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requirements introduced in FAR 25 Amendment 92 and JAR 25 Change 15.
Boeing and Airbus do currently not support correlation between friction testing devices and aircraft
performance. Nevertheless a lot of research has been done for decades and is still ongoing at the
moment. Examples are the Joint FAA/NASA Runway Friction Program and the Joint Winter Runway
Friction Measurement Program. A literature survey by NLR on behalf of CROW concludes that
correlation between friction measurement and aircraft stopping performance remains difficult,
especially on wet surfaces.
The ICAO Airport Services Manual provides a method in Appendix 1, based on NASA Combined
Viscous/Dynamic Hydroplaning Theory relating non-dimensional tire-to-ground friction ratios (μ-
actual/μ-ultimate) for both friction testing device and aircraft tire. An aircraft effective braking
coefficient is derived from the resulting aircraft tire-to-ground friction value. The μ-ultimate value
must be derived from low-speed testing on a dry surface.
ESDU Data Item 99015 provides a statistical method for relating measured friction values on wet
runways and aircraft braking performance. This method also assumes a groundspeed friction
relation where the zero speed value on a wet runway approximates the dry friction value.
CROW Report 03-10 and the Airport Services Manual Appendix 1 contain dry friction values in the
form of μ-ultimate or μ-datum values in the range from 0.95 to 1.15. The Airport Services Manual
states an ultimate friction value for the Saab Surface Friction Tester as used on EHRD of 1.10. Actual
maintenance friction measurements on EHRD are some 20 – 30 % lower.
NASA TP 2917 contains the following conclusions: A lower macrotexture surface creates less friction
than a rough macrotexture surface, wet grooved or PFC runways improve the wet runway friction
capability, however it is not the same as a dry runway and the runway friction on a damp runway
was reduced compared to a dry runway. These conclusions are reproduced in the Boeing
Performance Engineer Operations Course Notes.
Conclusions
VNV does not support the conclusion that the runway surface of EHRD can be considered dry for
dispatch purposes when rain is forecasted based on the following:
This conclusion is based purely on a textual interpretation of the definition of a dry runway
currently in JAR-OPS and cannot be substantiated by friction measurements, flight testing or
scientific research.
Although the definitions of runway state in JAR-OPS should be amended in line with
CS/JAR/FAR 25 (see DNPA-OPS 47), the current definition of a dry runway merely includes an
unattainable condition, i.e. ‘… maintained to retain ‘effectively dry’ braking action …’ This
applies to aircraft and not friction testing equipment.
There is no maintenance program on EHRD comparable with FAA AC 150/5320-12C or CAAUK
CAP 683, which are industry wide accepted as being an acceptable standard for accepting
performance credit on grooved/PFC runways.
The maintenance program on EHRD is not designed to retain effective dry friction and does
not consider preventive maintenance and as such does not prevent friction levels from
dropping below MFL.
The MFL assures safe operations when the runway is considered wet. The equivalent braking
action associated with wet runways in Boeing terminology is GOOD. Braking action GOOD is
approximately equal to half the braking action of a dry runway.
The maintenance standard for the texture depth at EHRD is 1.3 mm. This is well within the
range of tested surfaces considered in ESDU Data Item 71026 and used for CS/JAR/FAR 25
grooved/PFC performance credit. As such there is no reason to expect significant better
performance than considered for certification requirements.
There is no scientific research or flight test data available showing equivalent dry
performance of jet aircraft on these surfaces and maintenance friction values measured
cannot be used for operational interpretation of aircraft stopping distances.
JAR-OPS addresses aircraft performance and not performance of friction measuring devices.
As such landing distances shorter than that for a wet runway can only be used according to
JAR-OPS 1.520 when specific information on landing distances on wet runway is contained in
the Aircraft Flight Manual. This information will have to be derived from flight testing and
such information is available from manufacturers (e.g. Airbus A320 for operations on
Funchal, Madeira).
Correlation between friction testing devices and aircraft performance is problematic,
especially on wet runways and currently not supported by Boeing and Airbus. Amongst
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