The tragic overrun of Southwest Airlines Flight 1248 at Chicago Midway in December 2005 sparked a global effort to understand a critical discrepancy: why an average runway friction reading of approximately 0.40µ would result in an actual aircraft braking coefficient of only 0.08µ.

In response, initiatives such as FAA’s Takeoff and Landing Performance Assessment (TALPA) and ICAO’s Global Reporting Format (GRF) were developed to standardize runway condition reporting limited to contaminant coverage, type and depth as initial decision-supporting tools.

These frameworks helped inform and support operational decisions while encouraging the aviation industry to explore the effects of runway contaminants on takeoff performance and modern antiskid braking systems (ASBS) during rejected takeoffs and landings.

Steve McKeown  is is celebrating 50 years helping airports enhance airfield operations and improve safety, efficiency and sustainability. Since founding the Team Eagle companies in 2000, he has collaborated with the industry’s largest stakeholders and has deployed equipment and services in more than 100 countries. The companies’ products include several systems to assess and improve runway conditions.

Key findings emphasize the importance of drag on takeoff, and reduced braking on landings and rejected takeoffs. For successful takeoffs, contaminants primarily increase drag, which can impact acceleration but is generally manageable with appropriate runway length and thrust. The bigger concern arises during landings and rejected takeoffs. Contaminants such as snow, slush or water significantly reduce or even eliminate tire adhesion and cohesion with runway pavement. This not only lowers available grip but also interferes with the efficiency of modern ASBS.

The Role of Antiskid Braking Systems

Modern ASBS are designed to limit braking forces to prevent tire damage rather than optimize braking, and enhance directional control by, among other things, preventing wheel lockup and asymmetric braking forces. However, on slippery contaminated runways, these systems face critical limitations:

• Lower Brake Pressure Limits: ASBS reduce brake pressure when slip or wheel deceleration thresholds are reached. On slippery surfaces, these thresholds are reached more quickly and at lower levels of braking forces.
• Extended Recovery Time: Once brake pressure is reduced, it takes longer for the system to reapply braking force because it waits for the wheel speed to more slowly recover on the slippery surface to theoretical threshold levels.
• Reduced Braking Time: This cycle means the system spends more time “recovering” and less time actively braking, resulting in longer stopping distances.

Why “Friction” Becomes Fiction

Friction is traditionally defined as the force resisting the sliding of one surface over another. On a clean, dry runway, this concept works well. But when contaminants like snow, sand or slush create a “third surface,” friction transforms into complex tribological forces. These forces—combined with the dynamic, modulated braking of ASBS—make traditional notions of friction irrelevant.

Thus, on contaminated runways, braking performance is no longer limited by “friction” but by the ability of ASBS to manage the degraded surface conditions. The mismatch between an average friction value of 0.40µ and an aircraft braking coefficient of 0.08µ underscores this reality.

As a result, modern fully modulating ASBS are the “elephant in the room” for understanding runway braking performance. As runway slipperiness increases, ASBS efficiency declines, leading to longer stopping distances. Recognizing this limitation is critical to improving safety on contaminated runways.

Taking Action

Following the fateful overrun at MDW, industry leaders entered into Cooperative Research and Development Agreements with the FAA to investigate the effects of runway contaminants on modern aircraft braking systems.

I’m pleased to report significant achievements regarding two primary goals outlined in the agreements. The Braking Availability Tester (BAT) is an aircraft antiskid braking control system and landing gear mounted into a ground vehicle designed to emulate the braking dynamics of a modern aircraft with fully modulating ASBS. It was evaluated by several aviation stakeholders and met the objective of accurately reconciling objective, empirical BAT ground vehicle measurements with modern fully modulating aircraft in-situ braking coefficients.

The Aircraft Deceleration Early Warning (ADEW) system leverages “aircraft as a sensor” technology to provide real-time braking availability insights. While traditional systems focus on runway condition reporting, ADEW highlights both the advantages and limiting challenges of using onboard aircraft data to monitor aircraft braking in real time (but, unfortunately, cannot be used to actually report runway conditions because of the limited portion of the runway used during normal landings).

Cooperative Research and Development Agreements also led to development of the In-Cockpit Early Warning System and Adaptive Braking and Directional Control System.

Together, these innovations represent significant progress in understanding and addressing the challenges posed by contaminated runways and their impact on modern aircraft braking systems.