Slippery When Wet: The Case for More Conservative Wet Runway Braking Coefficient Models

摘要

Over the past eight years, the NTSB has led or participated in the investigation of a number of runway overrun accidents and incidents that occurred after the airplanes involved landed on wet runways. An analysis of the airplane stopping performance during these events indicates that the wheel braking friction coefficient (μ_B) achieved during the landing roll was significantly less than the μ_B predicted by industry-accepted models, and less than the μ_B assumed in the wet-runway landing distance advisory data provided in the manufacturers' Airplane Flight Manuals. In recognition of this problem, the Federal Aviation Administration has issued a safety alert to operators warning them that the advisory data for wet runway landings may not provide a safe stopping margin under all conditions, and tasked an Aviation Rulemaking Advisory Committee with reviewing the wet runway stopping performance requirements contained in the 14 Code of Federal Regulations (CFR) Part 25 airworthiness standards and guidance (the μ_B that can be assumed on a wet runway during an aborted takeoff is specified by 14 CFR 25.109, and the 25.109 model has been proposed for computing landing distances on a wet runway). This paper presents six wet runway landing overrun events (all involving turbojet airplanes) and compares the μ_B achieved by the airplane in each event with the μ_B predicted by several models and inherent in advisory data. The results indicate that current models and advisory data can significantly overestimate the μ_B achievable on wet runways (and therefore underestimate the required runway length for landing on these runways). Consequently, these events underscore the need for more conservative models of μ_B, and more conservative use of existing advisory data and models in the meantime.