Despite statistics showing almost one runway excursion per week worldwide, pilots tend to think that it will never happen to them. In many cases, they are right. Some, though, will be exposed to a situation of an uncontrollable aircraft leading to a runway excursion. There are many reasons that can explain these events. Understanding that there is never one single reason for a runway excursion, but rather a combination of causal factors will help pilots and air traffic controllers work together to avoid them.
First, we need to consider the quality of performance parameters. Some are accurate (aircraft weight, fuel load, runway length), while others are unreliable (braking efficiency, wind, runway friction). All these parameters determine the braking performance of the aircraft and therefore adverse or inaccurate variables will increase the risks of runway excursions.
One particularly important parameter is the surface condition. Indeed, just like roads, a wet, slippery, moist runway will decrease the braking capacity of the aircraft and this, combined with, for example, a short runway, can easily lead the aircraft to drift off it. Wind and vortices are also important. For instance, a wind shift during the approach will make it more difficult for pilots to make a stable approach.
In addition, there are several bad – but usual – practices. The restrictions on the use of reverse thrust for environmental reasons increase the brake temperatures and thus decrease the braking performance. Also, in order to avoid discomfort for passengers, pilots are encouraged to perform soft (and thus possibly long) landings, which increases the chances of an incorrect flare followed by floating. This, combined with adverse parameters, can lead to a runway excursion.
With numerous factors explaining the unfavourable statistics, it is crucial that pilots, air traffic controllers and rulemakers understand them, in order to better mitigate the risks. Pilots need to be correctly informed of the parameters to take the appropriate decisions. Air traffic controllers have to understand the characteristics of a stable approach and allocate the runway accordingly. Finally, rulemakers have to accept that adequate margins are essential to cover for the imperfections of the theoretical system. It is only by working together that we will eventually succeed in decreasing the number of runway excursions.
Based on an article by Capt. Rob van Eekeren: for the full version of this article, click here