Just How “Slippery When Wet?”

John O’Callaghan

If you’ve developed a personal relationship with the National Transportation Safety Board (NTSB), chances are you’ve had a bad day. Thankfully, for most in the aviation industry, the work of the NTSB will remain something they read about and benefit from, but have no occasion to observe first-hand. Nonetheless, many do become personally acquainted with the agency; airport operators often get their chance as the result of a runway overrun. 

Poor runway friction can contribute to runway overruns, and while it is well known that friction will deteriorate with the accumulation of frozen contaminants (ice, snow, slush), one might be surprised by how much more slippery a runway can become even if it is merely wet. NTSB investigations of several wet-runway overruns indicate that the runway friction in each case was significantly worse than that predicted by industry models. The investigations also indicate that Continuous Friction Measuring Equipment (CFME) devices can help identify slippery-when-wet runways, and even be used to predict aircraft stopping distances on wet runways.

John O’Callaghan   

John O’Callaghan serves as the NTSB’s National Resource Specialist for Aircraft Performance. He came to the agency in 1997 after six years as a stability and control engineer with a major aircraft manufacturer. 

The 2015 FAA Safety Alert for Operators (SAFO) 15009 summarizes the wet-runway friction problem, noting that overrun events “occurred on both grooved and ungrooved or non-Porous Friction Course overlay (PFC) runways,” and that “applying a 15% safety margin to wet runway time-of-arrival advisory data, as recommended by SAFO 06012, may be inadequate in certain wet runway conditions.” The SAFO states that “the root cause of the wet runway stopping performance shortfall is not fully understood at this time; however issues that appear to be contributors are runway conditions such as texture (polished or rubber contaminated surfaces), drainage, puddling in wheel tracks and active precipitation. Analysis of this data indicates that 30% to 40% of additional stopping distance may be required in certain cases where the runway is very wet, but not flooded.” The SAFO encourages aircraft operators to “consider additional conservatism in their time-of-arrival assessment,” such as “assuming a braking action of medium or fair when computing time-of-arrival landing performance or increasing the factor applied to the wet runway time-of-arrival landing performance data.”

SAFO 16009, issued in October 2016, implemented the use of the Runway Condition Assessment Matrix (RCAM). The RCAM is “used by airport operators to perform assessments of runway conditions and by pilots to interpret reported runway conditions…the RCAM replaces subjective judgments of runway surface conditions with objective assessments tied directly to contaminant type and depth categories.” The RCAM provides two codes for classifying wet (but not flooded) runways: code 5, corresponding to “Wet (Includes damp and 1/8-inch depth or less of water)” and equivalent to a pilot reported braking action of “good,” and code 3, corresponding to “Slippery When Wet (wet runway),” equivalent to a pilot report of “medium.” There is no mechanism in the RCAM for a wet runway to be classified as code 4, equivalent to a pilot report of “good to medium” braking action, even though wet runways may in fact perform in this range.

According to AC 150/5200-28F, to be classified as RCAM code 3 “Slippery When Wet,” a runway must be surveyed with CFME and “[fail] to meet the minimum friction level classification specified in AC 150/5320-12.” Consequently, if airport personnel do not have access to CFME, they will not be able to determine (except by pilot reports) that their runways should be classified as RCAM code 3 instead of code 5 when they are wet. By default, RCAM code 5 will be reported in wet conditions, even though the runways may perform below that level.

 The NTSB’s analyses of wet-runway overrun events and wet-runway friction research indicate that CFME devices can also be used to determine airplane stopping distances on wet runways more reliably than other friction models (for the technical details, see https://dms.ntsb.gov/public/52500-52999/52556/618065.pdf). Monitoring the friction levels of runways with CFME may help airport operators issue more accurate RCAM reports, and perhaps forestall a visit from the NTSB. 

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