DETERMINATION OF FRICTION CHARACTERISTICS OF

WET PAVED RUNWAYS

1 The friction of a wet paved runway should be measured to:

(a) Verify the friction characteristics of new or resurfaced paved runways when wet (Subpart F, 139.309(a)(23));

(b) Assess periodically the slipperiness of paved runways when wet (Subpart L, 139.341(e));

(c) Determine the effect on friction when drainage characteristics are poor ( Subpart L, 139.341(h)); and

(d) Determine the friction of paved runways that become slippery under unusual conditions (Subpart E, 139.307(i)(8)).

2 Runways should be evaluated when first constructed or after resurfacing to determine the wet runway surface friction characteristics. Although it is recognized that friction reduces with use, this value will represent the friction of the relatively long central portion of the runway that is uncontaminated by rubber deposits from aircraft operations and is therefore of operational value. Evaluation tests should be made on clean surfaces. If it is not possible to clean a surface before testing, then for purposes of preparing an initial report a test could be made on a portion of clean surface in the central part of the runway.

3 Friction tests of existing surface conditions should be taken periodically in order to identify runways with low friction when wet. Table A-1 defines what minimum friction level it considers acceptable to ECAA before a runway is classified as slippery when wet. When the friction of a runway is found to be below this reported value, then such information should be promulgated by NOTAM. Table A-1 provides also a maintenance planning level, below which, appropriate corrective maintenance action should be initiated to improve the friction. However, when the friction characteristics for either the entire runway or a portion thereof are below the minimum friction level, corrective maintenance action must be taken without delay. Friction measurements should be taken at intervals that will ensure identification of runways in need of maintenance or special surface treatment before the condition becomes serious. The time interval between measurements will depend on factors such as: aircraft type and frequency of usage, climatic conditions, pavement type, and pavement service and maintenance requirements.

4 For uniformity and to permit comparison with other runways, friction tests of existing, new or resurfaced runways should be made with a continuous friction measuring device provided with a smooth tread tire. The device should have a capability of using self-wetting features to enable measurements of the friction characteristics of the surface to be made at a water depth of at least 1 mm.

5 When it is suspected that the friction characteristics of a runway may be reduced because of poor drainage, owing to inadequate slopes or depressions, then an additional test should be made, but this time under natural conditions representative of a local rain. This test differs from the previous one in that water depths in the poorly cleared areas are normally greater in a local rain condition. The test results are thus more apt to identify problem areas having low friction values that could induce aquaplaning than the previous test. If circumstances do not permit tests to be conducted during natural conditions representative of a rain, then this condition may be simulated.

6 Even when the friction has been found to be above the level set (see Table A-1) to define a slippery runway, it may be known that under unusual conditions, such as after a long dry period, the runway may have become slippery. When such a condition is known to exist, then a friction measurement should be made as soon as it is suspected that the runway may have become slippery.

7 When the results of any of the measurements identified in 7.3 through 7.6 indicate that only a particular portion of a runway surface is slippery, then action to promulgate this information and, if appropriate, take corrective action is equally important.

8 When conducting friction tests on wet runways, it is important to note that, unlike compacted ice conditions, in which there is very limited variation of the friction coefficient with speed, a wet runway produces a drop in friction with an increase in speed. However, as the speed increases, the rate at which the friction is reduced becomes less. Among the factors affecting the friction coefficient between the tire and the runway surface, texture is particularly important. If the runway has a good macro-texture allowing the water to escape beneath the tire, then the friction value will be less affected by speed. Conversely, a low macro-texture surface will produce a larger drop in friction with increase in speed. Accordingly, when testing runways to determine their friction characteristics and whether maintenance action is necessary to improve it, a speed high enough to reveal these friction/speed variations should be used.

9 ECAR Part 139 specifies two friction levels as follows:

(a) A maintenance friction level below which corrective maintenance action should be initiated; and

(b) A minimum friction level below which information that a runway may be slippery when wet should be made available.

Table A-1 provides guidance on establishing the design objective for new or resurfaced runway surfaces and maintenance planning and minimum friction levels for runway surfaces in use.

10 The friction values given above are absolute values and are intended to be applied without any tolerance. The two friction measuring tires mounted on the Mumeter were smooth tread and had a special rubber formulation, i.e. Type A. The tires were tested at a 15 degree included angle of alignment along the longitudinal axis of the trailer. The single friction measuring tires mounted on the Skiddometer, Surface Friction Tester, Runway Friction Tester and TATRA were smooth tread and used the same rubber formulation, i.e. Type B. The GRIPTESTER was tested with a single smooth tread tire having the same rubber formulation as Type B but the size was smaller, i.e. Type C. The specifications of these tires (i.e. Types A, B and C) are contained in EAC 139-19. Friction measuring devices using rubber formulation, tire tread/groove patterns, water depth, tire pressures, or test speeds different from those used in the programme described above, cannot be directly equated with the friction values given in the table. The values in columns (5), (6) and (7) are averaged values representative of the runway or significant portion thereof. It is considered desirable to test the friction characteristics of a paved runway at more than one speed.

11 Other friction measuring devices can be used, provided they have been correlated with at least one test equipment mentioned above. EAC 139-19 provides guidance on the methodology for determining the friction values corresponding to the design objective, maintenance planning level and minimum friction level for a friction tester not identified in the above table.