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applicable in all conditions. If the surface is affected by snow
or 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.
Estimated
Measured braking
coeficient action Code
6.3 The measurement of the friction coefficierit provides 0.40 and above Good 5
the best basis for determining surface friction conditions. The 0.39 to 0.36 Medium to good 4
value of surface friction should be the maximum value which 0.35 to 0.30 Medium 3
occurs when a wheel is slipping but still rolling. Various 0.29 to 0.26 Medium to poor 2
friction measuring devices may be used. As there is an oper- 0.25 and below Poor 1
ational need for uniformity in the method of assessing and
reporting runway friction conditions, the measurements should 6.7 It has been found necessary to provide surface friction
preferably be made with equipment which provides continuous information for each third of a runway. The thirds are called
ATT A-5
Annex 14 -Aerodromes Volume l
A, B and C. For the purpose of reporting infomation to c) determine the effect on friction when drainage characaeronautical
service units, section A is always the section teristics are poor (Chapter 10, 10.2.6); and
associated with the lower runway designation number. When
giving landing infomation to a pilot before landing, the d) determine the friction of paved runways that become
sections are however refened to as first, second or third part slippery under unusual conditions (Chapter 2, 2.9.8).
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 fiction 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 Aiqon' Services Manual, Part 2).
Note.- Where applicable, figures for stopway friction
value should also be made available on request.
6.8 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 snow- and ice-covered runways.
A decelerometer (e.g. Tapley Meter or Brakemeter - Dynometer)
may be used on certain surface conditions, e.g.
compacted snow, ice and very thin layers of dry snow. Other
friction measuring devices can be used, provided they have
been correlated with at least one of the types mentioned above.
A decelemmeter should not be used in loose snow or slush, as
it can give misleading friction values. Other friction measuring
devices can also give misleading friction values under certain
combinations of contaminants and airlpavement temperature.
6.9 The Airport Services Manual, Part 2 provides
guidance on the unifom use of test equipment to achieve
compatible test results and other information on removal of
surface contamination and improvement of friction conditions.
7. Determination of friction characteristics
of wet paved runways
7.1 The friction of a wet paved runway should be
measured to:
a) verify the friction characteristics of new or resurfaced
paved runways when wet (Chapter 3,3.1.24);
b) assess periodically the slipperiness of paved runways
when wet (Chapter 10, 10.2.3);
7.2 Runways should be evaluated when first conshvcted
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
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