Definition
Braking action refers to how well an aircraft can slow down when the brakes are applied. It is also about how easily the pilot can control the direction of the aircraft during this process.
Description
Although there isn’t an official definition from the International Civil Aviation Organization (ICAO), the term “braking action” is commonly used in aviation. It describes two main things: the ability of an aircraft to stop using its wheel brakes, which pilots report based on their experience, and the surface friction of the runway, which can be measured by special devices.
The SNOWTAM format mentions “estimated surface friction,” which is a way for ground staff to assess how slippery the surface is, based on all available information. However, it’s important to note that “braking action” and “surface friction” are not the same. The latter only serves as a rough guide to help understand the former.
The relationship between braking action and surface friction can be complicated. It depends on various factors, such as the type of friction measuring equipment, the condition of the runway surface, and who is taking the measurements. The following table was created from data collected on compacted snow and ice and should not be seen as absolute values for all situations:
| Measured Coefficient | Estimated Surface Friction | Code |
|---|---|---|
| 0.40 and above | Good | 5 |
| 0.39 to 0.36 | Medium to good | 4 |
| 0.35 to 0.30 | Medium | 3 |
| 0.29 to 0.26 | Medium to poor | 2 |
| 0.25 and below | Poor | 1 |
It’s crucial to understand that if the runway is covered with snow or ice, and the estimated surface friction is labeled as “good,” pilots should not expect conditions to be as ideal as on a clean, dry runway. In fact, the friction on a clean runway may exceed what’s necessary for safe landing. The term “good” is relative, suggesting that pilots should not encounter significant issues with braking or directional control, particularly during landing.
The values in the “Measured Coefficient” column are just guidelines. Each airport can create its own specific table based on the friction measuring device they use and the standards set by the governing body. The PI values will differ depending on both the device and the surface being measured, as well as the speed at which the measurements are taken. Generally, friction readings should only be part of a complete assessment of runway conditions.
A key difference exists between decelerometer devices and others; when using a decelerometer, the operator plays a vital role in the measurement. Not only do they conduct the measurement, but they also feel how the vehicle behaves during the process, which adds valuable insight into the overall assessment.
To provide a clearer understanding of runway conditions, surface information, including estimated surface friction, must be provided for each third of the runway, labeled A, B, and C. For reporting purposes, section A corresponds to the lower runway designation number. However, when informing pilots before landing, these sections are referred to as the first, second, or third part of the runway. The first part always means the initial third of the runway from the direction of landing (the touchdown zone).
When giving pilots information about braking action or surface friction, the following standard phrasing is used:
ESTIMATED SURFACE FRICTION (descriptive term).
Example: ESTIMATED SURFACE FRICTION MEDIUM TO GOOD BRAKING ACTION REPORTED BY (aircraft type) AT (time).
For instance: BRAKING ACTION REPORTED BY A320 AT 1000 MEDIUM.
This standard phrasing emphasizes that the surface friction is an estimate, while the braking action is based on pilot reports.
