Friction Testing – the way ahead


As friction is a dimensionless coefficient, it needs surfaces to be in contact to create a measurable value. The UK Civil Aviation Authority (CAA) has developed a policy document (CAP 683) to flesh out the requirements needed to maintain adequate grip on wet runways. A Friction Task Force has also recently been formed.

Friction-testingThe ultimate goal in friction testing is to develop a process that can translate readings taken from a runway and convert the data into meaningful figures to help aircrew recalculate performance figures when conditions change en route. This still remains a high priority for the industry. Finding a common reporting format to disclose conditions from runway assessments is also a major objective.

One idea currently under consideration is to merge the NOTAM and SNOWTAM systems and use technology to create and promulgate reports directly from the runway into flight information distribution systems.

Another issue is how to ensure repeatability and reproducibility of all Continuous Friction Measuring Equipment (CFME) are to an agreed standard. This is a major challenge due to a variety of makes and designs which use differing data gathering methods.

Harmonisation, however, may not be beneficial in terms of the readings each machine produces. If CFME is used as part of a planned maintenance programme as a trend monitor, the numerical values produced do not necessarily have to match a neighbouring aerodromes machine. The variability of conditions makes that virtually impossible which is why national fiction indices are no longer computed.

If build standards were produced to ensure all manufacturers followed the same criteria, the reproducibility and repeatability of families of machines would be improved. A good example of this is the SAE Standard for the design and manufacture of ‘locked wheel devices’ used by the US Highways Department. A major obstacle to this would be the need to competing manufacturers to cooperate on drafting a standard; something that would be difficult to enforce.

Central to the Civil Aviation Authority’s regulation of friction testing is the need for aerodromes to have a trend monitoring tool. Data gathered on a regular basis should trigger actions to restore levels of grip across the full length and width of the runway to help deliver adequate braking in wet conditions.

There have been no changes to the procedures set out in the regulations, though a number of established European CFME manufacturers have either supplied or are in the process of supplying test data sufficient to allow their machine to be added to the CAA’s data bank.

Our role as a regulator means that we have to be even handed across all licensed aerodromes. The production of submissions by prospective manufacturers is a commercial contract between them and one of two accredited test houses, hence our need to remain at arm’s length. CAA inspectors work closely with their allocated aerodromes to review records of assessment runs and offer clarification on policy if needed.

Where CFME manufacturers hold a user group meeting the CAA always supports this by attending and giving updates on regulatory developments. One recent example was a briefing on the EASA Implementing Rules.

The CAA has a seat on the Friction Task Force and was a founder member for phase one. This saw the publishing of an ICAO Circular which reported on the state of current techniques and for the first time classified the differing needs for maintenance testing and the more tactical use in winter weather. Phase two has started and the CAA has already contributed to the work programme. This has included providing facilities for the rapporteur and recovery of a vast number of files from the National Archive which show how the advent of the Jet Age accelerated the science of friction.

Paul Fraser-Bennison is the Aerodrome Standards Officer at the UK Civil Aviation Authority.

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