CFME – Continuous friction measuring equipment

There is general concern over the adequacy of the available friction between the aeroplane tires and the runway surface under certain operating conditions, such as when there is snow, slush, ice or water on the runway and, particularly, when aeroplane take-off or landing speeds are high. This concern is more acute for jet transport aeroplanes since the stopping performance of these aeroplanes is, to a greater degree, dependent on the available friction between the aeroplane tires and the runway surface, their landing and take-off speeds are high, and in some cases the
runway length required for landing or take-off tends to be critical in relation to the runway length available. In addition, aeroplane directional control may become impaired in the presence of cross-wind under such operating conditions.

 

Introduction to Continuous Friction Measuring Equipment (CFME)

 

A measure of the seriousness of the situation is indicated by the action of national airworthiness authorities in recommending that the landing distance requirement on a wet runway be greater than that on the same runway when it is dry. Further problems associated with the take-off of jet aeroplanes from slush- or water-covered runways include performance deterioration due to the contaminant drag effect, as well as the airframe damage and engine ingestion problem. Information on ways of dealing with the problem of taking off from slush- or water-covered runways
is contained in the Airworthiness Technical Manual (Doc 9051).

Further, it is essential that adequate information on the runway surface friction characteristics/aeroplane braking performance be available to the pilot and operations personnel in order to allow them to adjust operating technique and apply performance corrections. If the runway is contaminated with snow or ice, the condition of the runway should be assessed, the friction coefficient
measured and the results provided to the pilot. If the runway is contaminated with water and the runway becomes slippery when wet, the pilot should be made aware of the potentially hazardous conditions.

Before giving detailed consideration to the need for, and methods of, assessing runway surface friction, or to the drag effect due to the presence of meteorological contaminants such as snow, slush, ice or water, it cannot be overemphasized that the goal of the airport authority should be the removal of all contaminants as rapidly and completely as possible and elimination of any other conditions on the runway surface that would adversely affect aeroplane performance.

 

Importance of runway surface friction characteristics / aeroplane braking performance

 

Evidence from aeroplane overrun and run-off incidents and accidents indicates that in many cases inadequate runway friction characteristics/aeroplane braking performance was the primary cause or at least a contributory factor. Aside from this safety-related aspect, the regularity and efficiency of aeroplane operations can become significantly impaired as a result of poor friction
characteristics. It is essential that the surface of a paved runway be so constructed as to provide good friction characteristics when the runway is wet. To this end, it is desirable that the average surface texture depth of a new surface be not less than 1.0 mm. This normally requires some form of special surface treatment.

 

Adequate runway friction characteristics are needed for three distinct purposes:

a) deceleration of the aeroplane after landing or a rejected take-off;
b) maintaining directional control during the ground roll on take-off or landing, in particular in the presence of cross-wind, asymmetric engine power or technical malfunctions; and
c) wheel spin-up at touchdown.
With respect to either aeroplane braking or directional control capability, it is to be noted that an aeroplane, even though operating on the ground, is still subject to considerable aerodynamic or other forces which can affect aeroplane braking performance or create moments about the yaw axis. Such moments can also be induced by asymmetric engine power (e.g. engine failure on take-off), asymmetric wheel brake application or by cross-wind. The result may critically affect directional stability. In each case, runway surface friction plays a vital role in counteracting these forces or moments. In the case of directional control, all aeroplanes are subject to specific limits regarding acceptable cross-wind components. These limits decrease as the runway surface friction decreases.

Reduced runway surface friction has a different significance for the landing case compared with the  rejected take-off case because of different operating criteria.

On landing, runway surface friction is particularly significant at touchdown for the spin-up of the wheels to full rotational speed. This is a most important provision for optimum operation of the electronically and mechanically controlled anti-skid braking systems (installed in most current aeroplanes) and for obtaining the best possible steering capability. Moreover, the armed autospoilers which destroy residual lift and increase aerodynamic drag, as well as the armed autobrake systems, are only triggered when proper wheel spin-up has been obtained. It is not
unusual in actual operations for spin-up to be delayed as a result of inadequate runway surface friction caused generally by excessive rubber deposits. In extreme cases, individual wheels may fail to spin up at all, thereby creating a potentially dangerous situation and possibly leading to tire failure.

Generally, aeroplane certification performance and operating requirements are based upon the friction characteristics provided by a clean, dry runway surface, that is, when maximum aeroplane braking is achievable for that surface. A further increment to the landing distance is usually required for the wet runway case.

To compensate for the reduced stopping capability under adverse runway conditions (such as wet or slippery conditions), performance corrections are applied in the form of either increases in the runway length required or a reduction in allowable take-off mass or landing mass. To compensate for reduced directional control, the allowable cross-wind component is reduced.

To alleviate potential problems caused by inadequate runway surface friction, there exist basically two possible approaches:

a) provision of reliable aeroplane performance data for take-off and landing related to available runway surface friction/aeroplane braking performance; and
b) provision of adequate runway surface friction at all times and under all environmental conditions.

The first concept, which would only improve safety but not efficiency and regularity, has proved difficult mainly because of:
a) the problem of determining runway friction characteristics in operationally meaningful terms; and
b) the problem of correlation between friction-measuring devices used on the ground and aeroplane braking performance. This applies in particular to the wet runway case.

The second is an ideal approach and addresses specifically the wet runway. It consists essentially of specifying the minimum levels of friction characteristics for pavement design and maintenance. There is evidence that runways which have been constructed according to appropriate standards and which are adequately maintained provide optimum operational conditions and meet this
objective. Accordingly, efforts should be concentrated on developing and implementing appropriate standards for runway design and maintenance.

 

Need for assessment of runway surface conditions

Runway surface friction/speed characteristics need to be determined under the following circumstances:

a) the dry runway case, where only infrequent measurements may be needed in order to assess surface texture, wear and restoration requirements;
b) the wet runway case, where only periodical measurements of the runway surface friction characteristics are required to determine that they are above a maintenance planning level and/or minimum acceptable level. In this context, it is to be noted that serious reduction of friction coefficient in terms of viscous aquaplaning can result from contamination of the runway, when wet, by rubber deposits;
c) the presence of a significant depth of water on the runway, in which case the need for determination of the aquaplaning tendency must be recognized;
d) the slippery runway under unusual conditions, where additional measurements should be made when such conditions occur;
e) the snow-, slush-, or ice-covered runway on which there is a requirement for current and adequate assessment of the friction conditions of the runway surface; and
f) the presence and extent along the runway of a significant depth of slush or wet snow (and even dry snow), in which case the need to allow for contaminant drag must be recognized.

Note.— Assessment of surface conditions may be needed if snowbanks near the runway or taxiway are of such a height as to be a hazard to the aeroplanes the airport is intended to serve. Runways should also be evaluated when first constructed or after resurfacing to determine the wet runway surface friction characteristics.

 

The above situations may require the following approaches on the part of the airport authority:

a) for dry and wet runway conditions, corrective maintenance action should be considered whenever the runway surface friction characteristics are below a maintenance planning level. If the runway surface friction characteristics are below a minimum acceptable friction level, corrective maintenance action must be taken, and in addition, information on the potential slipperiness of
the runway when wet should be made available (see Appendix 5 for an example of a runway friction assessment programme);
b) for snow- and ice-covered runways, the approach may vary depending upon the airport traffic, frequency of impaired friction conditions and the availability of cleaning and measuring equipment. For instance:
1) at a very busy airport or at an airport that frequently experiences the conditions of impaired friction — adequate runway cleaning equipment and continuous friction measuring equipment (CFME) to check the results;
2) at a fairly busy airport that infrequently experiences the conditions of impaired friction but where operations must continue despite inadequate runway cleaning equipment — measurement of runway friction, assessment of slush contaminant drag potential, and position and height of significant snowbanks; and
3) at an airport where operations can be suspended under unfavourable runway conditions but where a warning of the onset of such conditions is required — measurement of runway friction,
assessment of slush contaminant drag potential, and position and height of significant snowbanks.

 

Contaminant drag

There is a requirement to report the presence of snow, slush, ice, or water on a runway, as well as to make an assessment of the depth and location of snow, slush or water. Reports of assessment of contaminant depth on a runway will be interpreted differently by the operator for the take-off as compared with the landing. For take-off, operators will have to take into account the contaminant
drag effect and, if applicable, aquaplaning on take-off and accelerate-stop distance requirements based on information which has been made available to them. With regard to landing, the principal hazard results from loss of friction due to aquaplaning or compacted snow or ice, while the drag effects of the contaminant would assist aeroplane deceleration.

However, apart from any adverse effects from contaminant drag which may occur on take-off or loss of braking efficiency on landing, slush and water thrown up by aeroplane wheels can cause engine flame-out and can also inflict significant damage on airframes and engines. This is further reason to remove precipitants from the runway rather than, for instance, devoting special efforts
towards improving the accuracy of measurement and reporting the runway friction characteristics on a contaminated runway.

 

Explanation of terms

It is not possible to discuss methods of measuring friction and assessing contaminant depth without first considering some of the basic phenomena which occur both under and around a rolling tire. For the sake of simplicity, these can, however, be given in a qualitative manner.

Percentage slip

Brakes in the older aeroplane models were not equipped with an anti-skid system; i.e. the harder the pilot applied the brakes, the more braking torque developed. In applying the brake pressure, the wheel slowed down and, provided there was sufficient braking torque, could be locked. Assuming an aeroplane speed of 185 km/h (100 kt) and the speed of the tire at its point of contact with the ground 148 km/h (80 kt), the tire would slip over the ground at a speed of 37 km/h (20 kt). This is termed 20 percent slip. Similarly, at 100 per cent slip, the wheel is locked. The importance of this term lies in the fact that as the percentage slip varies, so does the amount of friction force produced by the wheel, as shown in diagrammatic form in Figure 1-1 for a wet runway. Therefore the maximum friction force occurs between 10 to 20 per cent slip, a fact which modern braking systems make use of to increase braking efficiency. This is achieved by permitting the wheels to slip within these percentages.

The importance of this curve from the viewpoint of runway friction coefficient measurement is that the value at the peak of the curve (termed μ maximum) can, when plotted against speed, represent a characteristic of the runway surface, its contamination, or the friction-measuring device carrying out the measurement and is, therefore, a standard reproducible value. This type of device can thus be used to measure the runway friction coefficient. On snow- or ice-covered runways, the measured value can be given in a meaningful form to a pilot. On wet runways, the
measured value can be used as an assessment of the friction characteristics of the runway when wet.

Locked wheel

The term “locked wheel” is exactly as implied and the friction coefficient μ skid produced in this condition is that at 100 per cent slip in Figure 1-1. It will be noted that this value is less than the μ max attained at the optimum slip. Tests have shown that for an aeroplane tire, μ skid varies between 40 and 90 per cent of μ max, subject to runway conditions. Even so, vehicles using a
locked wheel mode have also been used to measure the runway friction coefficient. In this case, the measured value would be indicative for the wheel spin-up potential at touchdown.

Side friction coefficient

When a rolling wheel is yawed, such as when a vehicle changes direction, the force on the wheel can be resolved in two directions — one in the plane of the wheel and the other along its axle. The side friction coefficient is the ratio of the force along the axle divided by the vertical load. If this ratio is plotted against the angle of yaw on different surfaces, a relationship similar to Figure 1-2 is
established.

When the wheel is yawed at an angle greater than 20 degrees, the side friction coefficient cannot be used to give a number representing the runway friction coefficient. Allowing for certain other considerations, the wheel can in effect be made to work at μ max. Depending on tire pressure, stiffness (construction) and speed, the relationship between side force and yaw angle will vary.

“Normal” wet friction and aquaplaning

When considering a wet or water-covered runway, there are certain separate but related aspects of the braking problem. Firstly, “normal” wet friction is the condition where, due to the presence of water on a runway, the available friction coefficient is reduced below that available on the runway when it is dry. This is because water cannot be completely squeezed out from between the
tire and the runway, and as a result, there is only partial contact with the runway by the tire. There is consequently a marked reduction in the force opposing relative motion of tire and runway because the remainder of the contacts are between tire and water. To obtain a high coefficient of friction on a wet or water-covered runway, it is, therefore, necessary for the intervening water film to be displaced or broken through during the time each element of the tire is in contact with the runway. As the speed rises, the time of contact is reduced and there is less time for the process to
be completed; thus, friction coefficients on wet surfaces tend to fall as the speed is raised, i.e. the conditions, in effect, become more slippery. Secondly, one of the factors of most concern in these conditions is the aquaplaning phenomenon whereby the tires of the aeroplane are to a large extent separated from the runway surface by a thin fluid film. Under these conditions, the friction coefficient becomes almost negligible, and wheel braking and wheel steering are virtually ineffective. A description of the three principal types of aquaplaning known to occur is given below.

The typical reduction of friction when a surface is wet and the reduction of friction as aeroplane speed increases are explained by the combined effect of viscous/dynamic water pressures to which the tire/surface is subjected. This pressure causes partial loss of “dry” contact, the extent of which tends to increase with speed. There are conditions where the loss is practically total and
the friction drops to negligible values. This is identified as either viscous, dynamic or reverted rubber aquaplaning. The manner in which these phenomena affect different areas of the tire/surface interface and how they change in size with speed is illustrated in Figure 1-3, which is based on the three zone concept suggested by Gough. In Zone 1 where there is dynamic pressure and in Zone 2 where there is viscous pressure, friction is virtually zero, whereas one can assume dry friction in Zone 3. Zone 3 will gradually decrease in size as speed increases and the friction coefficient μ will be reduced in proportion to the reduction in the size of Zone 3. It can be assumed that the proportion between the zones will be the same if two wheels are running at the same fraction of their aquaplaning speed.

In the case of viscous aquaplaning, loss of traction can occur at relatively low speeds due to the effect of viscosity in preventing water from escaping from under the tire footprint. However, a very smooth runway surface is required; such a surface can be encountered in areas that have become heavily coated with rubber deposited by tires during wheel spin-up at touchdown or that have been subjected to polishing by traffic. Viscous aquaplaning is associated with damp/wet runways or on wet ice-covered runways and, once begun, can persist down to very low speeds. Viscous aquaplaning can occur during the braking portion of either a rejected take-off or a landing ground roll.

Dynamic aquaplaning will occur beyond a critical speed which is a function of tire pressure. The condition is a result of an inertial effect of the water in which the downward pressure (inflation pressure) of the tire is insufficient to displace the water away from the footprint in the short time of contact. Dynamic aquaplaning can occur on a runway with inadequate macrotexture at speeds
beyond the critical aquaplaning speed provided the fluid is deep enough. It is associated with a coverage of fluid of measurable depth on the runway and occurs at a critical velocity which is a direct function of the tire pressure. The higher the tire pressure, the higher the velocity at which (dynamic) aquaplaning will occur. However, the trade-off will be that with increasing tire pressure, the achievable wet friction will generally decrease in the speed range up to aquaplaning. Dynamic aquaplaning is experienced during the higher speeds of landing and take-off ground roll. As
little as 0.5 mm of standing water has been found to be sufficient to support dynamic aquaplaning. This relatively small depth can occur in heavy rain showers or can result from water pools due to surface irregularities.

There is still much to be learned regarding rubber reversion, but present thinking indicates that superheated steam is generated between the tire footprint and the runway surface at a temperature of approximately 200°C, which results in the melting of the affected area of the tire tread. One theory is that the melted rubber acts as a seal preventing escape of high-pressure steam. Following
incidents when rubber reversion is known to have occurred, white marks have been observed on the runway surface characteristic of the “steam cleaning” action. Reverted rubber aquaplaning can develop in any situation and at any speed where a tire is non-rotating (braked or unbraked) for a prolonged period of time. Accordingly, avoidance of wheel lock-up appears to be the important preventative measure in this case. Additional material on the viscous/dynamic aquaplaning theory is contained in Appendix 1.

Coefficient of friction

The coefficient of friction is defined as the ratio of the tangential force needed to maintain uniform relative motion between two contacting surfaces (aeroplane tires to the pavement surface) to the perpendicular force holding them in contact (distributed aeroplane weight to the aeroplane tire area). The coefficient of friction is often denoted by the Greek letter μ. It is a simple means used to
quantify the relative slipperiness of pavement surfaces.

Braking system efficiency

Modern anti-skid braking systems are designed to operate as near to the peak friction value (μ max) as possible. Aeroplane brake system efficiency, however, usually provides only a percentage of this peak value. The efficiency tends to increase with speed; tests on an older type of system on a wet surface gave values of 70 per cent at 56 km/h (30 kt), rising to nearly 80 per cent at 222 km/h
(120 kt). Even higher values have been claimed for the more modern systems. For anti-skid systems in use on many transport aeroplanes, the effective braking coefficient, μ eff, has been empirically established as:

and μ eff = 0.2 μ max + 0.7 μ max2 for μ max less than 0.7
and μ eff = 0.7 μ max for μ max = 0.7 or greater

Rolling resistance

Rolling resistance is the drag caused by the elastic deformation of the tire and a supporting surface. For a conventional, bias-ply, aeroplane tire, it is approximately 0.02 times the vertical load on the tire. For the tire to rotate, the coefficient of rolling friction must be less than the friction coefficient between the tire and the runway.

Friction/speed curves

Water is one of the best lubricants for rubber, and displacement of water and penetration of thin water films in the tire contact area take time. There are a number of runway surface parameters that affect the drainage capability in the tire contact area. If a runway has a good macrotexture allowing the water to escape beneath the tire, then the friction value will be less affected by speed.
Conversely, a low macrotexture surface will produce a larger drop in friction with increase in speed. Another parameter is the sharpness of the texture (microtexture), which determines basically the friction level of a surface.

As speed increases, the friction coefficients of the two open-textured surfaces A and D drop slightly, whereas the friction coefficients for surfaces B and C drop more appreciably. This suggests that the slope of the friction/ speed curve is primarily affected by the macrotexture provided. The magnitude of the friction coefficient is predominantly affected by the roughness of the asperities, A
and B having a sharp microtexture, C and D being smooth. From the friction point of view, therefore, runway surfaces should always provide the combination of sharp and open textures. A friction/speed curve is, therefore, indicative of the effect of speed on the wet surface friction coefficient, particularly if it includes higher velocities, i.e. approximately 130 km/h (70 kt) and over.

Surface texture

The surface texture between the tire and the runway depends on a number of factors, such as speed, surface texture, type of runway contamination, depth of contamination, tire rubber compound, tire structure, tire tread pattern, tread surface temperature, tire wear, tire pressure, braking system efficiency, brake torque, wheel slip ratio and season of the year. Some of these factors have
effects on each other, and their individual effect on the magnitude of the friction coefficient varies in significance. The parameter, however, that determines most significantly the magnitude of achievable wet friction and the friction/speed relationship is runway surface micro/macrotexture. Additional information on the influence of surface micro/macrotexture characteristics on tire friction performance is given in Appendix 2.