b'74 INDUSTRY INSIDERContaminated Runways:When Friction Becomes Fiction The tragic overrun of Southwestreached. On slippery surfaces, these Airlines Flight 1248 at Chicagothresholds are reached more quicklySteve McKeownMidway in December 2005and at lower levels of braking forces.is is celebrating 50 sparked a global effort to understandyears helping airports Extended Recovery Time: Onceenhance airfield a critical discrepancy: why an averagebrake pressure is reduced, it takesoperations and improve runway friction reading of approximatelylonger for the system to reapplysafety, efficiency 0.40 would result in an actual aircraftbraking force because it waits for theand sustainability. Since founding the braking coefficient of only 0.08.wheel speed to more slowly recoverTeam Eagle companies in 2000, he In response, initiatives such as FAAson the slippery surface to theoreticalhas collaborated with the industrys Takeoff and Landing Performancethreshold levels.largest stakeholders and has deployed Assessment (TALPA) and ICAOs GlobalReduced Braking Time: This cycleequipment and services in more than 100 countries. The companies products Reporting Format (GRF) were developedmeans the system spends more timeinclude several systems to assess and to standardize runway condition reportingrecovering and less time activelyimprove runway conditions. limited to contaminant coverage, type andbraking, resulting in longer stopping depth as initial decision-supporting tools.distances. Im pleased to report significant These frameworks helped inform and support operational decisionsWhy Friction Becomes Fictionachievements regarding two primary Friction is traditionally defined as thegoals outlined in the agreements. The while encouraging the aviation industryBraking Availability Tester (BAT) is an to explore the effects of runwayforce resisting the sliding of one surfaceaircraft antiskid braking control system and contaminants on takeoff performance andover another. On a clean, dry runway,landing gear mounted into a ground vehicle modern antiskid braking systems (ASBS)this concept works well. But whendesigned to emulate the braking dynamics during rejected takeoffs and landings.contaminants like snow, sand or slushof a modern aircraft with fully modulating Key findings emphasize the importancecreate a third surface, friction transformsASBS. It was evaluated by several aviation of drag on takeoff, and reduced brakinginto complex tribological forces. Thesestakeholders and met the objective of on landings and rejected takeoffs. Forforcescombined with the dynamic,accurately reconciling objective, empirical successful takeoffs, contaminantsmodulated braking of ASBSmakeBAT ground vehicle measurements with primarily increase drag, which cantraditional notions of friction irrelevant. modern fully modulating aircraft in-situ impact acceleration but is generally Thus, on contaminated runways,braking coefficients. manageable with appropriate runwaybraking performance is no longer limitedThe Aircraft Deceleration Early Warning length and thrust. The bigger concernby friction but by the ability of ASBS to(ADEW) system leverages aircraft as a arises during landings and rejectedmanage the degraded surface conditions.sensor technology to provide real-time takeoffs. Contaminants such as snow,The mismatch between an averagebraking availability insights. While traditional slush or water significantly reduce or evenfriction value of 0.40 and an aircraftsystems focus on runway condition eliminate tire adhesion and cohesion withbraking coefficient of 0.08 underscoresreporting, ADEW highlights both the runway pavement. This not only lowersthis reality.advantages and limiting challenges of using available grip but also interferes with theAs a result, modern fully modulatingonboard aircraft data to monitor aircraft efficiency of modern ASBS.ASBS are the elephant in the roombraking in real time (but, unfortunately, for understanding runway brakingcannot be used to actually report runway The Role of Antiskid Brakingperformance. As runway slipperinessconditions because of the limited portion of Systemsincreases, ASBS efficiency declines,the runway used during normal landings). Modern ASBS are designed to limitleading to longer stopping distances.Cooperative Research and braking forces to prevent tire damageRecognizing this limitation is critical toDevelopment Agreements also led to rather than optimize braking, and enhanceimproving safety on contaminated runways.development of the In-Cockpit Early directional control by, among other things,Warning System and Adaptive Braking preventing wheel lockup and asymmetricTaking Actionbraking forces. However, on slipperyFollowing the fateful overrun at MDW,and Directional Control System.contaminated runways, these systemsindustry leaders entered into CooperativeTogether, these innovations represent face critical limitations:Research and Development Agreementssignificant progress in understandingLower Brake Pressure Limits: ASBSwith the FAA to investigate the effects ofand addressing the challenges posed by reduce brake pressure when slip orrunway contaminants on modern aircraftcontaminated runways and their impact on wheel deceleration thresholds arebraking systems.modern aircraft braking systems. March | April 2025AirportImprovement.com'