FAA’s wet-runway guidance underestimates landing distances: NTSB

Safety agency calls for FAA to update a matrix that classifies runway conditions and resulting braking performance.

The National Transportation Safety Board (NTSB) is urging the Federal Aviation Administration to revamp how it classifies braking action on wet runways, a move re­sponding to a string of runway overruns.

­The NTSB also recommends that the regulator modify language used in weather reports, to better notify pilots about heavy precipitation.

The safety investigation agency disclosed the recommendations on 26 May, saying the rate at which pilots are able to slow aircraft by braking on wet runways can be substantially less than specified by the FAA’s in its “Runway Condition Assessment Matrix” (RCAM).

That matrix defines degrees of runway contamination and related braking performance, denoted by “wheel breaking coefficient”, a figure calculated by dividing a tire’s horizontal braking force by the vertical force on the tire.

The matrix, for instance, defines Code 3 runways as snow-covered and with “medium” braking action, while Code 1 runways have ice and “poor” braking action. Pilots, dispatchers and airport staff communicate runway conditions using those terms.

The NTSB now says “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 FAA takes NTSB recommendations seriously and will carefully consider those issued today,” the regulator says.

The NTSB identified the problem after developing a method of calculating actual wheel braking friction following a runway overrun in Owatonna, Minnesota in 2008. It then applied the method to 10 other incidents, including those involving business jets and commercial types like Boeing 737s.

In all but one of the 11 incidents, “the wheel braking friction coefficient achieved during the landing was substantially less” than the RCAM-coefficient, the NTSB says. “Consequently, the actual landing distance required was longer than that calculated.”

Operator errors also contributed to the accidents, the NTSB notes.

Additionally, the NTSB says the RCAM system inadequately accounts for how water depth degrades braking, saying some incidents evaluated involved aircraft that never should have attended to land. Those include a Miami Air Boeing 737-800 that plunged into the St Johns River after the pilots suffered “extreme loss of braking action” when landing amid heavy rain at Jacksonville Naval Air Station on 3 May 2019.

The NTSB’s response to the incidents includes a call for the FAA to update its runway condition matrix to account for how rainfall intensity affects braking.

It also urges the regulator to adopt additional descriptors of rainfall intensity, such terms like “heavy +” or “heavy ++”.

Under its current system, the FAA classifies rain as “light,” “moderate” and “heavy”.

The NTSB made its recommendations on 12 May but has only now disclosed them.