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In Search of Smooth Landings

Researchers Gather Friction Data for
Aviation Safety Standards

by Susan B. Costello

ASTM standards developed by Committee E17 on Vehicle-Pavement Systems play an important role in an ongoing international effort to create reliable data on airport runway friction in potentially slippery conditions.

Sometimes the term “friction” indicates an undesirable characteristic, but on an airport runway, friction is indispensable. Airport operators monitor runway conditions for friction and contaminants. A runway that has a surface condition other than bare and dry is termed “contaminated,” and any amount of contaminant may reduce friction. To maintain acceptable operating conditions, airports use plows, brooms, and blowers to remove loose contaminants from pavement surfaces and chemical agents to reduce the effects of runway ice and compacted snow. As an aircraft approaches for landing, the control tower relays information to the pilot about these surface conditions as well as information about current wind, visibility, precipitation, and air traffic.

The responsibility for the decision to land or not to land (the “go/no-go” decision), ultimately rests with the pilot. The pilot makes the decision based on the information supplied by the control tower and the pilot’s knowledge of the aircraft. Also for consideration in the go/no-go decision are factors such as the distance and accessibility of alternate landing sites, the conditions at those runways, and the amount of aircraft fuel remaining. Obviously, this decision can be critical; the pilot needs effective, consistent evaluation of runway conditions and a reliable means for relating those conditions to the aircraft’s stopping capabilities.

Getting the Program off the Ground

Runway water, ice, or snow was a factor in more than 100 aircraft accidents between 1958 and 1993. Inconsistent, ineffective reporting of winter runway conditions to airplane pilots has contributed to a disproportionate number of aircraft ground handling accidents. In spite of advances in technology and operational procedures, safe winter operations remain a challenge for airport operators, air traffic controllers, airlines, and pilots, who must coordinate their efforts under rapidly changing weather conditions.

Friction researchers in the aviation field have long campaigned for airport operators to use ground test vehicles to measure runway friction conditions. Although such test vehicles are not currently required, many airports do evaluate runway friction by means of vehicle-mounted or trailer-mounted friction testers. Because different airports use different friction testers, however, values applied to runway friction conditions have not been consistent from one airport to another. And because different types of aircraft behave differently on a given runway friction condition, identifying the stopping capability of an aircraft once the friction value is obtained has not been a clearly defined process.

In 1999, the Federal Aviation Administration (FAA) issued a revision to its guidelines on reporting pavement conditions. The revision states that, along with friction values, the runway condition report should identify by name the type of friction tester used. This additional information, however necessary, places a further burden on pilots: to know how the friction values from different testers relate to their aircraft.

Complicating the winter weather picture is that, for a given contaminated runway condition, criteria for safe operations differ from airport to airport due to differences in runway dimensions and pavement materials and textures. A step toward decreasing ground handling difficulties is to standardize and harmonize friction tester values so airports can provide pilots with uniform and reliable runway condition information that is independent of the type of measuring device.

In today’s economic climate, aviation industries are committed to affordable, cost-effective technology for improved safety and profitability. Aligning with this commitment, several government agencies have partnered in the Joint Winter Runway Friction Measurement Program to share cost, expertise, and facilities to achieve program objectives with industry’s guidance. The National Aeronautics and Space Administration’s (NASA’s) Langley Research Center and Transport Canada (TC) are leading this study with support from the FAA. Also participating are numerous government and industry organizations from North America and Europe and manufacturers of both aircraft and friction testing equipment.

Consistent, Well-Defined Measurement

The joint program integrates data from manual contaminant analyses, friction tester measurements, and aircraft instrumentation. Analysis of these data sets shows the effects of many parameters on aircraft and ground vehicle braking friction under various surface conditions.

The major program objectives are twofold:

--Harmonize friction tester measurements to report consistent friction value, or “index,” for similar contaminated runway conditions, and

--Establish reliable correlation between friction tester measurements and aircraft braking performance.

Accomplishing these objectives will give airport operators better procedures for evaluating runway friction and maintaining acceptable operating conditions and will contribute to reducing friction-related aircraft accidents.

The first objective of the Joint Winter Runway Friction Measurement Program involves harmonizing the friction measurements obtained by a variety of friction testers on a wide range of winter runway conditions. Many of these devices and test procedures are described in ASTM standards and practices prepared by ASTM Committee E17 on Vehicle- Pavement Systems. To ensure the accuracy of these different friction testers, Committee E17 formed a task group to prepare a standard that describes an international friction index calibration tester (referred to as the reference vehicle); the prototype underwent initial testing in early 2000.

Relating these harmonized friction tester measurements accurately to aircraft braking performance is the second objective of the program. Since testing began in January 1996, a variety of instrumented test aircraft have been involved.

During program testing, researchers monitor the test area environment and manually record numerous pre-run and post-run conditions: wind speed and direction; ambient temperature; temperatures of pavement surface and snow, slush, or ice; depth of cover material (water, snow, slush, ice); and in the case of snow or slush, specific gravity of cover material to determine density.

Testing thus far has used nine instrumented aircraft and 15 friction testers from Austria, Canada, France, Germany, Norway, Scotland, Sweden, Switzerland, and the United States at test sites in Canada, the United States, Norway, and Germany. Friction tester manufacturers, aircraft manufacturers, airports, airlines, and government agencies of eight countries have participated.

Joint Program Accomplishments

The Joint Winter Runway Friction Measurement Program will contribute significantly toward the reduction of aircraft accidents in adverse weather environments in two ways:

--By providing better tools for airport operators to use in evaluating runway conditions and

--By providing more accurate and reliable runway friction and aircraft stopping data for pilots to use in making their go/no-go decisions.

Joint program testing in the 1996–2000 winter seasons has provided a substantial friction database that includes nearly 400 instrumented aircraft test runs and more than 9000 friction tester runs under bare and dry, rain and artificially wet, artificially flooded, loose and compacted snow, smooth and rough ice, sanded and chemically treated ice, and slush conditions. Six weeks of warm-weather data from NASA Tire/Runway Friction Workshops (1994–1999), involving wet and dry conditions on different pavement materials, textures, and roughness, have been combined with the data from 19 weeks of winter testing. Results from the seventh annual NASA Tire/Runway Friction Workshop in May 2000 will be added to the friction database when data analysis is complete.

Data obtained during 1996 and 1997 helped define the methodology for an International Runway Friction Index (IRFI) to harmonize the friction measurements obtained by the different testers. Selected data from the first three years of testing were used to establish a Canadian Runway Friction Index (CRFI), an abbreviated version of the IRFI. Data from testing in 1998–2000 refined and improved the IRFI methodology.

Commercial friction testers can be grouped into three basic types: the fifth-wheel type (a standard automobile with a measuring wheel installed), the metered vehicle type (a standard automobile with a decelerometer installed to measure the vehicle’s wheel speed), and the trailer type (a self-contained measuring-wheel system pulled by a vehicle). Under similar runway conditions at the Canadian, United States, Norwegian, and German test sites, IRFI data from the different friction testers agreed closely, further substantiating the IRFI methodology. Researchers anticipate that all models of runway friction testers will be tested and harmonized with the IRFI to optimize their effectiveness. The CRFI is presently in use at Canadian airports as an interim measure to help pilots determine their aircraft stopping distance under compacted snow and ice conditions; Transport Canada plans to adopt the IRFI once the standard is approved by ASTM.

Data analysis in progress will further improve the harmonization of friction tester measurements for the IRFI and help determine a suitable Aircraft Friction Index (AFI). The AFI will be based on the stopping distances of different aircraft types for different IRFI values. Data indicate three probable type-categories: commuter aircraft, narrow-body transports, and wide-body transports. Primarily, the different types of landing gear and brake systems determine the three categories. Once testing correlates an aircraft’s stopping capabilities to the IRFI, pilots will be able to use the AFI to make the go/no-go decision. The expectation is that when the IRFI is accepted and in common use, aircraft manufacturers will want to test all aircraft types to determine aircraft braking performance for different IRFI values. Data from these tests will refine the different AFI categories.

During the aircraft test runs, a determination also has been made on the magnitude of runway contaminant-produced drag on aircraft ground performance. This type of drag results from contaminants thrown up by the aircraft tires during takeoff and landing and can affect aircraft ground handling. While contaminant-produced drag is a function of runway conditions, it does not relate to friction measurements or braking capability. It is, however, a factor of consequence in combination with other takeoff-landing factors: aerodynamic drag, aircraft accelerating-braking capabilities, runway friction, and runway length.

Program Impact

The proposed IRFI standard defining test procedures, data analysis methods, and accuracy requirements is currently being reviewed for approval by ASTM Committee E17 on Vehicle-Pavement Systems. Acceptance, dissemination, and implementation of the approved IRFI (and later, AFI) standard by the aviation community is expected through the guidance and assistance of several organizations, including ASTM, FAA, Transport Canada, the International Civil Aviation Organization, the Joint Aviation Authority, the International Federation of Airline Pilots, the United States and Canadian Airline Pilots Associations, the Air Transport Association, and the Airport Council International. Researchers hope the clear, effective standard will encourage more airport operators to heed the ongoing campaign for use of runway friction testers.

Worldwide acceptance of the IRFI would make friction standards consistent throughout the aviation industry. The IRFI will not only standardize runway friction values, but also help airport operators determine when to close a runway and, following treatment to restore friction, when to reopen the runway. In addition to improving aircraft ground operational safety, the overall results from this program are expected to increase the capacity of airports and may also apply to highway vehicle safety where winter conditions are severe.

On the Horizon

Although December 2000 was to mark the end of this ambitious five-year program, the various participating government organizations have agreed to extend the program by five years in order to include different aircraft types and explore more specific runway conditions. For example, a snow-covered runway exhibits different properties and gives a different IRFI value at –10 degrees C than at –30 degrees C. More aircraft and ground vehicle data are needed for slush-covered runway conditions also. And since reversing engine thrust can supplement wheel braking to enhance stopping capabilities, manufacturers and airlines are interested in reverse-thrust performance data. Aircraft braking performance and contaminant drag measurements at speeds from 120 to 170 knots have also been identified as part of future aircraft test run matrices, together with monitoring aircraft wheel brake torque variations during braking efforts.

Future testing in the joint program with new or improved friction testers and with other aircraft types, especially wide-body transports, will further validate the IRFI methodology and help produce an effective AFI to correlate different aircraft braking performance to the IRFI. Two wide-body transports have been committed for testing in the 2001 winter season; efforts are ongoing to enlist other types of aircraft for further winter season testing. //

Copyright 2000, ASTM

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Susan Costello, a member of the Society for Technical Communication, is a technical writer and editor at NASA Langley Research Center, Hampton, Virginia. Her earlier 18-year career as a highway construction inspector with the Virginia Department of Transportation stimulated a current interest in runway safety.

ASTM Committee E17 Standards Used in the Joint Winter Runway Friction Measurement Program

E 274, Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire

E 303, Test Method for Measuring Surface Frictional Properties Using the British Pendulum Tester

E 445, Test Method for Stopping Distance on Paved Surfaces Using a Passenger Vehicle Equipped With Full-Scale Tires

E 501, Specification for Standard Rib Tire for Pavement Skid-Resistance Tests

E 503, Test Methods for Measurement of Skid Resistance on Paved Surfaces Using a Passenger Vehicle Diagonal Braking Technique

E 524, Specification for Standard Smooth Tire for Pavement Skid-Resistance Tests

E 670, Test Method for Side Force Friction on Paved Surfaces Using the Mu-Meter

E 867, Terminology Relating to Vehicle-Pavement Systems

E 965, Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique

E 1136, Specification for a Radial Standard Reference Test Tire

E 1337, Test Method for Determining Longitudinal Peak Braking Coefficient of Paved Surfaces Using a Standard Reference Test Tire

E 1551, Specification for Special Purpose, Smooth-Tread Tire, Operated on Fixed Braking Slip Continuous Friction Measuring Equipment

E 1844, Specification for a Size 10 x 4-5 Smooth-Tread Friction Test Tire

E 1845, Practice for Calculating Pavement Macrotexture Mean Profile Depth