Understanding Runway Conditions – Slippery Wet

Mar 29, 2023 | Blog

Runway conditions are a critical aspect of aviation safety. The International Civil Aviation Organization (ICAO) introduced the term Slippery Wet alongside the GRF regulation to help aviators describe a specific type of runway condition. In this article, we will explore what slippery wet means and its significance for pilots, airport operators, and other stakeholders.

Before we dive into the specifics of slippery wet, it is important to understand runway condition codes (RWYCC). RWYCC is a system that classifies runway slipperiness based on various factors such as snow, ice, water, and other contaminants being one factor. The RWYCC’s range from 0 (less than poor) to 6 (dry). For example, a wet runway is assigned nominally a Runway Condition Code of 5. However, if there is a degradation of friction on the runway due to factors such as rubber build-up, the RWYCC is downgraded to 3, which is referred to as slippery wet. This reduction in friction can create difficulties for an aircraft to stop during landing or during aborted take-off.

Slippery wet is a very specific type of condition that is distinct from solely wet. Where wet surfaces might have enough friction for safe operations, this may be compromised when there are additional contaminants, such as rubber buildup under the water film.

To determine if a specific runway third is slippery wet, runway calibration measurements are carried out using Continuous Friction Measurement Equipment (CFME), such as Skiddometer BV11, with a smooth ASTM E1551 tire and a regulated 1.0mm water film in front of the measurement tire. If a 100m section or longer is below the minimum friction level, that specific third of the runway is reported as slippery wet. In some countries, the entire runway is reported as slippery wet in such cases.

It’s essential to issue a Notice to Airmen (NOTAM) when slippery wet runway conditions exist, until the surface is improved, as the condition poses a significant risk. When slippery wet conditions exist and NOTAM has been issued, every time the surface becomes wet due to rain, a Special NOTAM (SNOWTAM) must be published, and the Automatic Terminal Information Service (ATIS) is updated accordingly. When the surface dries up, a dry SNOWTAM is published, and ATIS is updated as it represents a significant improvement.

It’s worth noting that operational measurements are also taken during the summer period. These measurements are performed using CFME with a grooved T-520 tire without a regulated water film during rainfall to check the current situation on the runway. These measurements help determine if the runway’s RWYCC needs to be downgraded due to degraded friction levels, even if it’s currently assigned as wet or slippery wet. Remember that RWYCC downgrade is always possible, excluding dry conditions.

Although there are predetermined limits for friction values during runway calibration, there are no criteria for downgrading the RWYCC if the values are for example lower than the predetermined minimum levels. The downgrade is instead determined operationally when it rains, and the Skiddometer BV11 friction tester has guidelines on which friction levels require a downgrade.

In conclusion, understanding runway conditions is vital for pilots to make safe landings and take-offs. The introduction of the slippery wet term by ICAO helps provide more specific information for pilots about runway conditions. By following strict measurement and reporting protocols, airports can help to gain better safety.

Share this post