maximum wheel braking friction coefficient on wet runways if progressively higher rainfall intensities corresponded to progressively lower runway condition codes.

The rainfall intensity descriptors currently used in aviation weather reports do not identify the highest rainfall intensities that are possible at an airport, and therefore, these descriptors limit the ability of flight crews to accurately assess the runway condition and the required landing distance when such rainfall intensities are present.

A broader range of available rainfall intensity descriptors would help to more accurately associate progressively higher rainfall intensities with progressively lower runway condition codes in the runway condition assessment matrix.​

​​​As a result of this investigation, we made the following new safety recommendations.

To the Federal Aviation Administration:

• Update the runway condition assessment matrix (RCAM) runway condition codes assigned to wet runways to account for the progressive decrease in the wheel braking friction coefficient associated with increasing rainfall intensity. (A-26-61)
• Add additional rainfall intensity descriptors to be used in aviation weather reports to identify rainfall intensities that can substantially exceed the current heavy rain threshold of 0.3 inches per hour. (A-26-62)
• Once the rainfall intensity descriptors used in aviation weather reports are updated as recommended in Safety Recommendation A-26-62, incorporate these descriptors in the runway condition assessment matrix (RCAM). (A-26-63)

The NTSB notes that the update to the RCAM proposed in Safety Recommendation A-26-61 should account for the difference between smooth and grooved runways, and one possible method of doing so is proposed on page 11 of the report.