Chapter 1
General
Note-
The terms contaminant and debris are
used in this manual with the following meanings. A contaminant is considered to
be a deposit (such as snow, slush, ice, standing water, mud, dust, sand, oil,
and rubber) on an airport pavement, the effect of which is detrimental to the
friction characteristics of the pavement surface. Debris is fragments of loose
material (such as sand, stone, paper, wood, metal and fragments of pavements)
that are detrimental to aero plane structures or engines or that might impair
the operation of aero plane systems if they strike the structure or are
ingested into engines. Damage caused by debris is also known as FOD (foreign
object damage).
1.1
INTRODUCTION
1.1.1 There is general concern over the adequacy
of the available friction between the aero plane 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 aero plane take-off or landing speeds are high.
This concern is more acute for jet transport aero planes since the stopping
performance of these aero planes is, to a greater degree, dependent on the
available friction between the aero plane 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, aero plane directional control may become
impaired in the presence of cross-wind under such operating conditions.
1.1.2
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 aero planes 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 run ways is contained in the Airworthiness Technical
Manual (Doc 9051).
1.1.3
Further, it is essential that adequate information on the runway surface
friction characteristics/aero plane 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 water and the runway becomes slippery when
wet, the pilot should be made aware of the potentially hazardous conditions.
1.1.4
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 aero plane performance.
1.2 IMPORTANCE
OF RUNWAY SURFACE FRICTION CHARACTERISTICS/AEROPLANE BRAKING PERFORMANCE
1.2.1
Evidence from aero plane overrun and run-off incidents and accidents indicates
that in many cases inadequate runway friction characteristics/aero plane
braking performance was the primary cause or at least a contributory factor.
Aside from this safety-related aspect, the regularity and efficiency of aero
plane 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
1.2.2 Adequate runway friction characteristics
are needed for three distinct purposes:
(a) deceleration
of the aero plane 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.
1.2.3
With respect to either aero plane braking or directional control capability, it
is to be noted that an aero plane, even though operating on the ground, is
still subject to considerable
aerodynamic or other forces which can affect
aero plane braking performance or create moments about the yaw axis. Such
moments can also be induced by
asymmetric engine power (e.g. engine failure on takeoff),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 counter
acting these forces or moments. In the case of directional control, all aero
planes are subject to specific limits regarding acceptable cross-wind
components. These limits decrease as the runway surface friction decreases.
1.2.4
Reduced runway surface friction has a different significance for the landing
case compared with the rejected take-off case because of different operating
criteria.
1.2.5
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 aero planes) and for obtaining the best
possible steering capability. Moreover, the armed auto spoilers which destroy
residual lift and increase aerodynamic drag , as well as the armed auto
brake 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.
1.2.6
Generally, aero plane certification performance and operating requirements are
based upon the friction characteristics provided by a clean, dry runway
surface, that is, when maximum aero plane braking is achievable for that
surface. A further increment to the landing distance is usually required for
the wet runway case.
1.2.7
To compensate for the reduced stopping capability under 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.
1.2.8
To alleviate potential problems caused by inadequate runway surface friction,
there exist basically two possible
approaches:
(a) provision of
reliable aero plane performance data for take-off and landing related to
available runway surface "friction/aero plane braking performance;
and"
(b) provision of adequate runway
surface friction at all
times and under
all environmental conditions.
1.2.9
Tile 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 aero plane braking performance.
This applies in particular to the wet runway case.
1.2.
10 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.
1.3
NEED FOR ASSESSMENT OF RUNWAY SURFACE CONDITIONS
1.3.1
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; Runways should also be evaluated when first constructed or
after resurfacing to determine the wet runway surface friction characteristics.
1.3.2
The above situations may require the following approaches on the part of the
airport authority:
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);
1.4
CONTAMINANT DRAG
1.4.1
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 aero plane
deceleration.
1.4.2
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.
1.5
EXPLANATION OF TERMS
1.5.1
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 qualitative manner.
Percentage
slip
1.5.2
Brakes in the older aero plane 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 aero plane speed of
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.
1.5.3
The importance of this curve from the view point of runway friction coefficient
measurement is that the value at the peak of the curve (termed g 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 wet
runways, the measured value can be used as an assessment of the friction
characteristics of the runway when wet.
Locked
wheel
1.5.4
The term "1ocked wheel" is exactly as implied and the friction
coefficient U 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 U
max attained at the optimum slip.
Tests have shown that for an aero plane tire, U skid varies
between 40 and 90 percent of U 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
1.5.5
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.
1.5.6
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 U max. Depending on tire pressure, stiffness
(construction) and speed, the relationship between side force and yaw angle
will vary.
"Normal" wet friction and aquaplaning"
1.5.7
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 aero plane 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. Further guidance on water depth and its influence on aquaplaning is
contained in 2.1.
1.5.8
The typical reduction of friction when a surface is wet and the reduction of
friction as aero plane speed increases are explained by the combined effect of
viscous/dynamic water pressures to which the tire/surface is subjected.
Figure
1-1. Relation between percentage slip and friction coefficient on a wet runway
Figure
1-2. Typical variation of side friction coefficient with yaw angle
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 U 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.
1.5.9
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 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.
1.5.10
Dynamic aquaplaning will occur beyond 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 macro texture 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
1.5.11
There is still much to be learned regarding rubber reversion, but present
thinking indicates that super heated steam is generated between the tire
footprint and the runway surface at a temperature of approximately 20WC, 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
1.5.12
The coefficient of friction is defined as the ratio of the tangential force
needed to maintain uniform relative motion between two contacting surfaces (aero
plane tires to the pavement surface) to the perpendicular force holding them in
contact (distributed aero plane weight to the aero plane tire area). The
coefficient of friction is often denoted by the Greek letter U.It is a
simple means used to quantify the relative slipperiness of pavement surfaces.
Braking
system efficiency
1.5.13
Modern anti-skid braking systems are designed to operate as near to the peak
friction value (Umax) as possible. Aero plane 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
U eff = 0.2 U max + 0.7 U
max 2 for U max less than
and U
eff = 0.7 max for U max = 0.7 or greater
Rolling
resistance
1.5.14
Rolling resistance is the drag caused by the elastic deformation of the tire
and a supporting surface. For a conventional, bias-ply, aero plane 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.
Figure 1-3. Areas
of tire/surface interface
Friction/speed
curves
1.5.15 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 macro texture allowing the water to escape beneath
the tire, then the friction value will be less affected by speed. Conversely, a
low macro texture surface will produce a larger drop in friction with increase
in speed. Another parameter is the sharpness of the texture (micro texture),
which determines basically the friction level of a surface, as illustrated in
Figure 1-4.
1.5.16
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 macro texture provided. The magnitude of the friction coefficient is
predominantly affected by the roughness of the asperities, A and B having a sharp
micro texture, 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
Surface texture
1.5.17
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/macro texture. Additional information on
the influence of surface micr/macrotexture characteristics on tire friction
performance is given in Appendix 2.
Figure
1-4. Relationship between braking friction coefficient achieved with anti-skid
braking on different textured surfaces at certain operating conditions