DETERMINING AND EXPRESSING THE FRICTION CHARACTERISTICS OF ICE-COVERED PAVED SURFACES

1 There is an operational need for reliable and uniform information concerning the friction characteristics of ice- covered runways. Accurate and reliable indications of surface friction characteristics can be obtained by friction measuring devices; however, further experience is required to correlate the results obtained by such equipment with aircraft performance, owing to the many variables involved, such as: aircraft mass, speed, braking mechanism, tire and under-carriage characteristics.

2 The friction coefficient should be measured if a runway is covered wholly or partly by ice and repeated as conditions change. Friction measurements and/or braking action assessments on surfaces other than runways should be made when an unsatisfactory friction condition can be expected on such surfaces.

3 The measurement of the friction coefficient provides the best basis for determining surface friction conditions. The value of surface friction should be the maximum value which occurs when a wheel is slipping but still rolling. Various friction measuring devices may be used. As there is an operational need for uniformity in the method of assessing and reporting runway friction conditions, the measurements should preferably be made with equipment which provides continuous measuring of the maximum friction along the entire runway. Measuring techniques and information on limitations of the various friction measuring devices and precautions to be observed are given in EAC 139-19.

4 A chart, based on results of tests conducted on selected ice- covered surfaces, showing the correlation between certain friction measuring devices on ice- or covered surfaces is presented in EAC 139-19.

5 The friction conditions of a runway should be expressed as “braking action information” in terms of the measured friction coefficient p or estimated braking action. Specific numerical p values are necessarily related to the design and construction of each friction measuring device as well as to the surface being measured and the speed employed.

6 The table below with associated descriptive terms was developed from friction data collected only in compacted ice and should not therefore be taken to be absolute values applicable in all conditions. If the surface is affected by ice and the braking action is reported as “good”, pilots should not expect to find conditions as good as on a clean dry runway (where the available friction may well be greater than that needed in any case). The value “good” is a comparative value and is intended to mean that aeroplanes should not experience directional control or braking difficulties, especially when landing.

Measured coefficient	Estimated braking action	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

7 It has been found necessary to provide surface friction information for each third of a runway. The thirds are called A, B and C. For the purpose of reporting information to aeronautical service units, section A is always the section associated with the lower runway designation number. When giving landing information to a pilot before landing, the sections are however referred to as first, second or third part of the runway. The first part always means the first third of the runway as seen in the direction of landing. Friction measurements are made along two lines parallel to the runway, i.e. along a line on each side of the centre line approximately 3 m or that distance from the centre line at which most operations take place. The objective of the tests is to determine the mean friction value for sections A, B and C. In cases where a continuous friction measuring device is used, the mean values are obtained from the friction values recorded for each section. The distance between each test point should be approximately 10 per cent of the usable length of the runway. If it is decided that a single test line on one side of the runway centre line gives adequate coverage of the runway, then it follows that each third of the runway should have three tests carried out on it. Test results and calculated mean friction values are entered in a special form (see EAC 139-19).

Note: Where applicable, figures for stopway friction value should also be made available on request.

8 A continuous friction measuring device (e.g. Skiddometer, Surface Friction Tester, Mu-meter, Runway Friction Tester or Grip Tester), can be used for measuring the friction values for compacted ice-covered runways. A decelerometer (e.g. Tapley Meter or Brakemeter - Dynometer) may be used on certain surface conditions, e.g. compacted ice. Other friction measuring devices can be used, provided they have been correlated with at least one of the types mentioned above. A decelerometer should not be used in slush, as it can give misleading friction values. Other friction measuring devices can also give misleading friction values under certain combinations of contaminants and air/pavement temperature.

9 EAC 139-19 provides guidance on the uniform use of test equipment to achieve compatible test results and other information on removal of surface contamination and improvement of friction conditions.