… friction is a property of the tyre-surface interface There is no such thing as runway friction...

The Development ofThe Development of

RRunwayunway

FFrictionriction

TTestingesting

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… friction is a property of the tyre-surface interface

“There is no such thing as runway friction … ”

introductionintroduction

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 terminologyintroducing some terminologyintroducing some terminologyintroducing some terminology

All modern runway friction testers are of the type:

Continuous reading Friction Measuring Equipment

or

CFME

What was the first Continuous reading Friction Measuring Equipment

[CFME]

??When was it introduced

??

runway friction testing:runway friction testing: historyhistoryrunway friction testing:runway friction testing: historyhistory

the first CFME

designed by the UK Ministry of

Defence in the early 1960s

was the

runway friction testing:runway friction testing: historyhistoryrunway friction testing:runway friction testing: historyhistory

• 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

Is it as important to measure runway friction in summer as in winter??

YES!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 testingrunway friction testingrunway friction testingrunway 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 Most of these accidents need never have need never have

happenedhappened


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 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 & maintenanceoperational & maintenancetestingtesting


Should engineering decisions ever be made on the basis of operational testing?

No!No!Natural conditions are much too variable to provide a proper basis for engineering

decisions



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?

the role of ICAOthe role of ICAOthe role of ICAOthe role of ICAO

… friction is a property of the tyre-surface interface

“There is no such thing as runway friction … ”

Table A-1

Test tyre Test DesignTest water objective Maintenance Minimum

Pressure speed depth for new planning frictionTest equipment Type (kPa) (km/h) (mm) surface level level

(1) (2) (3) (4) (5) (6) (7)

Mu-meter Trailer A 70 65 1.0 0.72 0.52 0.42A 70 95 1.0 0.66 0.38 0.26

Skiddometer Trailer B 210 65 1.0 0.82 0.60 0.50B 210 95 1.0 0.74 0.47 0.34

Surface Friction B 210 65 1.0 0.82 0.60 0.50Tester Vehicle B 210 95 1.0 0.74 0.47 0.34

Runway Friction B 210 65 1.0 0.82 0.60 0.50Tester Vehicle B 210 95 1.0 0.74 0.54 0.41

TATRA Friction B 210 65 1.0 0.76 0.57 0.48Tester Vehicle B 210 95 1.0 0.67 0.52 0.42

GRIPTESTER C 140 65 1.0 0.74 0.53 0.43Trailer C 140 95 1.0 0.64 0.36 0.24

9/11/95

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

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)

the role of ICAOthe role of ICAOthe role of ICAOthe role of ICAO

runway friction testing : runway friction testing : historyhistory

runway friction testing : runway friction testing : historyhistory

the GripTester the GripTester the GripTester the GripTester

The measuring The measuring wheel is mounted wheel is mounted on a stub axle. on a stub axle. Strain gauges on Strain gauges on this axle measure this axle measure LOADLOAD and and DRAGDRAG: : from these, from these, friction, μ, is friction, μ, is calculated by the calculated by the formula:formula:

μ = DRAG / LOAD


There are now more than There are now more than 280 280

GripTesters operating worldwide, GripTesters operating worldwide,

including including 1515 with the US Air Force, with the US Air Force, 77

with the Canadian Air Force, with the Canadian Air Force, 66 with the with the

Turkish Air Force, Turkish Air Force, 22 each at most of the each at most of the

UK’s large airports, UK’s large airports, 22 at the new airport at the new airport

at Hong Kong – and so on on …at Hong Kong – and so on on …

The success of the GripTester is based on 8

carefully thought out design principles:-


Separating the measuring system from the automotive system gives greater flexibility and lower cost of

ownership

trailer or car with integral measuring system

??

design decision 1 design decision 1

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.

braked wheel or

slewed (angled) wheel

??


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).

measure LOAD or

assume it constant

??FRICTION = DRAG / LOAD


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.

measure DRAG directly or

measure torque and calculate drag

??


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.

fixed slip or

variable slip

??


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.

free rolling wheel = 0% slip ratio

locked wheel = 100% slip ratio

slip speed = test speed x slip ratio

which slip ratio

??


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 7design decision 7

design decision 8design decision 8design decision 8design decision 8Simple and Simple and efficient efficient calibration and calibration and maintenancemaintenanceCalibration of the Calibration of the GripTester is simple and GripTester is simple and direct: vertical and direct: vertical and horizontal forces are horizontal forces are applied to the measuring applied to the measuring axle and the readout axle and the readout adjusted if necessary.adjusted if necessary.

The GripTester’s modular The GripTester’s modular design helps to ensure that design helps to ensure that maintenance is also simple maintenance is also simple and efficient.and efficient.

policy decisionspolicy decisionspolicy decisionspolicy 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

… friction is a property of the tyre-surface interface There is no such thing as runway friction...

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