FRICTION TESTING

A closer look at the world of runway friction testing

Runway friction testing – Do you know what it is? Do you recognise its importance? There is no doubt in my mind that some of you will be well versed in this area, but I wouldn’t lay odds that that number would be particularly high. Airports, as we know, are a sum of their parts, with many different sections working on very specific and specialist jobs. Some might even say that airports are compartmentalised in this respect and it is difficult to see how they could operate in any other way. So, in order to enlighten those who perhaps are not familiar with this area of expertise, let’s take a look in a little more detail at the business of friction testing.
A good place to start has to be the obvious question: Why is friction testing so important and what does it tell airport operators that they need to know? In order to get an informed answer to this question, and the questions that follow, we must turn to the experts in the field.
Ron Hopkins, Director at Douglas SPD Ltd, comments: “In simple terms we are setting out to ensure that the aircraft can stop in the distance available! Of course, friction testing tells the airport ops team much more than that. It provides a guideline to plan runway maintenance. As the runway pavement wears, or ‘spin up’ rubber deposits build up, its texture changes and with it the coefficient of friction, which is an international standard from the laws of physics, so an operator can see some time ahead when he needs to have the surface retextured, remove rubber deposits or in extreme cases surface replacement.”
So wear and tear on the surface is a major factor it seems, but what of the weather?
“Wet tyre/pavement friction testing is important to be able to identify the runway or highway sections that are below acceptable levels and remedial treatment is needed ASAP to minimise the chance of accidents from occurring with possible loss of life. Similar testing needs to be expedited when more severe adverse weather conditions are accumulating such as snow, slush and or ice. If a pavement section is labelled “Slippery when wet” you may be sure that it will provide much less vehicle control when coated with these solid contaminants” says Tom Yager, Distinguished Research Associate at NASA Langley.
The bottom line when it comes to the importance of friction testing is summed up nicely by Fredrik Graflind of ASFT: “Airport operators need to know the status of their runways at all times and friction testing is a key parameter when it comes to assessing this.”


Accuracy
There are a number of options when it comes to equipment for measuring friction, from a trailer towed behind a vehicle to a fully integrated system within the vehicle itself, but how do you ensure that the readings you are taking are as accurate as possible?
“The only way to verify if a friction tester is reliable or not is to test its reproducibility.” (Reproducibility being when measurements are performed by different operators using different equipment of the same type, and with some lapse of time between them.) “Stable equipment (able to isolate the friction wheel from movements in the chassis and other external parameters) will produce reproducible measuring results” offers Graflind.
Paul Fraser-Bennison, Policy Officer for the UK CAA Safety Regulation Group, had this to say: “Continuous Friction Measuring Equipment (CFME) should be regarded as scientific instruments. As such, they are built to exacting tolerances, require pre-run calibration and check runs before and after deployment. Also the speeds at which they are used, and the depth of water film sprayed in front of the measuring wheel, have to be controlled accurately. Finally, there are limits on the weather conditions they can be used in too.”
Yager concurs, saying: “Accurate tyre/pavement friction readings are not easy to obtain. The measuring equipment must be properly calibrated, the hardware/software components must be in working order and the operators must be recently trained to collect and analyze the data. If any one of these three requirements is lacking or absent, tyre/pavement friction readings will not be correct and could be very misleading.”

The sharing of ideas and experiences is also of great importance to developing ever more accurate equipment and to this end there are numerous peer-reviewed workshops around the world where manufacturers gather to test equipment and compare results. Coming up this year for example are the 18th Annual Friction Workshop at State College, PA, USA, June 7-10, 2011 and the SWIFT Conference in Montreal, Quebec, CAN, September 11-15, 2011.

Tyre type
It would seem reasonable to expect friction readings to differ if the tyre types used in the equipment varied across the board. So, which tyres are best suited to the job?
“Test tyre type, inflation pressure and tread design can significantly affect tyre/pavement friction measurements along with the speed at which the measurements were collected. Under wet conditions, measurements at 65 km/hr will be higher than those measured at 95 km/hr; ribbed tread tyres will produce higher readings than those measured with smooth (Blank) tread tyres. The American Society for Testing and Materials (ASTM) recommends all pavement friction measurements be taken with a smooth tread measuring tyre” said Yager.

The future of friction testing
What, then, does the future hold for the world of friction testing? Our panel of experts on the subject offered these words in conclusion: Ron Hopkins: “We are working towards greater accuracy, and more importantly repeatability, and to increasing the range of information available to operators. We already have systems ready to test runway friction but also to offer a greater range of winter services information, such as snow depth, snow density , de-icing slush depth, and runway snow mapping that will enable more effective snow clearance and more effective, economic and environmentally sound use of de-icers .”
Fredrik Graflind: “It will be more important as traffic is increasing and putting existing infrastructure under more stress. There will also be more focus on training as airports are faced with higher employee turnaround and a general lack of accurately understanding the challenges with operating a busy runway.”
Paul Fraser-Bennison: “This is one of the biggest questions presently facing industry. The ICAO Friction Task Force, on which the CAA have a seat, is working on improving all aspects of measuring runway surface friction characteristics and consequently the quality of information made available to operators. Research to date points at differences across the globe in the understanding of the subject, the use of CFME and interpretation of the results. New thinking is also being brought to bear on the contribution inter alia surfaces’ mean texture depth (MTD) contributes to safe braking. There are new ways to measure this with LASER scanners developed for the highways. CFME will play its part but additional survey methods may be introduced to further enhance the quality of data.”
Tom Yager: “The Federal Aviation Administration (FAA) and the International Civil Aviation Organisation in recent months have increased their efforts at harmonizing the aviation regulations. More and more countries are recognising the need for highway tyre/pavement friction measurements and there are many more treatments available for remedial solutions.”
In conclusion, it seems to me that friction testing measurements are of crucial importance to the airports of today and that the various manufacturers, associations and Government agencies involved in this area are constantly striving to improve accuracy and safety for global passengers. So the next time your plane touches down, take a moment and spare a thought for the friction testing guys on the ground. They have you in mind every day!

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