Why measure friction
There are 2 major reasons for measuring friction. The first reason is of course for the safety and the second is a financial reason.
The friction on a runway can be affected by a number of reasons.
- The most common reason is snow or ice.
- Rain in combination with rubber build-up or pollution (oily products in the air) can have a very serious impact on the runway condition.
- Rain in combination with a worn down runway structure.
- Sand or similar on the runway could also mean that your friction is lower than expected.
The most important reason for measuring friction is to make sure that the aircraft have enough runway to make the aircraft come to a complete stop before the runway ends.
Take-off, “point of no return”
Friction is however also an important parameter for take-off. When the aircraft accelerates and comes to a certain position called “point of no return” on the runway it has to have a certain speed to assure takeoff further off on the runway. If the aircraft hasn’t reached this speed, it has to abort the take-off.
The “point of no return” can vary depending on the aircraft, weight, wind etc. but the most important factor is the friction. If the friction is good, the point of no return can be moved forward i.e. the aircraft needs less runway to come to a complete stop. This means that the carrier can take more payload, which is very beneficial for the airlines, without jeopardizing the safety issue.
Improving the friction
The friction can in most cases be improved by ploughing (snow), brushing, by blowing air or by chemical treatment. All lose objects such as snow, sand or similar will be removed and the friction will be restored to a satisfactory level.
However in many cases you have rubber built up at the runway ends. In rainy conditions this can be very hazardous. It is a very costly procedure to remove this rubber. Runway has to also be closed during this work. Therefore it’s obvious that you need to know when rubber removal is absolutely necessary to avoid unnecessary costs. To determine the friction in wet condition you have to simulate rain in a strictly controlled way. The agreed procedure states that water should be sprayed in front of the measuring wheel with a thickness of 1 mm. This procedure is called Runway Calibration.