Airside friction testing might only be a small part of the entire airside operational workflow, but it’s a vital one. And it’s an area in which a number of companies provide very specialised equipment
Maintaining runways in a safe condition is of course vital to avoid any potentially catastrophic accidents. It will also mean that better performance – especially under braking – can be achieved by aircraft using those surfaces; yet, runways and taxiways can be contaminated with fuel, tyre rubber and other manmade detritus, as well as natural materials deposited as part of the changing environment. Snow and ice are perhaps the main factors in this regard, elements that can close a runway and, in consequence, perhaps an airport if the adverse conditions are not identified and dealt with quickly.
Thus there is an ongoing need for reliable information on runways’ surface integrity and their impact on aircraft performance. Continuous friction measuring equipment, or CFME, is the equipment used by airport operators and their contractors to test, measure and record friction coefficients with a view to maintaining a gateway’s operations at full capacity. Once the recorded levels are below a safe minimum, the runway or taxiway must be closed until the necessary remedial action is taken.
While the 100% accuracy of CFME is contested by some on the grounds of what they regard as an inability of equipment to achieve truly consistent ongoing results under the same conditions, there is no doubt that they are widely perceived and used as a vital method of assessing changing grip conditions and therefore a critical tool in making the aviation industry as safe as it is today.
The test of time
The Mu-Meter – mu is the Greek letter symbolising the coefficient of friction – is described by manufacturer Douglas as “the world’s best-selling continuous friction measuring equipment”. According to Douglas’ defence sales director, Ron Hopkins: “It’s the original and still the best.”
The Mu-Meter was designed in the 1960s for the UK Ministry of Defence as a means to assess the viability of jungle grass airstrips during the Malaya campaign. Plenty of scientific research prior to that allowed for the development of a piece of equipment that has – with, admittedly, significant technological improvement over the decades – survived the test of time very admirably.
The currently available variant is the Mk. 6, a wheeled system that has no need for a specialised towing vehicle and that features its own onboard computer, which allows the display of easy-to-understand data on runway condition in real time. Instant runway condition reports can then be radioed by the operator to airport authorities that need it or be printed out for more detailed analysis and historical record. The results can be displayed in different formats and languages as chosen by the operator, while the Mu-Meter can be operated in both dry and self-wetting modes.
It may have 40 years under its belt already, but improvements continue to be made to the Mu-Meter. For example, the Mk. 7, which will be available by the start of next year, will incorporate a sophisticated undercarriage/wheel impinged drag measurement capability, suitable for use in snow and slush.
Tested in Iceland and ideal for airports facing harsher winter conditions that need to assess impinged drag for take-off performance calculations and friction coefficients in difficult operating conditions, the new model will require no leads connecting it to the towing vehicle for the transfer of data – the information will be passed wirelessly. The Mk. 7 will also carry its own integrated power supply and be even more simple to operate and user-friendly than ever before, Hopkins assures.
The new variant should add further to sales that already number 1,000 or so over the last four decades. User airports number in the hundreds, he says, many of which have been using the equipment for many years; indeed, Hopkins notes, tough and resilient Mu-Meters have come back to Douglas for upgrade after 25 years or more of ongoing use.
‘Each and very day’
The Mu-Meter has its competitors, however. According to Robert Sims, director of sales at Findlay Irvine, a majority of today’s runway friction tester users turn to a product called GripTester for Runways. “GripTester is the world’s leading trailer-based continuous surface friction measuring device,” he argues.
Since its introduction 20 years ago, GripTester has been used each and every day around the world to reduce the risk of wet skidding incidents and to monitor contamination build-up; it is now used at more than 300 airports worldwide, Sims insists. The latest version, GripTester Mk2 is fully compliant with ICAO and UKCAA standards and can be used in all weather conditions to monitor such variables as the build-up of rubber on the surface; as a surface performance indicator; to measure contaminants; to assess skid resistance of high-speed run-offs; to analyse the effectiveness of de-icing treatments; during resurfacing work; to assess the success or otherwise of rubber removal treatments (before and after); and during retexturing (before and after).
The GripTester achieves consistent results both in straight lines and on road curvatures, making the tasks of testing and analysis significantly easier.
Findlay Irvine, established in Scotland in 1960, is a multi-disciplinary engineering, design and manufacturing company that serves several market sectors as well as the transport/aviation business. Its products designed for the airport sector also include the new Micro GT designed for friction measurement on Helidecks, runway alignment systems for use while surveys are carried out, as well as weather monitoring systems that measure atmospherics.
Reliable and green
Neubert Aero Corp (NAC) is another manufacturer active in this area that has enjoyed some success of late. It recently delivered 32 of its Dynamic Friction Testers (DFTs) to the General Directorate of State Airports Authority (DHMI) in Turkey, for example, accompanied with associated training.
Florida-based NAC had been awarded the contract to supply 32 DFT systems back in late 2011. According to NAC, the DFT is expected to be the primary friction measuring device used by the airports operated by DHMI. A towed, self-wetting unit, the DFT offers real-time reporting of ambient/surface temperature and Mu factor. It graphically plots recorded friction classification values every 250 feet on a program within Windows XP, featuring touch-screen technology and powered either by AC or solar charging. The data collected can be transmitted to the tow vehicle either by cable or wirelessly.
Containing no hazardous materials, NAC describes the system as “a leader in reliability and environmental friendliness”, while its built-to-last design is also said to dramatically reduce maintenance and operating expenses.