Abstract

Tire skid marks on runways create a hazardous landing surface and a safety concern.  A rotating tire upon touchdown could prevent skid marks.  An innovative new tire design could be the solution to excessive skid marks.  This design extends tire life, decreases “burning rubber” pollution, and alleviates the jarring ground transition.  The FAA, EPA, and the airlines would likely support this implementation.

 

Skid Solution: Eliminating Runway Skid Marks

Chapter 1

The Problem

     Airport markings are very distinct and very recognizable to pilots.  Most markings are wanted, but some create hazards to aircraft.  One such zone of markings is the black skid marks in the touchdown zone at larger airports.  Black skid marks pose several hazards as relating to aircraft safety.  A few markings pose no threat, but when a few turns into many, serious issues arise.

A precision approach touchdown zone has many distinct markings.  The Jeppesen Instrument Commercial textbook (2006) lists several markings on a precision runway: runway number, centerline, threshold markings, side stripe, touchdown zone markings, 1,000 foot aiming point markings, and length markings to 3,000 feet (p.3-2). Other markings include blast pads, displaced thresholds, and all usually include runway lighting.  Black tire skid marks often begin to cover any of the markings in the touchdown zone.  This portion of the runway has very important landing information for the pilot—especially in bad weather.  Without seeing these markings, pilots may as well be landing on an unmarked road.  An analysis document published by the Federal Aviation Administration (2007) lists “Airport signage and markings” as the 11^th of 33 leading causes of “Wrong Runway Contributing Factors” (p.16). Beside the superseding human factors, the next portion capable of change is the design and layout of the runways.

Runways are a rather permanent airport feature.  Redesigning and repositioning runways would be irrefutably more expensive than improving signage and marking.  The existing markings and signs are made to be very obvious in broad daylight, but they are designed that way for bad weather.  Pilots are supposed to check runway markings before beginning the take-off roll, but many are distracted with multiple pressures.  A black ambiguous rubber zone detracts from the clarity of these runway markings.  The majority of skid marks do not usually cover the number markings, but can obscure other critical markings such as the centerline or distance markings.  The FAA report (2007) concludes that with other advancements, “…enhanced surface markings offer significant risk elimination” (p.30). Keeping current markings visible contributes significantly to this conclusion.

The majority of skid marks obscuring runway markings occur at airports with larger traffic influx, namely international airports.  This report will focus on alleviating the skid mark problem particularly at larger airports and larger aircraft.

Runway Design

Runways are specially designed to provide the maximum braking, and allow moisture to flow off easily.  Build up of rubber skid marks on a runway surfaces actually creates a surface different than the intended design.  Early studies at the Langley Research Center in 1968 eventually yielded to an FAA advisory circular (1997) that concluded, “Pavement grooving was the first major step in achieving safer pavement surfaces for aircraft operations in wet weather conditions” (p.7). Keeping these grooves clean and clear is critical for the water to flow off.  Runway skid marks can fill up these grooves, allowing water to puddle on the runway.

Wikipedia (2009) gives an accurate description of runway surfaces and moisture combating friction:

Runway pavement surface is prepared and maintained to maximize friction for wheel braking. To minimize hydroplaning following heavy rain, the pavement surface is usually grooved so that the surface water film flows into the grooves and the peaks between grooves will still be in contact with the aircraft tires. To maintain the macro-texturing built into the runway by the grooves, maintenance crews engage in airfield rubber removal or hydro-cleaning in order to meet required FAA friction levels. (Wikipedia, 2009)

Since the majority of aviation accidents happen during take-off and landing, it is wise to consider the criticality of the runway in that environment.  The runway surface is the main concern for the safety of an existing runway.  A couple runways will be examined to illustrate.  A History Channel documentary on runways (2002) describes how critical the surfaces of aircraft carriers are:

Keeping [aircraft carrier landing] failures to a minimum today has a lot to do with paying constant attention to the surface of these cruising airstrips. Skid problems are a huge concern, more so than on conventional runways since the aircraft carrier exists in an all wet environment that can pitch and roll at any time.  We need to have an environment where the planes can move safely on the flight deck and provide good traction.  Every eighteen months the steel plated runway is resurfaced with non-skid; a texturized coating made up of metal and sand.  The application of this special surface takes weeks to put on.  It has to be stripped down to bare metal, and then once the environmental conditions are right, we will lay this down with very specialized paintbrushes to ensure that the grain is correct and it takes about four days to cure to the required hardness so this will stay up to the pounding of aircraft and the tail hooks slamming down on the surface (pt.3).

This same documentary on runways (2002) indicates the special design of a most critical runway at the KennedySpaceCenter: “The Kennedy space center shuttle landing runway was originally designed with an incredibly rough surface in order to provide a greater anti-skid if moisture were on the runway” (pt.4).  Tough enough to combat up to three inches of rain an hour, which would flood any other runway in the world (History, 2002, pt.4). The space center’s runway is extremely rough because the shuttle only gets one chance to land.  A wet runway could cause the demise of this two billion dollar aircraft (History, 2002, pt.4).  Scientists at Langley discovered a balance between a smooth slippery surface verses an overly rough runway surface that would cause great tire-wear. (History, 2002, pt.4)  Aviation airports currently use texture and grooves for braking reasons, but skid remains can frustrate the purpose of these designs.

Current Rubber Removal Methods and Problems

Speidel Construction Inc. (2002) published an article on Airfield rubber removal and the causes for needing such:

The material that accumulates on the runway surface is no longer rubber like that on the tires of the airplane that put it there; that rubber is relatively soft and flexible and designed to absorb some of the shock of the landing aircraft. The aircraft tires are stationary just before they touch the ground, but at the moment they touch, literally where the rubber meets the runway, and for about 1000 feet or 300 meters, that tire is gaining rotation speed. During that time, the tires are under thousands of pounds of pressure between the tire and the surface, right at the interface, that causes considerable friction and heat. This time is referred to as “spin up speed”. The heat created causes a polymerization of the rubber, or a chemical reaction, turning it into a very hard material that is spread on that 300 meters of runway surface in a thin layer. Seven hundred grams of rubber, or about 1.4 pounds, are deposited per tire per landing of each large aircraft, such as a 747 or an L-1011. With repeated landings of aircraft, this hardened rubber fills the micro-texture of the pavement giving it a smooth, almost glass like surface that can make landing the aircraft and stopping difficult, or even dangerous, particularly when the pavement is wet. It will also begin to fill the macro-texture of the pavement surface, which will diminish the ability of the grooves to adequately drain the water during a rain event, increasing the likelihood of hydroplaning. (Speidel, 2002, p.3)

Hydroplaning is when a tire separates from the runway surface to glide on top of the standing water, creating a no steering or braking environment.  Without the texture and grooves, the wet runway would be like a dangerous Slip and Slide® waiting for disaster.  The article by Speidel (2002) explains:

The most significant reason to be concerned about the build up of rubber deposits is safety.  When an aircraft skids off of the runway, whether the pilot is unable to brake soon enough upon landing or when having to abort a takeoff, the pavement surface will be carefully examined as a contributing factor. (Speidel, 2002, p.2)

Speidel (2002) describes five major methods for rubber removal techniques: high-pressure water-blasting, ultra-high pressure water-blasting, chemical removal, high velocity impact removal {shot-blasting}, and mechanical removal.  All these methods require the closure of the runway, deterring or eliminating the possibility to use it in an emergency. High pressure ideally requires multiple vehicles for cleaning.  Ultra-high pressure water-blasting “cleans the rubber deposits and anything else within its path, leaving a very clean, but often smoother surface” later described as a “polishing effect” (Speidel, 2002, p.5). This very well means that the paint is also removed which will require reapplication, requiring a longer runway closure.  Speidel’s publication (2002) claims that chemicals have been developed that are environmentally safe, yet are not biodegradable, nor is the debris left behind.  The article continues to refute the environmentally safe chemical by saying,

The method of clean up for the chemical removal operation is to flush off the soapy residue at the completion of the work, along with the rubber debris, onto the shoulders and into the surrounding soil. Over time the debris accumulates and may eventually cause an environmental problem requiring remediation (p.5).

Chemicals also require a curing time, “…once the process begins, the runway cannot be reopened until the process is complete because it is ‘soapy’ and slippery” (Speidel, 2002, p.6). The chemical process also costs twice as much as other processes (Speidel, 2002, p.6). A shot-blaster, “better suited for paint removal operations”, retextures the pavement along with rubber removal by using abrasive particles, loose contaminants, and dust.  Speidel claims however that this process “is very expensive to mobilize and operate” (p.6). This process also may very easily remove the paint on the runway, creating a longer closure for repainting.  The mechanical removal grinds 1/8 to 3/16 of an inch off the runway surface, which will “smooth out a rough ride”.  This process also requires a prolonged runway closure as the surface needs new paint and new grooves to allow moisture runoff (Speidel, 2002, p.6).

Chapter 2

Prevention

Prevention of rubber skid deposits or surface contamination completely alleviates any need for extensive rubber removal.  If the tires were spinning prior to touchdown, rubber deposits on the runway would be drastically reduced.  The frictional heat from the dramatic inertial change of a non-rotating tire contacting runway would also be eliminated, creating a dramatically longer tire life.

A 747 has 4 tires per landing gear assembly with individual tire loads on around 50,000 pounds (History, 2002). It is apparent why this creates large skid marks and a big skid problem with so many aircraft.  The 1.4 pounds of polymerized rubber deposited per tire per landing of each large aircraft not only negatively affects the runway, but also the life of the tire.  History Channels documentary (2002) says that a 747 tire costs around $800 and can only touchdown 250 times before needing replacement (pt.4). There is a great difference between these tires and those of the Kennedy space center shuttle landing tires.

The shuttle tires can only be used once because the surface of the runway is “designed with an incredibly rough surface in order to provide a greater anti-skid if moisture were on the runway.” (History, 2002) Ultimately, if little to no wear and tear occurred on aircraft tires upon touchdown, runways could be outfitted with rougher more skid resistant surfaces in the future.

When a non-rotating tire touches a runway, there is a jolt in the aircraft as the tires are spinning up to match the airplane groundspeed.  If the tires were already spinning upon touchdown, the transition from air to ground would be less jolting.  Passengers would experience greater comfort without this jarring transition.  The jarring effect also imposes stresses on the aircraft.  This smoother transition allows greater constant controllability for pilots.

When an aircraft touches down there is a plume of smoke that comes from the tire spinning up.  This is more dramatic with large, heavy, and fast airliners.  The smoke comes from the non-rotating tires contacting a fast runway; an effect commonly referred to as “burning rubber”.  Of all the aircraft around the world “burning rubber”, the contaminating effect is not much different than setting fire to a tire recycling plant once in a while.  Fast spinning tires touching a respectively fast moving runway do not create “burning rubber” smoke.  This small change is positive for the environment.

Another advantage to a rotating tire is that of icing conditions.  Ice cannot freeze to a moving object.  If an aircraft is landing in icing conditions, moving tires may help prevent ice buildup on the tires or landing gear.  The tires will already be rotating, creating friction and heat.  The short amount of time rotating will not be enough to melt ice, but may be enough to help prevent its formation.

Chapter 3

Innovation

In consideration that the majority of the problem areas occur at larger airports with larger aircraft, the concentration will be toward implementing a change in the large aircraft category.  There are two feasible solutions for alleviating this skid mark problem: wheel motors, and/or aerodynamically designed air scoops on the sides of the tire.

Motors

Small wheel motors could be outfitted to each tire or wheel in the landing gear configuration.  These motors would be wired directly to a power source, or auxiliary turbine driven.  They would also be wired directly to the air data computer and/or the global positioning system.  This would be the reference or control indication that would dictate the velocity of the tire rotation.  The motors would match the exact groundspeed of the aircraft upon touchdown as calculated by the already existing aircraft data.  This is very advantageous because the groundspeed can often vary during an approach because winds vary.  The motor would allow a very accurate match of rotation speed and a very smooth transition to the runway.

Another advantage to motors as opposed to air driven tire rotation is the constant source of power.  For instance, rain has a greater weight and density than air.  An air driven tire is designed to rotate with pure air, but if raindrops exist, the tire may spin faster than intended.

Disadvantages possible for the motors include the additional weight.  Though motors can be made small, a single motor would be required for at least ever set of wheels.  The Antinov-225 main landing gear has seven sets of two wheels per main landing gear.  This equals 14 motors added to the empty weight of the aircraft, or the weight before any payload is added (Khantsis, 2005).  The vast majority of aircraft have fewer wheels than this, but the added weight is a consideration.  This could lower the loading capacity of the aircraft, but only by a small amount, considering the large aircraft size.

The spin-up of the wheels would begin upon the automatic closing of a circuit after all the gear is in the down and locked position.  Upon landing, the motors would be automatically disengaged with an already existing switch known as a squat switch.  This requires no extra buttons or tasks for the pilots.  Depending on the design of the motor it is possible to also use it as a type of braking for slowing the aircraft down.

Tire Design

The second possibility in addition to or in place of the motors is a tire design with air scoops.  The concept is the same as a wind velocity indicator on a weather station, called an anemometer.  From a report of the one of the first weather observatories in the United Kingdom, Director Bailey (Bailey) credits Thomas Romney Robinson, astronomer and meteorologist, for constructing the first anemometer. (Bailey, 2002) A cup anemometer consists of three or four half spherical cups mounted on rods that rotate about a vertical shaft (Wikipedia, 2009). A measurement is taken of the average rotation speed and given as a wind velocity.

Tire Aerodynamics

The reason the cups rotate has to do with the shape directed into the wind.  The wind moving has energy that can be transferred to objects.  Objects, in turn, have a certain potential for collecting or transferring energy.  A paper cup has more potential for transferring water, or energy, than an upside-down paper cup.  Similarly, the open half sphere collects more wind than the sphere facing the opposite way.  The wind flowing into the open cup would be momentarily stagnated and its energy would be transferred to rotating the arm around which it is mounted.  The wind flowing over the concave, or closed side of the cup would be deflected around the sides, thus keeping its own energy.

These cups would be manufactured on the sides of the landing gear tire, the open scoop on the lower portion, facing forward.  Once the landing gear is extended, the moving air would begin to rotate the tire in a forward direction.  The complication comes with matching the rotational speed to the groundspeed of the aircraft.  Because there are no controllable variables in flight, the tires would need to be designed per the groundspeed of the individual aircraft make and model.  Tires of all different shapes and sizes exist, but the design variables are numerous.

The attributes affecting the velocity of rotation include, tire weight, axle lubrication, tire diameter, scoop positioning, scoop size or surface area, scoop shape and scoop orientation.  Similar to airframe design, all these factors are interdependent on one another.  For instance, for a smaller scoop to have the same force it would need to be in a different position.  These variables are all adjustable depending on the specific specifications of the aircraft.  A good balance of variables would be to calculate the rotation speed to be the lowest possible touchdown speed.  The precaution should err on the side of the lowest conceivable airspeed, in case of emergencies.  To create the desired speed, there are specific variables that can be altered in the design.

Newton’s second law states that an object in motion tends to stay in motion, or that an object at rest tends to stay at rest (Costello, 2009). This tendency is called inertia.  The weight of the tire indicates how much inertia must be overcome to begin rotation; greater weight equals greater inertia.  A few teaspoon sized scoops could not rotate a three hundred pound tire sitting at rest.

Specifications

To begin tire rotation, at least two things must be examined: air velocity and good axle lubrication.  First to be examined is, “At what airspeed does the gear normally extend?”  The constant for the design should be normal airspeed at which the pilot would extend the landing gear.  Emergency extension situations and airspeeds should also be factored into the design: “Will the pilot ever need to lower the landing gear at a lower than normal airspeed?”

Two critical air velocities exist: the airspeed required to begin the tire rotation, and the airspeed to yield the maximum tire rotation.  There needs to be a scoop large enough to begin the rotation of the tire, but not so large that it spins the tire faster than the landing speed of the aircraft.  Once the tire is rotating, a constant airspeed will yield what is called a terminal velocity.  This terminal velocity is the maximum rotation speed achieved for a given airspeed.  Since a constant airspeed is used by pilots for the entire approach, this terminal velocity speed must be near the groundspeed upon touchdown.

The second design factor is based upon axle lubrication: “How easily can the tire begin rotating?”  If there is any stickiness initial resistance to motion in the lubrication material, the scoop size will need to be larger, or the scoop positioning farther from the axle.

Scoop Placement: The distance away from the center of rotation or axle will change the force.  The farther the scoop is from the axle, the more effect it will have.  If for instance the tire is relatively small and the scoop is placed on the outermost part of the tire, another factor will need to be changed, such as the size of the scoop.

Scoop size is the most dramatic variable.  The force that is applied to wheel rotation will be the product of the placement and the size of the scoop.  The greater surface area the scoop has, the more effect it will have to transfer wind energy to tire rotation energy.  More surface area to contact wind also creates slightly more drag on the convex side.  The drag on the open cup side will always be greater than the drag on the closed cup side.

Different scoop shapes can be implemented if any one shape is found to be more effective for the given scoop placement or tire diameter.  Each shape has a coefficient of drag, or amount of energy it will catch in an air flow such as wind.  Jeff Scott, (2005) an aerospace engineer and chief editor of Aerospaceweb.org, gives drag coefficients for eleven different three dimensional shapes.

For instance, air flowing over a concave or open cup half sphere has a coefficient of 1.42, which is a valueless number.  Conversely, flow over the same convex or closed cup is only 0.38 with the same diameter. Mr. Scott’s tests found that the half sphere has both the greatest drag on the open side, and the lowest drag on the closed side (Scott, 2005). This shape, even split in half yields most effective.  This means that of all the shapes tested, it will require the smallest size or area required to produce the needed drag for tire rotation.

Scoop orientation refers to which the angle at which the cup is mounted on the tire.  This angle can vary the level of wind energy that is transferred to tire rotation.  For the most dramatic affect, the cup should be open directly perpendicular to the oncoming air.  The air will flow and come to a momentary stop at a point called the stagnation point.  This 90 degree orientation provides the best force because the stagnation point is most center in the cup at the most effective angle.  The stagnation point will have a constant change because the tires will be rotating constantly in the airflow.  This perpendicular orientation will give the greatest amount of time for the stagnation point to be on the innermost portion of the scoop for the most time.  This provides the maximum amount of energy to transfer from the wind to the tire rotation.  If the amount of rotation is too fast for the given airspeed, fine adjustments could reduce the air energy that is transferred.

Possible Flaws

Even with an effective design there may be some extremely unusual circumstances that would over rotate the tire.  Having a tire spinning faster than the ground would cause some to imagine a dangerous acceleration of the aircraft upon touchdown.  However, if the tires were rotating at a speed faster than the aircraft, the large mass of the aircraft would quickly match the speed of the ground.

For instance if a twenty knot headwind were encountered on approach for a design that was accidentally calculated for zero wind, the tires would be spinning twenty knots faster than what the aircraft actually lands.  This, however, is still an improvement to the large aircraft landing at high speed today.  A twenty knot difference is much smaller than a 140 knot difference that currently exists with large aircraft.  The spin-down time would be dramatically lower and still provide a greater measure of safety.

The configuration of some aircraft may have more constraints.  For instance, a landing gear that is not retracted into the aircraft after takeoff will have constant tire rotation.  This adds to the drag of the aircraft and decreases performance.  The design of the wheel will be configured for the landing portion of the flight, and might cause excessive speed during other portions of the flight.  This also will create unneeded friction on the wheel axles for the entirety of the flight.  Since the tires will have a greater width, the wheel wells of retractable landing gear may require more room to fit the wider tires.  On the other hand, the rubber exerts on the tire could be flexible and collapsible to allow little wheel well modification.  This flexibility could however decrease the durability.

All aircraft have different specifications, which makes it difficult to say exactly how every particular aircraft can implement this design.  Aerodynamic designs can be very particular for aircraft.  It would be convenient for the FAA to say that now everyone has to buy these new tires that save our runways, but because there are such a variety of aircraft, this may not be simple.  Considering that the mandated aircraft will be so large, it is unlikely that this addition will make a substantial difference on the aerodynamic design.  At worst, the landing approach may require slightly more power, well within the capability of these aircraft.  With the numerous design variables previously mentioned, all large aircraft manufacturers will be capable of implementing the new design.

Conclusion

The Federal Aviation Administration, also known as the FAA, has a goal to improve safety at airports.  Skid marks on the runway are a safety hazard for landing aircraft.  The FAA is therefore likely support movement toward eliminating such hazards.  A time frame could be created whereby all aircraft could comply.  The weight of large aircraft and the faster touch down speed point to these as the leading cause of skid marks.  The requirement mandated by the FAA only need apply to aircraft above 12,500 pounds.  The mandated change would require implementation to this new tire design, and adjust weight, balance and any configurations if necessary.  Considering the large mass of these aircraft, the change to configurations would be minimal.

The FAA also has a goal to promote the aviation industry.  A move to require these tires may promote positive growth in aeronautical engineering and aviation tire manufacturing.  The Environmental Protection Agency may also support this requirement because it will eliminate toxic rubber removing chemicals from being used, burned rubber smoke from being emitted into the air, and smoother landings when they need to travel.  The airlines may support the mandatory change because it creates lower long term costs for replacing tires so often, increases passenger comfort, and promotes a better safety record.

Runway skid marks are a standing safety hazard at airports.  An FAA mandated implementation of either motors or tires to be added to all large aircraft could eliminate skid marks on runways.  This implementation would increase safety, improve comfort, reduce long-term tire replacement costs, promote the aviation industry, and protect the environment.

The rotating tire design virtually eliminates skid marks on runways.  This design requires no additional tasks for pilots.  Pilots only need to be made aware that the new features exist.  After implementation, airports will no longer have rubber removal costs.  Long term costs will be cut for airports and airlines.  Passenger comforts are improved.  Safety is improved.

© J. Kyler Dalton      All Rights Reserved

References

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Costello, Dr. John. (11 June 2009). Newton’s First Law of Motion. Retrieved June 18, 2009, from

FAA. (18 March 1997). Advisory Circular. Retrieved June 15, 2009, from

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History Channel. (2002). Modern Marvels: Runways. Retrieved May 21, 2009, from http://www.youtube.com/watch?v=XSi8txfD2i4&NR=1

Jeppesen Sanderson. (2006). Guided Flight Discover: Instrument commercial.Englewood, CO: Jeppesen Sanderson, Inc.

Khantsis, Sergey. (2005). Antinov-225 main landing gear.jpg. Retrieved June 16, 2009, from

Scott, Jeff. (2005). Drag of Cylinders & Cones. Retrieved June 18, 2009, from http://www.aerospaceweb.org/question/aerodynamics/q0231.shtml

Speidel, Donna J. (February 2002). Airfield Rubber Removal. Retrieved June 16, 2009, from

Wikipedia. (2009). Runway. Retrieved June 15, 2009, from http://en.wikipedia.org/wiki/Runway#Pavement_surface

Wikipedia. (2009). Anemometer. Retrieved June 18, 2009, from http://en.wikipedia.org/wiki/Anemometer#cite_note-0