Lecture 03 and 04 ASII AIRCRAFT TIRES AND TUBES AIRCRAFT TIRES AND TUBES `TIRE CLASSIFICATION • Aircraft tires are classified according to their type, size, and ply rating and whether they use tubes or are tubeless. • The United States Tire and Rim Association has established nine types of aircraft tires, but only three of these types are of primary concern today. TYPE • Type III is the most popular low-pressure tire found today on pistonpowered aircraft. The section width is relatively wide in relation to the bead diameter. This allows lower inflation pressure for improved cushioning and floatation. Tube type • Type VII extra-high pressure tires are the standard for jet aircraft. They have exceptionally high load-carrying ability. Tube type • Type VIII tires are used for high performance jet aircraft with their extremely high takeoff speeds. They use extra high inflation pressure and have a low profile. Because of their strength and rigidity, such tires, whether tubed or tubeless, are normally fitted only to divided or detachable-flange wheels. Tube less AIRCRAFT TIRES AND TUBES TIRE SIZE • A tire size could be shown on a tire as follows • A only (for example, 44”), or • B-C only (for example, 8.50-10), or • AxB only (for example, 49x17), or • AxB-C (for example, 49x19.0-20) or • AxBRC (for example, 46x17R20) for radial-ply tires. • The size specifications shown above are the standards used for tires of new design. • Dimensions A, B and C could be inches or millimeters. For example, for a tire size specified as 750x23015, dimension A = 750 mm, dimension B = 230 mm, and dimension C = 15 in. • An inner tube size is specified the same as the size of its related tire. AIRCRAFT TIRES AND TUBES AIRCRAFT TIRES AND TUBES • • • • • • PLY RATING In the past, tires were rated for strength by the number of fabric plies used in the construction of their carcass. But newer materials have comparatively so much greater strength that fewer actual layers of material are needed to get the same strength. Therefore, now tires are given a ply rating, rather than specifying the actual number of layers of fabric material used in the carcass. The ply rating of a tire relates to its maximum static load and its inflation pressure. TUBE OR TUBELESS Aircraft tires are manufactured as both tube type and tubeless, with the basic difference between the two being the inner liner. Tubeless tires have an inner liner that is about one-tenth of an inch thick that serves as a container for the air, while tube-type tires have no such liner, but are somewhat smoother on the inside so the tube will not be damaged by chafing against the inside of the tire. Some of the advantages derived from the use of tubeless tires include about 7½% saving in weight compared with using a tire and tube, a reduction in permeability losses, cooler running by about 100C Tubeless tires are identified by the word TUBELESS on their sidewall, and the lack of identification signifies that a tube should be used in the tire. Because it is necessary to keep the bead areas in good condition, tubeless aircraft tires are not fitted to well-base wheels. AIRCRAFT TIRES AND TUBES TIRE CONSTRUCTION • Automobile and truck tires are required to operate for long periods of time, carrying a relatively large but steady load at reasonably high rotational speeds. Therefore these tires are allowed to have only a relatively small amount of deflection. • Airplane tires, on the other hand, must be strong enough to absorb the tremendous loads encountered on touchdown, and while they must operate at very high speeds, far higher than that of a car or truck, their ground rolls are of a limited duration. Because of these severe requirements, aircraft tires are allowed a deflection of more than twice that allowed for automobile tires. AIRCRAFT TIRES AND TUBES BEAD • The most important part of a tire is the bead, for it is the bead that anchors the carcass and provides a firm mounting for the tire on the wheel. Ultimately, all of the ground forces on the tire terminate in the bead. • The beads are made of bundles of high strength carbon steel wire, with one, two, or three of these bundles used in each side of the tire. • Rubber apex strips streamline the round bead bundles, so the fabric will fit smoothly around them with no voids AIRCRAFT TIRES AND TUBES AIRCRAFT TIRES AND TUBES CARCASS (BIAS TIRE) • The carcass of the tire is made up of layers or plies of rubbercoated nylon-cord fabric. This fabric is cut into strips on the bias, meaning that the cords of the fabric run at an angle of about 450 to the length of the strip. These strips extend completely across the tire and lap back over the beads to form the ply turnups. Each successive ply of the fabric is put on in such a way that the cords cross each other at about 90 degrees, so the strength of the carcass will be balanced. • Chafers of fabric and rubber wrap around the edges of the carcass plies and enclose the entire bead area. They also provide good chafe resistance between the bead and the wheel. • An undertread, which is a layer of specially compounded rubber, is placed over the carcass to provide good adhesion between the tread and the carcass • Tubeless tires are lined with a special compound of rubber that is less permeable than the rubber used in the rest of the tire. • In tube-type tires, a thin coating of rubber over the inner ply cords protects the tube from chafing. AIRCRAFT TIRES AND TUBES AIRCRAFT TIRES AND TUBES • CARCASS (RADIAL TIRE) • The casing of a radial-ply tire is not the same as that in a bias tire. Casing fabric plies of a radial-ply tire is laid so that the cord direction is from bead to bead (at approximately 900). (Figure A) • The breaker package (also known as the belt plies) is a number of plies attached below the tread of a radial-ply tire. The breaker package gives stiffness in the tread area, and helps to keep the circumference constant as the tire expands when it is inflated. AIRCRAFT TIRES AND TUBES • • • • THE TREAD The tread is the wearing surface on the outer circumference of the tire. It is made of specially compounded rubber and has a pattern of grooves molded into its surface to give the tire the required traction characteristics with the type of runway surface the aircraft will encounter. One of the problems encountered with modem jet aircraft is that of hydroplaning. This condition occurs when the tire rides on the surface of water or slush, rather than contacting the runway surface. The brakes stop the wheel, and the tire rides on the water, in much the same way as a water ski. When this happens, braking action is nil, and extreme heat is built up in a highly localized area of the tire. Hydroplaning is minimized by the proper design and placement of the grooves in the tread, so the water can pass under the footprint of the tire, and the tread can contact the runway surface. AIRCRAFT TIRES AND TUBES • • • • PLAIN TREAD The plain, or smooth tread was popular for tires used on airplanes with no brakes, or for aircraft whose brakes were used primarily as a taxi aid, rather than for slowing the aircraft in its landing roll. Today, this type of tread is found only on some helicopters and on very light airplanes. ALL-WEATHER TREAD A diamond-shaped tread pattern is effective for aircraft operating on grass or hard-packed dirt, but good braking action can be obtained on both hard surfaced and dirt runways with a tire having a rib tread in the center and diamonds molded into the shoulders. RIB TREAD This is the most popular tread pattern found on aircraft today. It is designed especially for use on hard surface runways and gives long tread wear, good traction, and exceptionally good directional stability. The width and depth of the grooves and their placement on the tread are factors used to adapt the tire to the operating conditions of the aircraft for which they are designed. AIRCRAFT TIRES AND TUBES AIRCRAFT TIRES AND TUBES DEFLECTOR • Jet aircraft with aft-mounted engines have a problem with water or slush being thrown up by the nose wheel and entering the engines, causing damage or flameout. To prevent this, tires used on the nose wheels of these aircraft have chines, or deflectors, molded in the upper sidewall to deflect the water or slush away from the engine intakes. Tires for dual nose wheel installations have chines on one side only, while single nose wheel installations have dual deflectors, one on either side of the tire. AIRCRAFT TIRES AND TUBES • • • • THE SIDEWALL The sidewall is a rubber covering that extends from the tread down to the bead heel to protect the carcass from such minor injuries as cuts or bruises and from exposure to moisture and ozone. Tubeless tires have an inner liner that is designed to hold air, but some air does seep through. If the sidewall trapped this escaping air in the body plies, it could expand when the tire was heated and cause ply separation and possibly allow the tread to be thrown from the tire. To prevent this, tubeless tires have vent holes in their lower with a green or gray spot and must be kept open when the tire is retreaded. Tube-type tires are also vented to release air that is trapped between the tire and the tube when the tire is mounted AIRCRAFT TIRES AND TUBES TIRE MARKINGS • Tires have certain markings imprinted on their sidewalls for identification purposes. These markings vary according to the manufacturer but usually include size, part number, serial number, date of manufacture, tubed/tubeless, speed rating, ply rating, and the type and number of retreads carried out PART NUMBER • The part number usually includes the manufacturer's identification, the drawing to which the tire is manufactured, and letters to indicate the tread type, and whether it is tubed or tubeless. The part number is the only positive means of identifying a tire, and size markings alone should not be used for this purpose. Example: DR 7153 T. SERIAL NUMBER • The serial number is usually marked in conjunction with the date of manufacture, which may be in the form of a code indicating the day, week, or month, and the year. Example: 2283 Nov 72 or 23202283. AIRCRAFT TIRES AND TUBES SPEED RATING • Most high-speed tires (those which may be used at speeds over 160mph) have the speed rating imprinted on the tire to indicate the maximum speed for which they are designed, e.g. 200mph. OTHER MARKINGS • Other markings that may be found on new tires include the following: – A Green or gray spots indicating the positions of the awl vents. – A red spot or triangle indicating the light part of the tire. RETREADS • Retreaded tires are usually marked in accordance with a system peculiar to each manufacturer. The markings usually include the tire part number, the name of the retreader, the number and date of the last retread, and in the case of retreads in which the sidewalls are covered with new rubber, the tire serial number, manufacturer, speed, size and ply rating. AIRCRAFT TIRES AND TUBES AIRCRAFT TIRES AND TUBES TIRE INSPECTION ON THE AIRCRAFT INFLATION • The greatest enemy of an aircraft tire is heat, either the heat that is generated within the tire as it flexes when rolling over the ground, or that from such external sources as the brakes or hot runway surfaces. • It is the internally generated heat that causes damage that is not likely to be discovered until it results in a tire failure. Aircraft tires are designed to withstand the heat generated by normal flexing for a reasonable amount of time. • Deflection due to under-inflation of the tire will cause excess heat to be generated within the tire that it is not designed to withstand and therefore can cause internal carcass damage. Tires that have been operated with low inflation pressure will have their tread worn away on the shoulders more than in the center, and any tire showing this pattern of wear should be carefully examined for evidence of hidden damage. AIRCRAFT TIRES AND TUBES • • • • INFLATION Over-inflation causes accelerated centerline wear on the tread while leaving rubber on the shoulder. When a tire is worn in this way, it has much less resistance to skidding than it has when its tread wears uniformly. The importance of maintaining the proper inflation pressure in a tire makes pressure checks one of the most important parts of routine preventive maintenance. Tire pressure should ideally be checked daily and before each flight. The proper inflation pressure is that specified by the airframe manufacturer in his service manuals Inflation pressure of a tire varies with the ambient temperature by about one percent for every five degrees Fahrenheit. If an airplane is to fly into an area where the temperature is much lower than that of the departing point, the pressure should be adjusted before the airplane leaves. The airframe manufacturer's manual should be consulted before adjusting the pressure. AIRCRAFT TIRES AND TUBES AIRCRAFT TIRES AND TUBES TREAD CONDITION TREAD DEPTH AND WEAR PATTERN • Since the basic strength of the tire is in its carcass, a tire loses none of its strength as long as the tread does not wear down into the body plies of the carcass. But when the tread is worn away, tire traction characteristics are seriously affected. • A tire that has been properly maintained and operated with the correct inflation pressure will wear the tread uniformly, and it should be removed for retreading while there is still at least 1/32” of tread left at its most shallow point. When the tire is removed at this point, there is still enough tread left to provide traction and handling during wet runway operation. AIRCRAFT TIRES AND TUBES • If the center ribs are worn away while the shoulder ribs still have an appreciable depth, the tire has been operated in an over- inflated condition, and as such is highly susceptible to cuts and bruises. It should be carefully checked for this type of damage. (Figure C) • Under-inflation will cause the shoulder ribs to wear more than those in the center. Any tire showing this wear pattern should be carefully inspected for signs of bulges, which could indicate, ply separation. (Figure D) • Tread that has been worn until the body plies are visible indicates poor maintenance. If it is worn only to the point that the tread reinforcement is showing, it is possible that the tire can be salvaged by retreading. But if it is worn into the body plies, it has to be scrapped. (Figure E) AIRCRAFT TIRES AND TUBES TREAD DAMAGE • Any time the tread is cut more than halfway across a rib, or any of the carcass plies are exposed, the tire should be removed. (Figure A) • When a wheel locks up on a watercovered runway and rides on the surface of the water, a tremendous amount of heat builds up at the point of contact and actually bums the rubber. Tires showing this type of damage should be removed from service. (Figure B) • Operating on grooved runways can cause chevron-shaped cuts across the ribs of a tire, and, as with any type of cut, if they extend across more than one-half of the rib, the tire should be removed from service. (Figure D) AIRCRAFT TIRES AND TUBES • The main purpose of the sidewall of a tire is to protect the carcass plies from damage, either from mechanical abrasion or from deterioration by chemicals or by the sun. • Small snags or cuts or weather checking in the sidewall rubber that do not expose the cords are not normally considered a cause for removal of the tire, but if any of the ply material is exposed, the tire must be removed. • The liner of a tubeless tire is there to hold the air, but some of the air diffuses through into the body plies. • The sidewalls of these tires are vented to allow this air to escape, but if the vents do not adequately relieve the pressure, ply separation may occur. AIRCRAFT TIRES AND TUBES • • • • TIRE RETREADING The great amount of abrasion a tire experiences on its tread each time the airplane lands or taxis on a hard surfaced runway wears the tread away long before the carcass is worn out, and so it is standard practice for commercial aircraft tires to be retreaded. When a tire is received by the retreading agency, it is thoroughly inspected. The tread, sidewalls, and beads are checked for cuts, bruises, other damage, or wear, and air is injected into the sidewall to check for any ply separation. The tire is checked for fabric fatigue and for any indication of contamination by oil, grease, or hydraulic fluid. The tires that pass this inspection then have their old tread rubber removed by contour buffing, which produces a smooth shoulder-to-shoulder surface. New tread rubber and reinforcement are then applied to the buffed carcass, and the tire is placed in a heated mold and cured. The tire is identified as a retreaded tire and a record made of the number of times it has been retreaded. Some tire manufacturers may specify the number of time a tire can be retreaded but for some tires there is no specific limit to the number of times, but is determined by the condition of the carcass. Side wall damaged tire can’t be repaired AIRCRAFT TIRES AND TUBES TIRE STORAGE • All new and retreaded tires should be stored in a cool, dry area, out of direct sunlight and away from any electrical machinery. • The storage room should not have extremes of temperature, but should be maintained between 320 and 800 F (00 and 270 C). • Whenever possible the tires should be stored vertically in racks, with the tire supported on a flat surface AIRCRAFT TIRES AND TUBES TUBE CONSTRUCTION AND SELECTION • A great number of aircraft tires, ranging, from some of the small type Ill up to large type VII are of the tube type. • Tubes for these tires are available in either non reinforced rubber for normal applications or as a special heavy-duty reinforced tube that has a layer of nylon fabric molded to its inside circumference to protect it from chafing against the rim and from heat caused by brake application. • It is extremely important that only the tube recommended for a particular tire be used with it. If the tube is too small for the tire, its splices will be overstressed and the tube will be weakened. AIRCRAFT TIRES AND TUBES TUBE INSPECTION • If a tube is suspected of leaking, first check the valve by spreading a drop of water over the end of the valve, and watch to see if a bubble forms. If a bubble does form, the valve core should be replaced. If the leak is not in the valve, the tire must be deflated and demounted and the tube removed. • If the tube is not too large, inflate it and submerge it in water to find the source of bubbles. If the tube is too large for the available water container, flow water over the surface of the tube as you look for the leak. • Examine the inside circumference of the tube for evidence of chafing against the toe of the bead or by corrosion on the wheel. Any tube that is chafed enough to lose some of its thickness in spots should be replaced. AIRCRAFT TIRES AND TUBES • • • • • TUBE STORAGE Tubes should be stored in their original cartons whenever possible, but if their cartons are not available, they should be dusted with tire talcum and wrapped in heavy paper. Tubes may also be stored inflated by putting them in the proper size tire and inflating them just enough to round them out The inside of the tire and the outside of the tube should be dusted with tire talc to prevent the tube sticking to the tire. Tubes should never be stored by hanging them over nails or pegs, or supporting them in any way that would cause a sharp fold or crease, as these creases will eventually cause the rubber to crack. Tubes with creases should not be put into service. AIRCRAFT TIRES AND TUBES TIRE BALANCING • As aircraft takeoff speeds increase, the vibration caused by unbalanced wheels becomes annoying. And this vibration is especially noticeable on nose wheels, since they extend quite a distance below the airplane on a slender strut, and they usually do not have a brake to help dampen the vibrations.
