` The Development of Runway Friction Testing

“There is no such thing as runway friction … ” … friction is a property of the tyre-surface interface Why are there different thresholds for the different testers? because friction is always a property of a pair of surfaces and of the process of moving them in relation to each other (under process I would include the speed of their relative motion and the amount of water or other lubricant involved). because the measuring systems of the different CFMEs differ: and because it is relatively easy to design a device that will measure friction but more difficult to design a device that will measure only friction (that is, will be unaffected by such factors as water drag). because one type of CFME is not generally calibrated in the same way as other types.

introduction Findlay, Irvine Ltd is an innovative engineering company founded in 1960 and located near Edinburgh, Scotland. The company specialises in the design and manufacture of technically advanced products for the transport industry. Its runway friction tester, the GripTester, is known and used all over the world. My name is Delia Harverson and I have been a Director of the company since 1981.

introducing some terminology All modern runway friction testers are of the type: Continuous reading Friction Measuring Equipment or CFME

runway friction testing: history What was the first Continuous reading Friction Measuring Equipment [CFME] ? When was it introduced

the first CFME was the designed by the UK Ministry of Defence in the early 1960s

runway friction testing: history Why was the MuMeter developed? Mainly for testing in winter conditions

1962: a Canberra lands on a winter contaminated runway ...

… and comes safely to a stop

runway friction testing Is it as important to measure runway friction in summer as in winter? YES! In rain, a runway with poor texture and rubber deposits can be just as slippery as a runway contaminated with ice and snow

runway friction testing Every year, substantially more overrun accidents occur on wet runways than on winter contaminated runways These accidents cause airports and airlines expense, embarrassment and loss of customer confidence Most of these accidents need never have happened

runway friction testing Regular use of an approved, well maintained CFME will not guarantee that an overrun accident won’t happen at your airport … … but it will make it less likely

operational & maintenance testing Operational testing is carried out in natural conditions, in winter, in order to make immediate operational decisions. Maintenance testing is carried out in carefully controlled conditions, which include an exact amount of water put down in front of the measuring wheel, in order to make engineering decisions.

operational & maintenance testing Should engineering decisions ever be made on the basis of operational testing? No! Natural conditions are much too variable to provide a proper basis for engineering decisions

the role of ICAO How do we ensure that maintenance friction testing is consistent … … that the same results are obtained in Lima as in London Heathrow? … in Santa Cruz as in Singapore?

“There is no such thing as runway friction … ” … friction is a property of the tyre-surface interface Why are there different thresholds for the different testers? because friction is always a property of a pair of surfaces and of the process of moving them in relation to each other (under process I would include the speed of their relative motion and the amount of water or other lubricant involved). because the measuring systems of the different CFMEs differ: and because it is relatively easy to design a device that will measure friction but more difficult to design a device that will measure only friction (that is, will be unaffected by such factors as water drag). because one type of CFME is not generally calibrated in the same way as other types.

Maintenance testing table from the “Green pages” of ICAO Annex 14

the role of ICAO Friction is NOT a property of one surface (the runway) Friction is a property of a pair of surfaces (the runway and the skidding tyre) In fact, it is really a property of a process (runway, test tyre, speed, skid ratio, test water film)

runway friction testing : history

the GripTester The measuring wheel is mounted on a stub axle. Strain gauges on this axle measure LOAD and DRAG: from these, friction, μ, is calculated by the formula: μ = DRAG / LOAD

the GripTester There are now more than 280 GripTesters operating worldwide, including 15 with the US Air Force, 7 with the Canadian Air Force, 6 with the Turkish Air Force, 2 each at most of the UK’s large airports, 2 at the new airport at Hong Kong – and so on on …

8 the GripTester The success of the GripTester is based on carefully thought out design principles:-

car with integral measuring system design decision 1 trailer or car with integral measuring system ? Separating the measuring system from the automotive system gives greater flexibility and lower cost of ownership

? design decision 2 braked wheel or slewed (angled) wheel A braked measuring wheel reflects rolling friction better and more directly than a slewed measuring wheel. The MuMeter was designed with slewed measuring wheels because, in the 1960s, strain gauges were much less advanced than they are today.

? design decision 3 measure LOAD or assume it constant FRICTION = DRAG / LOAD Whatever the weight of the tester, assuming the vertical force (LOAD) on the measuring wheel to be constant, instead of measuring it, can lead to inaccurate readings. Because it measures LOAD continuously, the the accuracy and reliability of GripTester’s readings are not compromised by its light weight (85 kilos).

? design decision 4 measure DRAG directly or measure torque and calculate drag ? The advantages of measuring DRAG and LOAD directly, by instrumenting the measuring axle, are highlighted by a recent US FAA “CERTALERT” warning of the effects of loose surface contaminants on devices using torque measurement.

? design decision 5 fixed slip or variable slip From an academic point of view, variable slip is attractive. From a practical point of view, it has been found to be awkward and unreliable.

? design decision 6 which slip ratio free rolling wheel = 0% slip ratio locked wheel = 100% slip ratio slip speed = test speed x slip ratio The slip ratio must be chosen with reference to the intended test speeds. For the ICAO specified speeds of 65km/h and 95km/h, a slip ratio of 15% gives maximum performance.

? design decision 7 a PC or a specialised data logger with a PC archiving is easy as computer design advances, options such as colour displays can be made available data storage is virtually unlimited printout is on standard paper rather than on paper rolls design decision 7

design decision 8 Simple and efficient calibration and maintenance Calibration of the GripTester is simple and direct: vertical and horizontal forces are applied to the measuring axle and the readout adjusted if necessary. The GripTester’s modular design helps to ensure that maintenance is also simple and efficient.

policy decisions A badly maintained or wrongly calibrated CFME is worse than no CFME All CFMEs should be given annual maintenance and calibration by qualified technicians working in a suitable environment. Findlay Irvine’s partners in China and the rest of Asia Pacific offer in depyh after sales support