Airfield runways must provide adequate skid resistance to ensure the safe braking of aircraft operating on the surface. The degree of skid resistance provided by a pavement is expressed in terms of the surface coefficient of friction (COF), which is defined simply as the ratio of the measured horizontal (braking) to the vertical (loading) forces acting on a test wheel, braked to give a predefined percentage of slip, as the wheel passes over the surface.
There are many variables that affect measured COF values. The first set is related to the pavement surface, the second set to the properties of the test tire itself. Controlling these variables (also known as the conditions of test) in a standard manner is critical to the achievement of consistent and reliable friction test results.
Because of the potential for so many different conditional states to exist at the pavement/tyre interface, there is no such thing as an 'absolute' or 'true' coefficient of friction value. The COF measured at the tyre pavement interface is valid only for the conditions in effect at the time of test.
However, by standardising the conditions of test and paying great attention to compliance with them, reliable and comparable friction measurements can be obtained. Friction readings obtained under the standard test conditions can then be meaningfully compared to national regulatory standards respecting the maintenance of minimum friction levels for runway surfaces.
MAINTENANCE VERSUS OPERATIONAL TESTING
Runway friction measurements made in the summer months are intended solely for use in detecting the deterioration of skid resistance and determining the need for and timing of maintenance action to restore friction to acceptable levels.
However, winter runway friction measurements made under ‘solid state’ surface conditions of snow, ice or compacted snow are made available to pilots as Canadian runway friction index (CRFI) values for the operational purpose of computing aircraft stopping distance.
FRICTION TESTING EQUIPMENT
In Canada, the surface friction tester (SFT) serves as the benchmark friction measuring device for the purpose of measuring and defining standard runway COF levels. The SFT has a fifth friction measuring wheel (see Figure 1), which retracts up into the trunk compartment when not being used for testing. This design provides mobility for highway travel between airport sites. The test wheel is braked by a chain attached to the rear axle which gives the tire a slip in the order of 15%.
RUNWAY FRICTION MEASUREMENT (CONDITIONS OF TEST)
Summer friction testing is conducted under ‘normal wet conditions’ which involves placing a thin film of water on the pavement just prior to measurement. Only enough water is placed to wet the surface without resulting in a flooded state which may cause hydroplaning.
Runway friction testing is performed by making four test runs along the length of the runway – two on each side of centreline at offsets of 3m (in the aircraft main gear wheelpaths). The depth of water placed in front of the test tyre by the SFT’s self-wetting system is 0.5mm thick. The test speed is 65kph (40mph). Other conditions of test are also applicable – it is extremely important that all specified conditions of test be adhered to in order that reliable test results are obtained. Tests take only several minutes and can be scheduled around airport flight operations.
Each friction test gives a trace of the COF profile for the runway. From the profile, areas of low friction which may require maintenance can be identified. The friction profile shown in Figure 2 indicates low friction values in the runway touchdown zones due to the build-up of rubber deposits caused by landing aircraft. From the traces, average friction values are calculated for each 100m section of runway and for the entire runway.
Runway friction measurements taken during the summer are used to identify runways that need to improve their surface friction characteristics. The build-up of rubber deposits in runway touchdown zones (see Figure 3) can obliterate much of the pavement surface texture. Such surfaces can become very slippery when wet.
Friction test results are compared to the friction maintenance standards shown in Table 1. These standards indicate when corrective action, such as the removal of rubber deposits, needs to be taken.
For hard-surfaced runways serving turbojet aircraft, it is a regulatory requirement that Canadian airport operating authorities measure the friction characteristics of their runway surfaces with a continuous friction-measuring device using self-wetting features.