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Introduction 1. Friction
1.1. Condition of a wheel rolling 1.2. Factors that influence friction
2 5 10
2. Friction measuring equipment
2.1. Regulations about the use of equipment
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3. Friction on the runways
3.1. Correlation with airplane stopping performance 3.2. Regulation about friction on the airport runways
29 29 33
4. Friction tester Dynatest
4.1. RFT 042
4.2. Tests carried out on airport runways
4.2.1. Tests performed on newly built track asphalt
4.2.1.1. Study about the variation of friction with load force equal to 100kg 4.2.2. Tests carried out on concrete airport pavement
4.3. Tests carried out on road
4.4. Elaboration of draft law about use of device
50 50 52 52 58 60 70 72 5. IFSTTAR
5.1. Tests performed analysis 5.2. IFSTTAR certification method
75 75 97
Conclusions 102
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Introduction
The thesis is based on the study in airport field of friction and of instruments that measure it. The introduction examines the physical phenomenon of friction and how this contributes to rolling motion of a wheel on the surface of the flooring; in particular, we have analyzed the factors that, in rolling motion, come into play and affect friction of the pavements: climate, materials and running speed. The climatic factors are related to temperature but, above all, to the presence of snow and ice water; material factors consist of the pavement characteristics (micro and macro texture), and of the tire tread. Current instruments for friction measurement are listed; it is made clear how they work and what kinds of measures carried out. We have also described the laser profilers, thanks to which we can measure the texture of the pavement.
The devices for friction reading continuously differ between them for the method by which investigations are conducted. In the thesis we have also summarized the regulations concerning the use of such instruments. These regulations are intended to standardize the survey of the measure, since friction phenomenon is influenced by many variables.
The specific study of the thesis focuses on importance of friction in airport field. We have analyzed international and national regulations governing friction measurements, particularly Annex 14 ICAO and ATP 10A.Friction should respect the minimum values required by the regulations both, when the runway is of new construction, to check that it has been performed in an optimal manner, both to verify the rubber buildup and tire wear due to traffic. The main factor determining a decrease of friction coefficient is the presence of contaminants which is constituted on airport runways by the rubber that aircraft tires release during landing. At touchdown it occurs most of the rubber loss due to the high speed of the aircraft while its wheels have zero speed on the contact point with the pavement: the amount of rubber deposited on the floor at this stage is very considerable. Rubber, besides cancelling the macrotexture, blocks spaces that allow the flow of water in case of rain (reduction of macrotexture). So presence of rubber on the airport runway affects friction when pavement is dry, but even more when pavement is wet. The regulations indicate how often the measurements should be performed depending on the number of landings on the runway and even where they need to be performed, that is, on which side of the longitudinal strip. Friction conditions are transmitted to the pilots on landing, even though these indications are treated differently by different countries.
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In this thesis we have studied a new high-performance measuring device of friction, described in its mechanical parts and in its operation. By this device we performed tests on airport runways both asphalt and concrete. On the asphalt runways the tests were carried out on 4-parallel alignments to 3 meters and 6 meters from the center line as required by law; the test speed was 65 km/h constant on all the 3000 mt. of the runway. As regards the concrete pavement, the tests were performed on 6 parallel longitudinal alignments corresponding to the 6 transverse plates which constitute the central part of the runway width. After practice on field data were used to verify whether friction of the flooring meets the minimum values required by the regulations and to evaluate the quality of the data. As for compliance with the rules, the Italian regulations, referring to ICAO, Annex 14, fixes minimum friction values for newly built airport runways.
To assess the quality of the data the device gives back to the operator, in a first analysis, we have both verified the uniformity of results, both compared these results to values of the load on the wheel. The device is able to read continuously the load on the measuring wheel; other measuring instruments work with a constant vertical load equal to 1000 N. The collected data is also compared with the texture data. The comparison of the values of friction, measured in wet conditions, with the texture data shows the relationship between ETD and friction coefficient. These data were compared on the concrete airport runway and on a road section under investigation, for which it was possible to make a double survey (macrotexture-friction) under the present work.
Friction data were also compared with the regularity of the longitudinal profile (IRI parameter); significant correlations for the case examined did not appear.
By comparing the available data we can see there is no correlation between IRI trend and vertical load measured during the friction survey, or on airport runway, or on road pavement.
In this thesis we have followed the certification procedures established by the National Board for the French Civil Aviation. This certification is required for all the instruments operating in the airports located in French territory. The certification is performed on a test circuit located in Nantes. The circuit is built in order to perform friction tests at different speeds and on different surfaces. In fact there is a long straight on which all the vehicles, one at a time, can reach the maximum speed required by the tests at 90 km/h and keep it constant when they are on the surface under sampling. Data acquisition process consists of detecting friction coefficient by the device under certification and by two other sample instrument. The sample device is IMAG: it is a drawn carriage which the measuring wheel is placed on. The measurement is performed with the
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constant slipping of the wheel at 14% (like to RFT 042). The surfaces, which data collection is performed on, are six, that are distinguished from one another by texture, composition and kind of inert. Among these kinds of pavements there are also surfaces, made of resins and paints, characterized by very low friction values: every surface is characterized by a different friction coefficient in order to cover the whole range of possible values. The first calibration test is performed on a pavement covered by epoxy resin at a speed of 40 km/h; the value of friction coefficient on this surface will have to be close to zero. Afterwards, on each surface we’ll realize 5 passages for each speed, for a total of 90 tests. Reprocessing data consists in identifying the most relevant data of each pavement and performing an average of the friction coefficient values for a total of 90 values. Later detected by RFT 042 instrument data were sent to IFSTTAR for certification and statistical comparison with the data in their possession, acquired by IMAG, and to check the existence of a mathematical relationship between the two instruments. During these tests we performed a preliminary data analysis based on checking coefficients and we discarded the passages where the value was out of the average; in fact in some sections six or seven passages were performed instead of five passages required. An initial certification is based on the comparison of friction values measured at different speeds for all pavements. We evaluated the accuracy of friction coefficient data by Z statistical test with the standard deviation imposed by STAC. The second study was conducted about repeatability of the values supplied by the device and it is based on the dispersion of results. The range of the acceptable results is constituted by the 5% limit value given by Fisher test. Passed Fisher test too, RFT 042 has been found compatible with the required value of measurement repeatability.
Once we have verified the collected data met the statistical tests, we performed a correlation between friction coefficients (between those detected by the examined device and those detected by the reference one) in order to establish a mathematical relationship that allows to switch from CFLimag to CFLRFT042. Through the correlation formula we were be able to define values of friction,
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1. Friction
1
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1 Condition of a wheel rolling
The forces acting during the motion of a wheel with a moment applied about the axis of it are The forces R of resistance to motion and the tangential reaction force, whose point of application is in the contact between the road and the tire , parallel to the motion.
The rolling motion is composed of the motion of rotation around its axis and the translation of the same parallel to the motion.
Distinguishing the driving wheel from the driven wheel, it is analyzed the motion.
Driving wheel
If the tensile stress is less than the resistance and less than the tangential reaction force, there is no motion; if the effort is smaller than the resistance but, at the same time, traction is greater than the tangential reaction, then you have no translation but only a rotational motion around its own axis: the wheel slips. The last case, in which the tensile force is greater than the resistance to motion but less than the tangential reaction, then you have simultaneously translation and rotation, that is, the rolling motion.
Driven wheel
In this case we consider a friction moment applied to the wheel axis; we have rolling until the resistant moment is less than the tangential reaction multiplied by the arm, equal to the radius of the wheel; otherwise you have skidding wheel.
The reaction force, which flows between wheel to plan viable, is called “friction force” and, as just shown by the balance of forces, has a key role in the movement of the wheel; since it represents a frictional force it is certainly proportional to the weight:
fa: friction coefficient
Pa: adherent weight
It ‘s called “adherent weight” the weight bearing down on the axle to which the moment is applied.
Fig. 1: performance of the pressure exerted by pneumatic
Friction coefficient is of complex definit
The main components, that come into play, are the friction between the wheel and the pavement and the tire hysteresis. The adhesion originates from the shear strength of the molecular bonds that are formed when the rubber is forced by pressure to a close contact with the surface; hysteresis derives from the deformation of the tire around the rolling stock plan asperities. Therefore the friction force is the sum of the force of hysteresis and adhesion, w
with the adherent weight, turn into their respective coefficients.
The two components are always present in the contact between tire and pavement, but they give a greater or lesser contribution depending on the variables involved. With a
free of roughness the hysteresis contribution is almost nil and friction force coincides with adhesion force; in the case of wet rough pavement adhesion is close to zero and thus the greatest contribution is given by hysteresis.
It ‘s called “adherent weight” the weight bearing down on the axle to which the moment is
performance of the pressure exerted by pneumatic on the pavement
Friction coefficient is of complex definition because of the many variables on which it depends. The main components, that come into play, are the friction between the wheel and the pavement and the tire hysteresis. The adhesion originates from the shear strength of the molecular bonds rmed when the rubber is forced by pressure to a close contact with the surface; hysteresis derives from the deformation of the tire around the rolling stock plan asperities. Therefore the friction force is the sum of the force of hysteresis and adhesion, w
with the adherent weight, turn into their respective coefficients.
The two components are always present in the contact between tire and pavement, but they give a greater or lesser contribution depending on the variables involved. With a
free of roughness the hysteresis contribution is almost nil and friction force coincides with adhesion force; in the case of wet rough pavement adhesion is close to zero and thus the greatest contribution is given by hysteresis.
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It ‘s called “adherent weight” the weight bearing down on the axle to which the moment is
on the pavement
ion because of the many variables on which it depends. The main components, that come into play, are the friction between the wheel and the pavement and the tire hysteresis. The adhesion originates from the shear strength of the molecular bonds rmed when the rubber is forced by pressure to a close contact with the surface; hysteresis derives from the deformation of the tire around the rolling stock plan asperities. Therefore the friction force is the sum of the force of hysteresis and adhesion, which, in relation
The two components are always present in the contact between tire and pavement, but they give a greater or lesser contribution depending on the variables involved. With a dry pavement and free of roughness the hysteresis contribution is almost nil and friction force coincides with adhesion force; in the case of wet rough pavement adhesion is close to zero and thus the greatest
Fig
Figura
A fundamental phenomenon of the rolling motion of a wheel , of which one must always take into account, is the sliding, which too has influence on friction. It highlights the fact that the rolling is always accompanied by a quantity of translational, in the event of braking, and by skidding, in case of acceleration.
Scorrimento con a>0:
Ψ=0 Pure rolling (it does not exist);
Ψ=1 Skidding: rotation around its axis,
Fig 2: Description of the adhesion phenomenon
Figura 3: Description of the hysteresis phenomenon
A fundamental phenomenon of the rolling motion of a wheel , of which one must always take into sliding, which too has influence on friction. It highlights the fact that the rolling is always accompanied by a quantity of translational, in the event of braking, and by skidding, in case
;
=0 Pure rolling (it does not exist);
=1 Skidding: rotation around its axis,
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A fundamental phenomenon of the rolling motion of a wheel , of which one must always take into sliding, which too has influence on friction. It highlights the fact that the rolling is always accompanied by a quantity of translational, in the event of braking, and by skidding, in case
Sliding with a<0:
Ψ=0 Pure rolling;
Ψ=1 Skidding: locked wheel.
As previously mentioned, friction coefficient depends, all other conditi sliding entity and it has the most influence with high values
Fig. 4: Trend of friction coeffic If also transverse forces act on the vehicl
have a transverse friction reaction, orthogonal to the rolling plane of the wheel. We thus have a longitudinal friction coefficient and a transversal one. The friction coefficient "fa" is not , stric speaking , the same in all directions; however, the small difference between the longitudinal and the transversal value can be neglected in practice and it can be assumed the hypothesis of polar symmetric (fa = fl = ft): the bond between the longitudin
transversal one "ft " can be represented by the friction ellipse that shows the trend of the friction coefficient as a function of the resultant of longitudinal and transverse forces applied to the tire.
The portion of the longitudinal and transverse traction grip y x , you can simultaneously deploy , is given by ellipse equation:
;
friction coefficient depends, all other conditions being equal, on the sliding entity and it has the most influence with high values of the friction coefficient.
rend of friction coefficient related to the value of slip
If also transverse forces act on the vehicle, for example the centrifugal force along a curve, we have a transverse friction reaction, orthogonal to the rolling plane of the wheel. We thus have a longitudinal friction coefficient and a transversal one. The friction coefficient "fa" is not , stric speaking , the same in all directions; however, the small difference between the longitudinal and the transversal value can be neglected in practice and it can be assumed the hypothesis of polar symmetric (fa = fl = ft): the bond between the longitudinal friction coefficient “ fl " and the transversal one "ft " can be represented by the friction ellipse that shows the trend of the friction coefficient as a function of the resultant of longitudinal and transverse forces applied to the tire.
of the longitudinal and transverse traction grip y x , you can simultaneously deploy , is
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ons being equal, on the of the friction coefficient.
e, for example the centrifugal force along a curve, we have a transverse friction reaction, orthogonal to the rolling plane of the wheel. We thus have a longitudinal friction coefficient and a transversal one. The friction coefficient "fa" is not , strictly speaking , the same in all directions; however, the small difference between the longitudinal and the transversal value can be neglected in practice and it can be assumed the hypothesis of polar al friction coefficient “ fl " and the transversal one "ft " can be represented by the friction ellipse that shows the trend of the friction coefficient as a function of the resultant of longitudinal and transverse forces applied to the tire.
The meaning of the friction ellipse? is extremely important because it allows to calculate, based on the coefficient of friction engaged in one direction, the one available in the orthogonal direction. In fact, between the tire and the pavement may develop, at the most, a force of adhesion
fa × P in any direction (unless the small difference that , as already mentioned above, it can be
neglected); this is broken down into its two components (along the direction of mo
transversely to it At) in order to evaluate the effective grip available to perform a specific maneuver. For example, if all the available adhesion
direction to brake (Alim = Al), there isn’t any transv
ellipse y = fl it follows x = 0) to compensate for any lateral forces. This means that, in case they arise (for example, a gust of wind or the need for a sudden steering), they will result in the loss of vehicle control. On the contrary, if all the available adhesion (
transverse direction (Alim = At) , for example to follow a curve , there is a longitudinal adherence reserve (putting in the equation of the ellipse
longitudinal forces. Even in this case, therefore, if they arise (for example the need for a sudden braking ), will result in the loss of vehicle control.
Figura 5: Trend of friction
The variables that determine the friction, as we have said, are several: a fundamental variable is the condition of dry or wet surface. The coefficient of adhesion, with wet surface, decreases significantly while the hysteresis coefficient is insensitive to the presence of water.
The meaning of the friction ellipse? is extremely important because it allows to calculate, based on the coefficient of friction engaged in one direction, the one available in the orthogonal direction.
tire and the pavement may develop, at the most, a force of adhesion in any direction (unless the small difference that , as already mentioned above, it can be neglected); this is broken down into its two components (along the direction of mo
) in order to evaluate the effective grip available to perform a specific maneuver. For example, if all the available adhesion Alim = fa × P is used in the longitudinal ), there isn’t any transverse grip reserve (putting in the equation of the ) to compensate for any lateral forces. This means that, in case they arise (for example, a gust of wind or the need for a sudden steering), they will result in the loss of
hicle control. On the contrary, if all the available adhesion (Alim = fa × P
) , for example to follow a curve , there is a longitudinal adherence reserve (putting in the equation of the ellipse x = ft it follows y = 0) to compensate for any longitudinal forces. Even in this case, therefore, if they arise (for example the need for a sudden braking ), will result in the loss of vehicle control.
rend of friction longitudinal and transversal coefficient related to the value of sli
The variables that determine the friction, as we have said, are several: a fundamental variable is the condition of dry or wet surface. The coefficient of adhesion, with wet surface, decreases
ficantly while the hysteresis coefficient is insensitive to the presence of water.
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The meaning of the friction ellipse? is extremely important because it allows to calculate, based on the coefficient of friction engaged in one direction, the one available in the orthogonal direction. tire and the pavement may develop, at the most, a force of adhesion Alim= in any direction (unless the small difference that , as already mentioned above, it can be neglected); this is broken down into its two components (along the direction of motion Al and ) in order to evaluate the effective grip available to perform a specific is used in the longitudinal erse grip reserve (putting in the equation of the ) to compensate for any lateral forces. This means that, in case they arise (for example, a gust of wind or the need for a sudden steering), they will result in the loss of
Alim = fa × P ) is used in the
) , for example to follow a curve , there is a longitudinal adherence ) to compensate for any longitudinal forces. Even in this case, therefore, if they arise (for example the need for a sudden
ient related to the value of slip
The variables that determine the friction, as we have said, are several: a fundamental variable is the condition of dry or wet surface. The coefficient of adhesion, with wet surface, decreases
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1.2 Factors which influence the friction
Factors influencing the friction can be distinguished between climatic factors (temperature, water, snow) and those due to mechanical adhesion (tire, road pavement). The variables are also collaborating with each other to help the final friction.
Temperature
Temperature is one of those factors affecting friction in the short term; it is classified as seasonal variation of the grip. The effects of temperature are divided into three different components: air temperature , pavement temperature, air temperature. The studies performed in the laboratory in order to control the temperature, checking with different ratios of sliding and different types of surfaces , have analyzed the effect on the coefficient of friction when the temperature changes. Keeping constant temperatures for the tire and for the air and by varying only the temperature of the pavement has been found a reduction of the coefficient increasing the temperature. For temperature variations from 0 to 60° has been checked a mean decrease of the friction coefficient of 0.11, in the case of porous conglomerate, a greater decrease for a SMA pavement up to 0.19.
By varying air temperature and leaving constant the temperature of the tire and the pavement, in any case one finds a decrease of the friction coefficient, but to a lesser extent. It is not affected by the sliding or the type of pavement: the decrease of the coefficient, around 0.08, is due to the hysteresis loss.
The third case provides a constant temperature of pavement and air and variation of the tire temperature: also in this case the decrease of the coefficient, produced by increasing in temperature, is indifferent with respect to the sliding and to the type of pavement. La variazione negativa del coefficiente di aderenza è pari a circa a 0,05 per un aumento di 10°C.
Finally, increasing the temperature, there is a reduction of the friction coefficient.
Water presence
Friction decrease drastically in the presence of water. The value of the coefficient , in the presence of water,, is also influenced by road surface, vehicle speed and wheel tread. When it is present, the water, trapped between the tire and the superstructure, goes into pressure thereby transferring part of the vertical load. In correspondence with the area occupied by the fluid
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tangential efforts will not be transferred. One can distinguish different types of contact depending on the amount of water present: with a thickness larger than 1 mm, the contact is said wet, while with thickness less than 0.1 mm , the contact is said wet. In both cases, the water prevents the direct contact between wheel and road.
The drainage capacity of a pavement and therefore the decrease of the thickness of the water film is due to the projections of the surface and to the macro - texture. Even the presence of voids inside the pavement helps the drainage allowing the storage, below the surface, of a part of precipitated rain. A part of the drainage is also recognized by the tire tread which is however inadequate to ensure a sufficient grip at high speeds. The theoretical interpretation of the phenomenon involves the formation of three zones under the footprint of the tire , characterized by different behaviors.
With reference to Fig . 5 , in Zone 1, the impact of the tire with the surface of the fluid generates a pressure sufficient to win the inertia. A significant amount of fluid is ejected or pushed through the tread pattern and the interstices offered by the macrotexture of the pavement. The entire surface of Zone 1 has a thin film of fluid that is interposed between the tire and the pavement. The pressure that is generated in this area is the main cause of two phenomena:
1. The contact surface of the tire undergoes an inflection inwards, the extent of which depends on the inflation pressure, the feed rate and the vertical deflection of the tire.
2. In the phase of rolling it occurs a forward displacement of the center of pressure with reference to the tire / superstructure contact. The magnitude of the displacement of the center of pressure from the static position (at the axis of the wheel) increases with speed, with the height of the layer of fluid and with its density. In the braking phase, it generates a further redistribution of the tensions, such that the center of pressure moves back.
Ultimately , the center of the pressures generated in the contact area , in conditions "wet " , can be in front or behind the axis of the wheel.
Zone 2 is a transition region. After most of the fluid has been moved, a thin film remains between the tire and the pavement. In the back of Zone 1 and Zone 2 viscous effects prevent a quick removal of the fluid. Such viscous effects are also needed to maintain pressure in the fluid. The thin film is broken at the points where the local pressure is high , for example in correspondence
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of the surface roughness. In the presence of certain fluids with lubricating effects, the friction coefficient of the rubber on hard surfaces decreases significantly compared to the condition "dry".
The tangential force, that can be generated in the presence of a fluid film, is low. In the model we don’t consider the contribution of the force that is generated in this section.
Zone 3 is the region in which a dry contact predominates; in this region it originates the greatest contribution to friction force. The trend to slipping, highlighted by the tread elements positioned in the rear part of the contact area, can be increased by the presence of the fluid at the sides of the contact area.
In "wet " conditions, the friction coefficient between tire and superstructure depends on the relative sizes of the zones 1, 2 and 3. These are influenced by: surface roughness , temperature of the contact zone, fluid layer thickness, fluid density, fluid viscosity, the tread pattern, hysteresis properties of the tire, tire inflation pressure, how long a tread element takes to flow through the contact area.
Figure 6 shows the effect of feed rate on the relationship between the extension of Zones 1, 2 and 3.
Fig. 6 refers to the effects of a higher feed rate than which Fig. 5a relates to, so that the Zone 1 extends further back within contact area, while the Zone 2 and 3 occupy a shaped region of the horse's hoof in the back. In Fig. 5c, at an even higher speed, the contact with the superstructure is almost completely lost. In these conditions, the tire develops a force of minimum grip. In Fig. 5d , finally, the tire is moving at a speed such that the Zone 1 extends throughout the area of contact. In this case no longer a dry contact is maintained with the surface and aquaplaning occurs.
The aquaplaning phenomenon occurs when separated by a layer of liquid.
Figura 6: Acquaplaning
The aquaplaning phenomenon occurs when between the wheel and the surface of contact remain
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We can discern:
The viscous aquaplaning triggers for the residual water veil thicknesses (thin layer of water remained on the pavement after that most of the fluid has alr
rolling) less than two millimeters and can be caused both by an insufficient texture (micro and / or macro) of the road surface and by a high degree of wear of the tire (poor depth of grooves) and it is independent of the speed. Zone C does not exist (there are Zone A and B).
The dynamic aquaplaning triggers for each thickness of the film of water higher than 2 mm, when it is exceeded the corresponding critical speed (also called speed of aquaplaning trigger). The contact area between tire and road surface is completely occupied by water under pressure (the value of this pressure is higher than the value of the tire inflation pressure), for which there is only the area A (the absence of B and C zone). The wheel is lifted fr
glide over the water film. There is no friction mechanism
Figura 7: Trend of friction coeffic
The viscous aquaplaning triggers for the residual water veil thicknesses (thin layer of water remained on the pavement after that most of the fluid has already been moved from the tread in rolling) less than two millimeters and can be caused both by an insufficient texture (micro and / or macro) of the road surface and by a high degree of wear of the tire (poor depth of grooves) and it
speed. Zone C does not exist (there are Zone A and B).
The dynamic aquaplaning triggers for each thickness of the film of water higher than 2 mm, when it is exceeded the corresponding critical speed (also called speed of aquaplaning trigger). The rea between tire and road surface is completely occupied by water under pressure (the value of this pressure is higher than the value of the tire inflation pressure), for which there is only the area A (the absence of B and C zone). The wheel is lifted from the road surface and free to glide over the water film. There is no friction mechanism.
rend of friction coefficient related to the value of velocity on wet pavements
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The viscous aquaplaning triggers for the residual water veil thicknesses (thin layer of water eady been moved from the tread in rolling) less than two millimeters and can be caused both by an insufficient texture (micro and / or macro) of the road surface and by a high degree of wear of the tire (poor depth of grooves) and it
The dynamic aquaplaning triggers for each thickness of the film of water higher than 2 mm, when it is exceeded the corresponding critical speed (also called speed of aquaplaning trigger). The rea between tire and road surface is completely occupied by water under pressure (the value of this pressure is higher than the value of the tire inflation pressure), for which there is om the road surface and free to
15 Texture
Fig. 8: Texture
The texture of the pavement is given by the variation in the height of the road surface profile due to the chaotic arrangement of aggregates present and to the different sizes of the same, as the ISO 13473 cites: "the deviation of the actual surface from a reference plane".
The microtexture is characterized by a variation of less than 0.5 mm; it represents the roughness of the individual stone elements used in the surface layer of the pavement. The roughness is generated by micro asperities present on the visible surface of the protruding aggregates on the surface and depends on both the petrographic and mineralogical nature of the elements , both the resistance to crushing of rocks, from which they have originated and then by the angularity of the crushed piece; for some mixtures , such as the concrete and the bituminous conglomerate, it also depends on the roughness of the mortar. The microtexture is sensitive to traffic action, atmospheric agents and weather and its persistence depends on the resistance to polishing of aggregates. Effects on friction at low speeds and on tire wear are essentially associated to the microtexture. The main role played by this kind of texture on the grip performance is the ability to penetrate the tread rubber, producing locally very high contact pressures and a true sliding friction force. In the presence of wet contact surfaces, the intervention of the micro roughness ensures the breaking of the water film that is interposed between the tire and the road , allowing the resulting contact "dry " interface. Ultimately the microtexture qualifies the contact between wheel and road.
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Macrotexture means deviation from the reference surface for sizes ranging from 0.5 to 50 mm; it consists in the roughness due to all the intergranular bumps, to the shape, size and positioning of the stone elements rising above the conglomerate; in the case of concrete pavements it is constituted by depth, spacing, width, by the symmetry and direction of any streaks. This weaving class is also connected to the way of putting in work and to the composition of the mixture; its durability comes from the mineralogical characteristics of the stones which inerts are made of. The tread of the tire, at the actual points of contact with the road surface, is slightly deformed by the macro surface roughness, for the order of magnitude 0.2 -0.4 mm; then the macrotexture is the main cause of deformation, due to hysteresis of the tire rubber, and the consequent development of horizontal reaction forces that oppose the wheel slip. The macrotexture is considered optimal, from the point of view of skid resistance, if its height is in the range 0.7-1.2 mm and the average distance between the particles of conglomerate tips varies between 6.5 and 12 mm. The macrotexture identifies the roughness of the floor which determines the surface drainage conditions , the deformation of the tread, the grip at high speed and finally the acoustic properties. At high speeds, because it becomes difficult and difficult to penetrate the water film in the time available, friction largely depends on the deformation component; the roughness must be sufficiently large to deform the tire even in the presence of a layer of water. The presence of asperities reveals therefore an essential characteristic for safety at high speeds and on wet coat because, on the surface of the pavement, it forms a more or less dense and continuous network of channels which, together the cross slope of the road platform, allows the disposal of rainwater, thus optimizing the contact between tire and pavement.
Vehicle speed
Speed greatly affect friction in wet pavement. We have already seen in the graph in Fig. 8 how the adhesion decreases significantly as the speed increases up to reach a critical speed after which aquaplaning occurs. Not considering the water, with the increase of speed, a friction decrease happens however: in special situations of the pavement, even without the presence of water, we can certify a sharp declines of the coefficient.
Relationship for friction/velocity/water
In this chapter we analyze in detail the variation of friction coefficient in the presence of water at different speeds and with different kind of pavement and the tread.
The effects of the three variables, above mentioned, are ex
the contact between wheel and pavement (through the tread pattern and the texture of the superstructure), which requires a certain time because it occurs. With the increase of the speed time decreases that a tread element takes to run through the tire contact area / superstructure and consequently available time decreases because these phenomena can take place.
Through the graphic below, which relates friction coefficient with the speed and with different surfaces, it can be noted that, with an open granulometry, you have bigger friction values but, speed increasing, there is bigger decay compared to a more closed texture, which, however, has smaller friction values anyway. It can also be deduced that , to check the ef
macrotexture , we must run at different speed tests
Figura 9: Trend of friction coeffic
Let’s come back to the definition of viscous and dynam
to the critical speed. If we define the speed of aquaplaning as that corresponding in the graph to the maximum point of the curve , which represents the trend of the resistance offered by the fluid to speed, this will coincide with the speed , at which , the tire stops rolling. It has been experimentally shown that the critical speed, at which the phenomenon of aquaplaning begins, is a function of the tire inflation pressure.
The phenomenon of aquaplaning is an u
speed, before the wheel returns to the rolling motion, the speed of the latter must be lowered well below the critical speed at which the phenomenon begins.
The effects of the three variables, above mentioned, are explained with the expulsion of water in the contact between wheel and pavement (through the tread pattern and the texture of the superstructure), which requires a certain time because it occurs. With the increase of the speed ement takes to run through the tire contact area / superstructure and consequently available time decreases because these phenomena can take place.
Through the graphic below, which relates friction coefficient with the speed and with different can be noted that, with an open granulometry, you have bigger friction values but, speed increasing, there is bigger decay compared to a more closed texture, which, however, has smaller friction values anyway. It can also be deduced that , to check the efficiency of the surface macrotexture , we must run at different speed tests
rend of friction coefficient related to the value of velocity and different pavements
Let’s come back to the definition of viscous and dynamic aquaplaning previously seen, particularly to the critical speed. If we define the speed of aquaplaning as that corresponding in the graph to the maximum point of the curve , which represents the trend of the resistance offered by the fluid s will coincide with the speed , at which , the tire stops rolling. It has been experimentally shown that the critical speed, at which the phenomenon of aquaplaning begins, is a function of the tire inflation pressure.
The phenomenon of aquaplaning is an unstable phenomenon and, once it reaches the critical speed, before the wheel returns to the rolling motion, the speed of the latter must be lowered well below the critical speed at which the phenomenon begins.
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plained with the expulsion of water in the contact between wheel and pavement (through the tread pattern and the texture of the superstructure), which requires a certain time because it occurs. With the increase of the speed ement takes to run through the tire contact area / superstructure and consequently available time decreases because these phenomena can take place.
Through the graphic below, which relates friction coefficient with the speed and with different can be noted that, with an open granulometry, you have bigger friction values but, speed increasing, there is bigger decay compared to a more closed texture, which, however, has ficiency of the surface
ient related to the value of velocity and different pavements
ic aquaplaning previously seen, particularly to the critical speed. If we define the speed of aquaplaning as that corresponding in the graph to the maximum point of the curve , which represents the trend of the resistance offered by the fluid s will coincide with the speed , at which , the tire stops rolling. It has been experimentally shown that the critical speed, at which the phenomenon of aquaplaning begins, is
nstable phenomenon and, once it reaches the critical speed, before the wheel returns to the rolling motion, the speed of the latter must be lowered
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2. Friction measuring device
In the first paragraph we described the importance and the factors that influence the friction in the rolling motion of a wheel on a pavement. In this section we’ll examine the tools used to measure the friction coefficient and those to measure the texture of the pavement. The
instruments for profile measuring can be divided between those that give an indirect response and those for direct measurement. The indirect measurement instruments measure the kinematic effect produced by the interaction between vehicle and floor on the car itself. The latter are called metric profile: they give back the road profile for points in a given interval.
.
Instruments for the detection of weaving
Skid tester
The skid tester consists of a pendulum with a strip of rubber at its end crawling on the surface previously dampened. The difference in height of the gravity center of the crawling end, between the horizontal position at the beginning of test and the highest position reached after the release, is used to calculate the energy loss due to friction phenomena. The skid tester then detects the sliding friction of the surface at a given point.
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Sand patch texture meter
Macrotexture point measurements are carried out using the method of the sand height that consists in placing above the road surface a certain volume of sand fine and uniform, capable of covering all of the surface irregularities. Performing a circle and calculating the area, through the relationship between volume and area, you get the average height of the texture.
Profilers
Fig. 10: Profilometers
The profilometers are devices, which use non-contact technologies, capable of providing a digital reading of the dimension of the macrotexture, which still represents an approximation of the real profile. The most used profilers are those in laser operation.
As for the laser profilometer, it consists of two basic elements: a source of emission and a sensor - transducer. The data output is an amplified signal of the height “h” that is the distance between the source of emission and the point of incidence of the laser on the pavement.
The measuring tool is located on a vehicle which, advancing along a direction parallel to the longitudinal axis of the street plan, allows to associate to each of the measured heights the value of the covered horizontal distance, for which one can obtain the graphic restitution of two-dimensional z (x) profile.
The definition of a parameter that uniquely identifies the regularity of road surfaces, is of fundamental importance for the classification of pavings. In the past, several indices have been proposed, in relation to specific issues, with the aim of identifying an indicator of road regularity that would provide the level directly relevant to study the phenomenon, but the aspect that has
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attracted greater attention is the swing of the vehicle traveling on a pavement not regular, for the analysis both of the dynamic load transmitted to the pavement, both of the oscillation of the passenger. The first aspect is connected directly to the damage caused to the road pavement, the second one to the driving comfort, as regards vehicles, and to the integrity of the goods carried, as regards lorries. The definition of an index of these characteristics has been proposed by the World Bank in 1986 and called the International Roughness Index (IRI). IRI is defined as the ratio between the sum of the relative vehicle-wheel displacements made by a standard vehicle that travels at the speed of 80km/h along a road route and the length of that path.
Friction measuring Instruments
The direct friction measurement instruments solve the problem of the numerous variables, on which this phenomenon depends, through the analysis of the forces directly employed. The tools, called Continuous Friction Measurement Equipment, are all based on the measurement of the
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forces to hold a standardized wheel braking on wet pavement, while the vehicle travels at a constant speed.
The various devices can be distinguished in 3 families depending of the relief method of adhesion: locked wheel, wheel with variable or fixed scroll, oblique wheel.
The first two methods allow the estimation of the longitudinal coefficient through the measure of the force applied to the wheel; the third method allows to obtain transverse friction coefficient. The results will be different due to the different methods of analysis.
Blocked wheel devices
Locked wheel devices provide the skid number (SN); it is based on one or more driven wheels or incorporated in the vehicle which are kept locked while the force of friction, that develops between paving and wheel, is measured. The test consists in measuring the force of drag determined by the locking of the wheel on wet pavement maintaining constant translation speed and load conditions on the wheel. The axis of the wheel will be perpendicular to the motion and to the pavement, the wheel will be parallel to the other median wheels. During the test frictionis measured.
The test results are reported as skid number SN = 100xfn. The procedure is standardized in ASTM E274 "standard method for Skid Resistance of paved surfaces using a full-scale tire" in which the tire too is normalized.
The friction properties are expressed using Friction Number FN, which represents the estimate of the longitudinal friction coefficient of the SN, which is defined as the ratio between the force to drag the locked wheel and the actual load on the wheel.
FN=F/Wx100
The locking of the wheel is carried out for a period ranging between one second and three seconds. The values are collected and a mean value is calculated.
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Adhera
The device combines the results of tests done at different speeds to determine the coefficient of longitudinal adherence. The measurement is performed with a standard car wheel, above which a vertical load is applied, pulled by a means at constant speed while it is pushed by water just before contact wheel-road. Before the measurement, the wheel begins to roll for a distance of about 20 meters. The measuring wheel and the brake system are carried by a rigid beam which is close to the components of a machine. The payload consists of a structure attached to the beam by a suspension system; the towing vehicle must have the water tank and the system for spraying the water to the different speed of the vehicle. During testing the truck and tractor are rigidly connected, the measures are read continuously by the operator. The ADHERA measures the CFL (coefficient friction longitudinal equivalent).
Oblique wheel devices
These instruments are capable of measuring the transverse friction coefficient (CAT) through the arrangement of the measuring wheel, inclined by a certain angle respect to the direction of motion. Transverse friction coefficient is very influenced by the angle that the wheel forms with the direction. The maximum coefficient in function of the angle of drift is around 15 degrees, so it is preferable to perform measurements with inclination of the wheel in the points where a difference in angle doesn’t involve a big friction difference.
Some devices that use this method are: SCRIM and Mu METER.
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It is one of the most used means for the relief of the friction conditions of the pavements especially in the airport field; it allows you to continuously measure the strength of transverse friction. Such force is determined through the analysis of the force which tends to separate the two symmetrically inclined wheels respect to the axis of the instrument. The tool consists of small cart with three wheels, wherein two wheels are gauge. The overall weight of the cart is 250 kg. The towing vehicle will be equipped with apparatus for data detection and with the water tank. The two wheels are placed at 7.5 degrees to the axis of symmetry towards the outside; each wheel carrying a load of 76 kg, the third wheel, in addition to equilibrate the system, measuring speed and distance traveled. The two measuring wheels have standardized smooth tires with a tire pressure of 0.7 kq/cm^2, the third wheel has a road tire and a pressure of 2.1 kg/cm^2. In front of each measurement wheel will be thrown water equal to 1.2 liters/min at a speed of 65 km/h; the water surface area affected is 25 mm wider than the footprint of the tire.
The returned values are in terms of Mu Number which is a function of the force of removal of the two wheels: depending on the value, the quality of the pavement is identified.
SCRIM
Lo scrim (sideway force coefficient routine investigation machine) is an instrument consisting of a truck on which a wheel is mounted, a container of water and all the essential tools for the measurement. The means is equipped with a tank of 2750 liters which allows the speed of 60 km/h h for about 70 km.
With the SCRIM it is possible to measure the transverse friction coefficient (CAT): it is defined as the ratio between the force N, perpendicular to measuring wheel rotation plane (with an angle of about 20° respect to the direction of motion) and the load P, acting on the wheel which is free to rotate, because it is not braking in any way. A complex of shock absorbers ensures that the load weighs continuously on the wheel. The water flow, at the 60 km/h, must be not less than 0.95 l/sec.
Measurement is performed for lengths of about 20 meters for an acquisition speed of 60 km/h; 10 meters for the speed of 70 km/h. The data is collected every 10 cm and an average is done for each measuring meter.
They are very sensitive to the pavement irregularities that can quickly ruin the tires. The instrument often requires checks to be calibrated and recalibrated.
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The values determined by the tests are then collected to identify the quality of the flooring according to the CAT.
Partially braked wheel devices(fixed-slip ASTM E2340)
Partially braked wheel devices, with fixed or variable flow, are the most common devices and have been produced in various models.
The tool consists of a standardized wheel, in front of which water is sprayed in a well-defined amount; the wheel is maintained at a constant sliding by a chain, which refers to the freewheels, or by the brakes. The sliding usually is fixed to the point of maximum of the curve friction - sliding, that is around 14% and it can generally varies from 12% to 20%.The wheel and the measuring instruments can stay on the same vehicle or the wheel can stay on a cart pulled. The collected data are the load weighing on the wheel and the resistance to dragging, which allow to calculate the longitudinal friction coefficient, defined by the average of the partial values, correlated with speed, temperature and quantity of water delivered. The measures are shown as friction coefficient and as FN. The tools that are able to control the load on the wheel are considered the best; this generally occurs in the devices that mount the measuring wheel directly in the main vehicle. So they can continuously control the load on the wheel by hydraulic systems, having as mass of contrast the total weight of the vehicle.Some devices use the partially blocked wheel: skiddometer, Griptester (UK), runwayfriction tester (USA), Dww trailer (NL), ROAR (Norway).
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The skiddometer BV11 is a device for measuring the surface friction coefficient in a predetermined slip condition (like in automotive systems ABS) and equal to 17%. It has two driving wheels and a measuring wheel; It can be equipped with two different types of measuring wheel: a low pressure tire for measuring on wet road and a high pressure tire for snow or ice-covered surfaces. For optimal performance, the Skiddometeris designed to operate with measuring wheel both at high and at low pressure. The central measuring wheel is subject to a constant stress of 1030N. It is possible to obtain a continuous monitoring through the direct recording of the horizontal resistance of drag (F) and of the vertical load weighing on the measuring wheel (P); the friction coefficient (F/P)*100, so evaluated is called Breaking Force Coefficient (BFC). Measurements are performed continuously and processed to obtain a mean value of the friction coefficient of the road surface every 50 m of analyzed lane.
Grip Tester
It is also of a cart being pulled. On the tow they are placed the water container and the data processing systems. The tester grip has a transmission that allows to act on the measuring wheel blocking sliding to 14.5%, or even locked wheel. The acquisition, which may occur with the recording of the horizontal drag force and the vertical force of the weight that weighs on the measuring wheel, allows the calculation of the friction coefficient given by the ratio between the two forces: that coefficient, so assessed, takes the name of number grip (GN).
The test speed can vary from 5 Km/h to 130 Km/h. For the airport flooring we have Indexes for 65 km/h and 95 Km/h, while for road paving the measuring speed is 50 km/h. The used tire is smooth and standardized by ASTM E1844-96.
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Saab friction tester
The Sarsys Friction Tester (SFT), built around a car, the Saab 9-5 family, is one of the most advanced systems to measure the adherence that there are on the market. It is designed specifically for the friction tests in the airport field, for runways and taxiways, but can be used for the calculation of the adherence even in the road field. The SFT is programmed to take measurements in accordance with rules laid down by authorities such as ICAO and FAA. The medium provides excellent performance and excellent maneuverability.
The measuring wheel in this tool is directly incorporated in the motorized means, together with the water tank and to all the measurement and data processing systems. The wheel is located almost in line with the axis of the rear wheels, which are non-driving. For the measurement of wet runways, the SFT mounts a tank that has a volume of 580 liters, sufficient for 7000 meters of track with 1mm film of liquid in front of the wheel. The test speed can vary from 25 km/ h to 165 Km/h; the instrument can mount both the high pressure wheel (700 kPa) or low pressure (ASTM E1551) (200kPa). The measurement is performed with a fixed slip that can vary from 13% to 17%.
Dynatest 6875 Runway Friction Tester
The DYNATEST 6875 is a device composed of a high power four-wheel drive pick-up, on which is mounted the water cistern, the measuring wheel and the data acquisition systems. The data of the traction force and of the load on the measuring wheel are read continuously. The DYNATEST 6875 meets the ICAO standards and FAA for adherence measures. The water tank may contain 1000 liters of water with which one can analyze 11000 meters of airport runway with a 1 mm liquid thread in front of the wheel.
The 6875 is a model that cannot be sold in Europe because of the laws on emissions of pollutants that the vehicle does not comply.
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Dynatest Runway Friction Tester 042
The RFT 042 is European version of the 6875; the device is mounted on a Ford Transit. Of this one we will study in detail its components later.
List of devices to measure the adherence
ASTM E-274 Trailer Lockedwheel BritishPortable Tester Slider DagonalBrakedVehicle (DBV) Lockedwheel
DFTester Slider
DW'W Trailer Fixedslip
Griptester Fíxed slip
IlvfACi Variablefixed slip
JapaneseSkid Tester Lockedwheel KomatsuSkid Tester Variablefixed slip
LCPC Adhera Locked Wheel
MuMeter Side force
Norsemeter Oscar Variable slip Norsemetel ROAR Variable slip Norsemeter SAUIAR Variable slip
Odoliograph Side force
Polish SRT-3 Locked Wheel
RunwayFïction'Ièster Fixedslip Saab Friction Tester (SFf) Fixedslip
SCRIM Side force
Skidclometer BV-8 Locked Wheel Skiddometer BV-l I Fixedslip
Stradograph Side force
StuttgarterReibungsmesser (SRM) Fixedslip, Locked Wheel
2.1 Regulations about the use of equipment
All measuring instruments and various types of them are normalized in order to do surveys. As mentioned in the first chapter, the variables for determining friction coefficient are many, so many standardizations are required for having comparable parameters. Therefore we will have rules both on the proof itself that the instrument makes, either on the setting of the instrument to perform measurements.
We list in this section only some of the rules concerning equipment. In the next chapter we look at the rules about the use of Airport field instruments.
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Measuring wheel
For testing on rain-wet or artificially wetted surfaces, the tread should be smooth with a pressure of 70 kPa for yaw-type friction-measuring devices; the tire must meet the specification contained in ASTM E670. With the exception of the Grip Tester, braking slip friction-measuring devices must use smooth tread tires made to ASTM E1551 specification and inflated to 210 kPa. The Grip Tester uses a tire made to ASTM E1844 Specification. For loose, wet or dry snow or compacted snow and ice covered surfaces, a trade pattern tire meeting ASTM E1551 specification, with a pressure of 700 kPa, should be used for fixed braking slip devices.
Device Measuring Tires Type
RFT 30 psi ASTM E1551
ASFT 30 psi ASTM E1551
SFT 30 psi ASTM E1551
Grip T. 20 psi ASTM E1844
BV-11 30 psi ASTM E1551
RUNAR 30 psi ASTM E1551
Mu-Meter 30 psi ASTM E670
The reference standards for each type of tire specify:
Materials and Manufacture;
Material Requirements;
Construction, Dimensions and Permissible Variations;
Workmanship;
Test Methods;
Precision and Bias;
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3. Friction on the airport runways
In this chapter we look at the importance of friction in Airport field and the rules in use, the controls for monitoring the friction, as a function of the traffic, and the types of action that you can use to partially restore the grip levels.
3.1 Correlation with airplane stopping performance
It is necessary to first determine the correlation between the friction data produced by the friction-measuring devices and the effective braking friction performance of different airplane types. Once this relationship is defined for the airplane flight crew should be able to determine airplane stopping performance for a particular runway landing operation by considering the other factors including touchdown speed, wind pressure/altitude and airplane mass, all of which significantly influence the stopping performance. At present, there is general agreement that success in this respect is greater for the compacted snow and/or ice covered surface conditions since fewer parameters affecting tire frictional behavior are involved compared to the more complex and variable wet runway case.
Statistical studies have shown the correlation between accident rates and friction coefficient. In Airport field the World Ascend Aircraft Accident has conducted investigations over a period of 10 years, identifying 141 accidents due to runway excursions occurred between 1998 and 2007. Of 141 accidents 120 took place during landing. The factors that lead to this kind of accidents mainly consist of crew errors, problems of aircraft, weather conditions and track conditions.
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Out of 120 accidents took place during landing, 77 are due to side-track outputs or long landings. Most of these have occurred in case of wet from rain track (including aquaplaning); however no incident is attributable to the ice.
The major cause that leads to this kind of accident is an inadequate grip in wet conditions and a bad drainage of the pavement. Good runway drainage is important to provide skid-resistance, improved runway friction, dissipate standing water, and prevent aquaplaning on water-affected runways.
Drainage can be assisted by: Runway cambering; Provision of adequate runway surface macrotexture by means of a suitable friction treatment (such as runway grooving or porous asphalt); Maintaining a runway surface free of irregularities such as depressions.
Landing on Contaminated Runways involves increased levels of risk related to deceleration and directional control. An approach to land on a contaminated runway requires a fully stabilized final approach and a firm (but not hard) touchdown within the prescribed touchdown zone. If either is not achieved, a go around or rejected is appropriate. The challenges of achieving a successful contaminated runway landing are such that there should be no indecision in either case.
Touchdown vertical speed needs to be sufficient to break through the layer of contaminant and find at least some friction so that wheel rotation speeds can reach normal levels quickly. This is necessary so that they will exceed the minimum required to prevent operation of the anti skid-system. A theoretical target for touchdown rate of descent is in the range 2 - 3 feet per second/120 - 180 fpm. Once main gear touchdown has occurred, de-rotation should start and thrust reverser deployment should occur. Both actions will increase wheel loading, which will ensure the achievement and/or continuation of wheel rotational speeds sufficient to allow lift spoiler deployment and brake activation. Deceleration is a function of both wheel spin up and braking efficiency. Once manual or automatic braking begins, its efficiency may also be indirectly affected by use of thrust reversers/reverse pitch and the manual or automatic deployment of lift spoilers. Spoiler activation will also be constrained by aircraft-on-ground logic and probably also by a wheel rotational speed - although usually a lower one than that needed to allow brake application. Absence of sufficient deceleration during a contaminated runway landing is much more likely to be due to low wheel rotational speeds than to brake system failure, (unless there are specific annunciations of this and/or related prior indications which have initiated doubt as to brake system integrity). Any memory drill action to select emergency braking channels should
therefore only be followed strictly in accordance w effects is likely to be the de-activation of the anti
locking the wheels; on surfaces contaminated with liquid water, this increases the risk of reverted rubber aquaplaning. Effective directional control, on a contaminated runway surface during landing, requires that all wheels are firmly on the ground without undue delay and that the control column/sidestick is then promptly centralized both longitudinally and laterally, so as to avoid inducing asymmetric main gear wheel loading and achieve adequate nose landing gear wheel loading. Once rudder effectiveness is lost at lower speeds, directi
a contaminated surface may increase, in contrast to what would be expected on a landing roll on a normal friction surface. This is because: The effects of even minor differential manual braking are likely to be greater; Thrust Reversers/Reverse Pitch are likely to be more de
nose landing gear wheel adhesion directly limits both steering input options and the usual directionally-stabilizing effect of the nose landing gear; Yaw effects arising from any differe braking effectiveness are exaggerated.
Having said all this, we understand the importance of being aware of the surface state of the track, at the level of micro and macro texture, in the presence of atmospheric precipitation. This information must be communicated, during landing and take off, to the crew so you can set all the parameters for a proper maneuver safely.
The monitoring and control operations are regularized by the various national and international bodies (ICAO, EASA, CAA).
Factors that reduce friction
therefore only be followed strictly in accordance with the associated criteria, since one of the activation of the anti-skid system and an attendant increased risk of locking the wheels; on surfaces contaminated with liquid water, this increases the risk of reverted . Effective directional control, on a contaminated runway surface during landing, requires that all wheels are firmly on the ground without undue delay and that the rol column/sidestick is then promptly centralized both longitudinally and laterally, so as to avoid inducing asymmetric main gear wheel loading and achieve adequate nose landing gear wheel loading. Once rudder effectiveness is lost at lower speeds, directional control difficulties on a contaminated surface may increase, in contrast to what would be expected on a landing roll on a normal friction surface. This is because: The effects of even minor differential manual braking are
Reversers/Reverse Pitch are likely to be more de
nose landing gear wheel adhesion directly limits both steering input options and the usual stabilizing effect of the nose landing gear; Yaw effects arising from any differe braking effectiveness are exaggerated.
Having said all this, we understand the importance of being aware of the surface state of the track, at the level of micro and macro texture, in the presence of atmospheric precipitation. This e communicated, during landing and take off, to the crew so you can set all the parameters for a proper maneuver safely.
The monitoring and control operations are regularized by the various national and international
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ith the associated criteria, since one of the skid system and an attendant increased risk of locking the wheels; on surfaces contaminated with liquid water, this increases the risk of reverted . Effective directional control, on a contaminated runway surface during landing, requires that all wheels are firmly on the ground without undue delay and that the rol column/sidestick is then promptly centralized both longitudinally and laterally, so as to avoid inducing asymmetric main gear wheel loading and achieve adequate nose landing gear onal control difficulties on a contaminated surface may increase, in contrast to what would be expected on a landing roll on a normal friction surface. This is because: The effects of even minor differential manual braking are Reversers/Reverse Pitch are likely to be more de-stabilizing; Reduced nose landing gear wheel adhesion directly limits both steering input options and the usual stabilizing effect of the nose landing gear; Yaw effects arising from any differential
Having said all this, we understand the importance of being aware of the surface state of the track, at the level of micro and macro texture, in the presence of atmospheric precipitation. This e communicated, during landing and take off, to the crew so you can set all the
