A. Natural contour. B. Taking a "set". C. Thinned out at edges.
FIGURE 9-55. Inner tube inspection.
Fabric Base Tubes In cases where brake drum heat is a recognizable factor, careful checking of the tube, as well as the tire beads, should be made to prevent a failure which might be disastrous. In such instances, fabric base tubes should always be used.
These have a layer of nylon cord directly imbedded in the inner circumference of the tubes to protect them from thinning out under brake drum heat. Additional protection is also provided against chafing action of the tire bead toes and from damage during mounting and demounting.
MOUNTING AND DEMOUNTING The object of these instructions is to show how to do the job as easily, and safely as possible, using proper tools, without damaging tires, tubes or wheels.
Almost every experienced aircraft tire service man has developed methods which are more or less his own, and undoubtedly some of these methods are as practical as the suggestions given here.
These instructions are intended to be simple, so that they can be carried out with tools which are commonly available, in contrast to the specialized equipment that is usually available only at larger airports or military installations.
Inspection, Tube Installation Before mounting any tire, examine the wheel carefully to make sure there are no cracked or injured parts. Naturally, the tire and tube should be carefully inspected, as described in the pages dealing with tire and tube inspection. A quick check should always be made to be certain that no foreign material is inside the tire or clinging to the inner tube.
Dust the inside of the tire and the inner tube outer surface with tire talc or soapstone before the tube is installed in the tire. This will prevent its sticking to the inside of the tire or to the tire beads.
Dusting also helps the tube assume its normal shape inside the tire during inflation, and lessens the chances of wrinkling or thinning out.
It is good practice to always mount the tube in the tire with the valve projecting on the serial side of the tire.
Lubrication Tubeless tires fit tighter on the wheel than the tube type. It is therefore desirable to lubricate the toes of the beads with an approved 10 percent vegetable oil soap solution, or plain water. This will facilitate mounting and accomplish proper seating of the tire beads against the wheel flanges so there will be no air loss. Care must be used, however, to make certain that none of the solution gets on the area of the bead making contact with the wheel flange.
On tube type tires, lubrication of tire beads may or may not be necessary, depending upon the type of wheel being used. An approved mounting solution, such as the 10 percent vegetable oil soap solution, or water mentioned above, can be used on the bead toes, and even on the rim side of the inner tube, to facilitate mounting.
Balance Balance in an aircraft wheel assembly is very important. From a wear standpoint, when the wheels are in landing position a heavy spot in a wheel assembly will have a tendency to remain at the bottom and thus will always strike the ground or runway first. This results in severe wear at one area of the tire tread and can necessitate early replacement. In addition, unbalanced tires can cause severe vibration which may affect the operation of the aircraft. In fact, pilots have reported that sometimes instruments could not be read because of such vibration.
A. Valve projecting on serial side of tire. B. Tube balance mark aligned with tire balance mark.
FICURE 9-56. Basic tire and tube assembly.
Balance marks appear on certain aircraft tubes to indicate the heavy portion of the tube. These marks are approximately ½" wide by 2" long.
When the tube is inserted in the tire, the balance mark on the tube should be located at the balance mark on the tire (figure 9-56). If the tube has no balance mark, place the valve at the balance mark on the tire.
When mounting tubeless aircraft tires, the red dot balance mark on the tire must always be placed at the valve that is mounted in the wheel.
Inflation Safety After the tire and tube are mounted on the wheel the assembly should be placed in a safety cage for inflation. The cage should be placed against an outside wall, constructed so as to withstand, if necessary, the effects of an explosion of either the tire, tube or wheel (figure 9-57).
The air line from the compressor or other air source should be run to a point at least 20 to 30 ft.
away from the safety cage and a valve and pres sure gage installed at that point. The line should A. Clip-on chuck permits inflation at B. Recommended for all aircraft tire shops.
safe distance from tire safety cage.
FICURE 9-57. Inflation precautions.
then be continued and fastened to the safety cage with a rubber hose extending from that connection.
A clip-on chuck is then fitted on the end of the hose for actual inflation purposes. This arrangement makes it unnecessary to reach into the cage to check air pressures or to be anywhere near the safety cage while the tire is being infiated.
Seating Tube Type Tires To seat the tire beads properly on the wheel, first inflate the tire to the pressure recommended for that particular size and for the aircraft on which it is to be mounted. Then the tire should be completely deflated and finally reinflated to the correct pressure (do not fasten valve to rim until this has been done). Use the valve extension for inflation purposes, it necessary.
This procedure accomplishes the following: it helps to remove any wrinkles in the tube and to prevent pinching the tube under the toe of the bead; it eliminates the possibility of one section of the tube stretching more than the rest and thinning out in that area; and it assists in the removal of air that might be trapped between the inner tube and the tire.
NOTE: With tubeless tires, it is not necessary to go through this inflation-deflation-reinflation procedure.
Let Stand-Then Recheck It is recommended that a newly-mounted assembly be stored away from work areas for at least 12 hours and preferably for 24 hours. This is for the purpose of determining if there is any structural weakness in either the tire, tube or wheel. This also permits rechecking of the tire, after the 12- or 24-hour period, to determine any drop in pressure and to judge whether this drop in pressure is in accordance with normal tire growth.
By such a test, when the assembly is mounted on the aircraft, it can be done with the assurance that each part of the assembly is satisfactory for service.
Demounting Safety Always be sure to deflate tires completely before demounting. There have been many serious accidents caused by failure to follow this important step. For even a better practice, it is recommended that tires be deflated before wheels are removed from the aircraft.
NOTE: Use caution when unscrewing valve cores, as the air pressure within the tube or tire can cause a valve core to be ejected like a bullet.
Handle Beads and Wheels With Care With any type of wheel, the tire beads must be loosened from the wheel flange and bead seat before any further steps are taken in demounting.
Be very careful not to injure the beads of the tire or the relatively soft metal of the wheel. Even with approved tools, extreme care must be taken.
A. Tubeless Split Wheels In the tubeless design, the tire and wheel are used to retain air pressure. Inflation is accomplished through a tubeless tire inflation valve installed in the wheel. The wheel valve hole, in which the tubeless tire inflation valve is mounted;
is sealed against loss of air by a packing ring or by an "O" ring. (See figure 9-58.)
Split wheels are sealed against loss of air by an "O" ring mounted in a groove in the mating surface of one of the wheel halves.
Demountable flange wheels are similarly sealed against loss of air by an "O" ring installed in a groove on the wheel base under the area covered by the demountable flange.
The air pressure contained in the tubeless tire seals the tire bead against the wheel bead seat to prevent loss of air.
Wheels used with disc brakes have thermal fuze (relief) plugs installed in the rotor drive area of the wheel as a protective measure against the tire blowouts due to excessive heat. The plugs have a fusible metal core that melts at a predetermined temperature to relieve the high pressure build-up.
Mounting Check tubeless tire inflation valve and thermal relief plugs for proper installation, and absence of damage. Refer to wheel manufacturer's manual for installation procedure.
Inspect "O" ring used to seal wheel for damage and replace if necessary.
Lubricate "O" ring (as specified by the wheel manufacturer) and install in wheel groove. Make sure the "O" ring is free of kinks or twists and is seated properly.
Mounting a demountable flange on a wheel base.
be careful not to dislocate or damage the "O" ring previously installed in the wheel base.
Mount tubeless tires in the same manner as tube type tires. Make sure that the wheel bead seats are clean and dry to insure proper sealing of the tubeless tire bead.
Assemble wheel halves of split type wheels with the light sides (impression stamped "L" on the 387
A. "O"-ring seal is vital - handle and install carefully. B. Inspect valve seals for signs of damage or deterioration.
FICURE 9-58. Split wheel seal inspection.
flanges) 180° apart from each other to insure minimum out-of-balance condition.
Be sure that nuts, washers, and bolts used to assemble split type wheels are in proper order and that the bearing surfaces of these parts are properly lubricated. Tighten to recommended torque values. See wheel manufacturer's manual for recommended procedures.
Demounting The procedure for demounting tubeless tires is generally the same as for tires with tubes. However, care must be taken to avoid damaging: (1) the wheel's "O" ring groove and mating register surfaces; (2) the flange area that seats the tire bead; and (3) the tubeless tire inflation valve hole sealing area. These areas of the wheel are critical and if damaged will result in failure of the wheel and tire unit to maintain required air pressure.
B. Tube Type Tires Mounting With the tube entirely deflated, insert it in the tire (folding makes this easier, particularly in small diameter tires), and inflate until the tube is just rounded out. The valve core should be in the valve during this operation.
Apply with brush or swab a 10 percent solution of vegetable oil soap to the rim of the tube extend ing well up into the tire. Use care not to lubricate any part of the bead which comes in contact with the rim flange.
Insert the valve hole section of the wheel into the tire and push the valve through the valve hole in the wheel.
Insert the other side of the wheel while holding the valve. in position. Be careful during this op eration not to pinch the tube between the wheel sections.
Inflate, deflate and reinflate to the recommended pressure.
Install the locking nut or nuts and tighten securely. Put on valve cap and tighten with fingers.
Demounting Remove valve core and fully deflate.
Do not use a pry bar, tire irons or any other sharp tool to loosen tire bead as the wheel may be damaged. Break bead before loosening tie bolts to prevent damage to register surfaces.
Use a bead breaker only to loosen tire bead from both wheel half flanges by applying pressure around the entire circumference of each sidewall.
Remove the tie bolts and the bolt nuts from the wheel and pull out both parts of the wheel from the tire.
C. Removable Side Flange Drop Center Wheels Mounting Fully deflate tube and line up tube balance mark with tire balance mark. Start tire over flange on an angle, being careful not to damage valve.
Be sure to remove valve extensions or valve fishing tools before wheel is installed.
Demounting Be sure to deflate fully.
Make full use of the wheel in pulling bead over flange on removable side.
Work wheel up and down to ease it out of tire.
For one-man demounting, tire can be leaned against a wall or bench, valve side out.
D. Smooth Contour Tail Wheel Tires Note: Smooth contour tires are usually harder to handle because of stiffer beads, small clearances and small diameters.
Mounting Inflate tube sufficiently to round out and be sure tube is worked in all around to avoid pinching.
Work bead opposite valve over edge of wheel first. Deflate tube.
Keep second bead on edge of wheel to allow for inserting valve in valve hole.
Inflate, deflate, reinfiate.
Demounting Be sure to use tools with good leverage.
Be careful not to damage soft metal of rim flange.
Keep tube inflated after lock ring is removed.
Work tube out carefully, using water as a lubricant before completing demounting.
F. Flat Base Wheels Removable Flange Locking Ring Mounting Examine wheel and flanges carefully for burrs or gouges.
Line up tube and tire balance marks.
Dust tube with talc.
Lean tire against bench or wall--flange outward.
Pry flange loose evenly to prevent binding.
Demounting Loosen bead carefully.
Use lead hammer or rubber mallet to loosen side ring.
Pry up side ring carefully and evenly.
Place wheel and tire on a wooden block about 14" high and large enough to fit over the wheel hub, so that wheel can be removed easily.
CAUSES OF AIR PRESSURE LOSS
IN TUBELESS AIRCRAFT TIRES There can be numerous causes for loss of air pressure within an aircraft wheel and tire assembly, therefore, it is economical and wise to follow a systematic check list. Without such a procedure, trial and error substitution of parts can needlessly increase tire maintenance costs.
For example, complaints of air loss in tubeless aircraft tire assemblies, while more common during cold weather, have no seasonal limitations. Factors which may seem distantly related to the problemchanges in tire maintenance personnel, inaccurate gages, air temperature fluctuations-are often the underlying causes of unsatisfactory tire service, further emphasizing the need for simple check procedures.
For guidance in setting up uniform inspection methods, there are general areas of the tire and wheel assembly which can be involved in air pressure loss. (See figure 9-59.)
E. One Piece Drop Center Wheels Mounting Insert wheel in tire, reversing the customary procedure. (Valve hole side goes in first.)
Pry bead over flange with small bites-use a fairly thin tool.
When first bead is on wheel, insert tube. Be sure no part of tube is caught under bead.
Inflate, deflate, reinflate.
Demounting After loosening bead, lay tire flat with wooden block, 3 to 4 inches high under sidewall. Work bead off in small bites. FIcURE. 9-59. Air loss in split wheels.
Damaged Beads-Check for exposure of the carcass cord body in bead toe area or under face of the bead.
Improperly Seated Beads-Condition can be caused by: (a) insufficient air pressure; (b) beads not lubricated; (c) kinked or distorted beads.
Cut or Puncture--Check for cut or puncture entirely through the carcass cord body and liner.
Air Temperature Was tire inflated in heated room and then stored outside? Air pressure will drop approximately 1 psi for every 4° drop in temperature. Tires should be checked and pressure adjusted for specific requirements when tires have reached the outside ambient temperature.
Venting of Tubeless Tires Tubeless aircraft tires are vented in the sidewall area to permit any air that has diffused through the liner and cord body to escape, thus preventing pressure build up within the carcass cord body and possible tread or ply separation. Rate of diffusion will vary by manufacturer and the maximum permissible is no more than 5 percent in any 24-hour period.
Vent holes penetrate the sidewall rubber to, or into, the carcass cord body and may vary in size, depth and angle. Therefore, the amount of air diffused through these holes will vary. Thus, when water or a soapy solution is brushed over the outside of an inflated tubeless tire, air bubbles form.
Some vent holes may emit a continuous stream of bubbles, where others may produce intermittent bubbles. This is normal and does not mean that there is anything wrong with the tire. In fact, as long as a tubeless tire is in an inflated condition, air will be coming out of these vent holes. Where the rate of loss exceeds 5 percent 24 hours, recheck for possible injuries. Vent holes may be covered or closed by spilled solvent or by the tire paint.
They may also be covered during the retreading process, Check for evidence that tubeless tires have been revented after being retreaded.
Initial Stretch Period All aircraft tires are of nylon construction and a certain amount of stretch occurs after the tire has been inflated. This, in itself, will reduce air pressure within the tire. It is absolutely necessary that the tire be inflated to its regular air pressure and let stand at least 12 hours in order to permit this expansion of the cord body. This may result in as much as 10 percent drop in air pressure.
Compensate by reinflating the tire to original pressure. Only after this initial stretch period can it be determined if there is any true air loss within the tire.
THE WHEEL Any of the following wheel conditions can contribute to air loss in the bead area of the tire:
Cracks Or Scratches In The Bead Ledge Or Flange Area Cracks can usually be traced to fatigue failure while scratches and gouges are the result of handling damage or the improper use of tire irons.
Exceptionally Smooth Enamel Surface On Bead Seat Ledges Corrosion Or Wear in Bead Ledge Area Usually occurs at the toe area of the tire bead.
Poor Seating In Bead Area May be caused by accumulation of rubber from the tire or dirt.
Knurls Wheels converted from tube-type use should have knurls removed.
Porous Wheel Assemblies Can be protected either by proper paint procedure and/or an impregnation process.
Holes for attachment of components of wheel assembly. In case of through bolts used to attach such items as drive lugs for brake assembly, etc., the mounting screws or bolts must be properly sealed. The recommendations of the wheel manufacturer should be followed.
Cracks in the wheel well area, in most cases, cannot be repaired.
Sealing Surfaces Look for damage or improper machining of sealing surfaces. Care should be taken to see that there is no handling damage. Any irregularities should be corrected before remounting wheel and tire. (See figure 9-60.)
Foreign material or paint can impair the sealing surface. Thus, all foreign material should be cleaned from the sealing surface before assembly of the wheel. A light, even coat of primer is per.
missible. However, surface must be free of runs or dirt inclusions.
FIGURE 9-60. Wheel inspection.
Improper Installation of "O" Rings Twisting or failure to provide lubricant when specified, may cause loss of air. The wrong size or type of "O" ring, or an "O" ring of the wrong compound for special low temperature service, may also cause leakage.
Inspect used "O" rings carefully. Be sure they are not thinned out, deformed, chipped, damaged or otherwise deteriorated.
Wheel Tie Bolts Proper torque and torquing procedure, as specified by the wheel manufacturer, should be followed to assure adequate compression of sealing "O" ring under all temperature conditions. Low torques, low temperatures, and shrinkage of the wheel halves may cause a significant lessening of compression on the "O" ring seal.
Tubeless Wheel Valve Holes Tubeless wheel valve holes and surrounding area must be free of scratches, gouges and foreign material.
The proper rubber grommet or "O" ring must be used as specified by the wheel manufacturer.
Seals other than those specified may not function properly under the compression loads and low temperatures required for good sealing. Tightening of tubeless valve should follow specific instructions of wheel manufacturer.
Valve core should be checked and replaced when found leaking.
Valve caps should be used and tightened finger tight.
Thermal Fuze Installation A faulty thermal fuze may cause leakage and require replacement. Usually this is the result of a poor bond between thermal melting material and bolt body.
Sealing surface for thermal fuze gasket must be clean and free from scratches and dirt. In some cases, surface can be repaired in accordance with manufacturer's instruction.
Be sure that sealing gasket is the one specified by the wheel manufacturer, sized and compounded for its specific job. Gasket should be free of dis tortion, cuts, etc.
To guard against air pressure losses before as sembly the best insurance is a careful and complete inspection. After assembly, if air loss occurs, the use of a soap solution (or, if possible, complete immersion of wheel-tire assembly) may pinpoint the exact source of leakage.
GOOD PRESSURE GAGE PRACTICE Quite often it is found that the differences in reported air pressures are entirely due to the difference in accuracy in different gages, rather than in any change in air pressure.
It is not unusual to find an inaccurate tire gage in constant use with a tag that states that the gage reads a certain number of pounds too high, or too low. Unfortunately, this error will change as ditferent pressures are checked. A tire gage reading 10 Ibs. high at 80 Ibs. pressure may very well read 25 Ibs. too high at 150 Ibs. pressure. Therefore, incorrect tire gages should either be repaired or replaced. They should not be continued in service.
Cold temperatures may also affect tire gages and cause pressure readings lower than they actually are. Occasionally, too, a gage has been mistakenly treated with oil or some other lubricant in expectation of making it work better.
will actually cause incorrect readings and probably render the gage unfit for further service.
It is good practice to have gages recalibrated periodically and to use the same gage for performing an inflation cycle-for the original 12- or 24- hour stretch period. Dial type gages, of good quality, are highly recommended for all tire maintenance installations-regardless of size!
Storing Aircraft Tires and Tubes The ideal location for tire and tube storage is a coo., dry and reasonably dark location, free from air currents and dirt. While low temperatures (not below 32° F.) are not objectionable, high room temperatures (80° F.) are detrimental and should be avoided.
Avoid Moisture, Ozone Wet or moist conditions have a rotting effect and may be even more damaging when the moisture contains foreign elements that are further detrimental to rubber and cord fabric. Strong air currents should be avoided, since they increase the supply of oxygen and quite often carry ozone, both of which cause rapid aging of rubber. Also, particular care should be taken to store tires away from electric motors, battery chargers, electric welding equipment, electric generators and similar equipment as they all create ozone.
Fuel and Solvent Hazards Care should be taken that tires do not come in contact with oil, gasoline, jet fuel, hydraulic fluids, or any type of rubber solvent, since all of these are natural enemies of rubber and cause it to disintegrate rapidly. Be especially careful not to stand or lay tires on floors that are covered with oil or grease. When working on engines or landing gear, tires should be covered so that oil does not drip Store In Dark The storage room should be dark, or at least free from direct sunlight. Windows should either be given a coat of blue paint or covered with black plastic. Either of these will provide some diffused lighting during the daytime. Black plastic is preferred as it will lower the temperature in the room during the warm months and permit tires to be stored closer to the windows.
Tire Racks Preferred Whenever possible, tires should be stored in regular tire racks which hold them up vertically.
The surface of the tire rack against which the weight of the tire rests should be flat and, if pos- Without With Tube Size Tube Inserted 26x6 5 33" 5 36" 44" 5 47" 3 4 sible, 3 to 4 inches wide in order that no permanent distortion to the tire is caused. If tires are piled on top of one another, they should not be stacked too high, as this will cause distortion of the tire and might result in trouble when the tires are put into service. This is particularly true of tubeless tires, as those on the bottom of the stack may have the beads pressed so closely together that a bead seating tool will have to be used to force the tire beads onto the wheel far enough to retain air pressure for inflation (figure 9-61).
Safe Tube Storage Tubes should always be stored in their original cartons, so that they are protected from light and air currents. They should never be stored in bins or on shelves without being wrapped, preferably in several layers of heavy paper.
Tubes can also be stored by inflating slightly and inserting them in the same size of tire. This of course, should only be done as a more or less temporary measure. However, before using such an assembly, the tube should be removed from the tire and the inside of the tire carefully examined, since quite often foreign material will get between the two and if not removed, could cause irreparable damage to both tire and tube.
Under no circumstances should tubes be hung over nails or pegs, or over any other object that might form a crease in the tube. Such a crease will eventually produce a crack in the rubber.
REPAIRING Many aircraft tires and tubes which become injured in service can be successfully repaired. Likewise, aircraft tires which have become worn out in service, or flat spotted and removed prematurely, Without With Tube Size Tube Inserted 56" SC and larger
12.50-16 4
15.00-16 3
17.00-16 3
15.50-20 3
17.00-20 3 4 smaller sizes of tubess tires may be stacked five high. This would include sizes througi 39 × 13. Larger sizes of tubless tires should not be stacked more than four nigh FIcURE 9-61. Permissible tire stacking.
can be recapped so that the new tread will give service comparable to the original tread. The recapping and repairing of aircraft tires has been practiced for many years and has saved aircraft operators considerable sums of money.
which might otherwise have been discarded have been safely reconditioned (many repeatedly) for continued service.
Recapping Aircraft Tires Recapping is a general term meaning reconditioning of a tire by renewing the tread, or renewing the tread plus one or both sidewalls. (See figure 9-62.) There are actually four different types of recapping for aircraft tires.
Top Capping--For tires worn to the bottom of the tread design, with no more than slight flat spotting and/or shoulder wear, the old tread is roughened and a new tread is applied.
Full Capping-For tires worn all around, those flat spotted to the cords, or those with numerous cuts in the tread area, the new tread material is wider than that used on a top cap, and comes down over the shoulder of the tire for several Three-Quarter Retread--For tires needing a new tread, plus renewing of the sidewall rubber on one side, due to damage or weather checking, a full cap is applied, and in addition, approximately 16" of the thickness of the old sidewall rubber is buffed off one side. New sidewall rubber is then applied from the bead to the edge of the new tread, on the buffed side only.
Bead-to-Bead Retread-A new tread and both new sidewalls are applied by this method.
Tires That May Be Recapped Tires with sound cord bodies and beads, or which meet injury limitations described under "Repairable Aircraft Tires."
Tires which are worn to 80 percent or more of their total tread depth.
Tires with one or more flat spots severe enough to cause an out-of-balance condition, regardless of the percentage of wear. Tires having so many tread cuts that repairing the tread rubber would be uneconomical.
Nonrecappable Tires Tires having injuries which would make them nonrepairable.
Tires with six full plies or more having any spot worn through more than one body ply. (It is generally not considered economical to retread 4 and 6 ply aircraft tires.)
Tires with weather checking or ozone cracking of tread or sidewall that exposes the cords.
Repairable Aircraft Tires When considering a tire for repairing only, the amount of service remaining in the tire is important. Any tire with at least 30 percent of tread life remaining, normally would be considered as having enough service left in it to warrant repair only.
Nonrepairable Aircraft Tires The following conditions disqualify a tire for repair:
Any injury to the beads, or in the bead area (except injuries limited to the bead cover or finishing strip as previously mentioned under repairable aircraft tires).
A. Sidewall cords damaged beyond B. Sidewall cords OK for retread. C. Worn through breaker only.
repair. OK for retread.
FIGURE 9-62. Operational damage 393
Any tire with protruding bead wire or badly kinked bead.
Any tire which shows evidence of ply or tread separation.
Any tire with loose, damaged, or broken cords on the inside.
Tires with broken or cut cords in the outside of the sidewall or shoulder area.
Tires that have gone fat, or partially flat, due to the melting or failure of fuze plugs in the wheels should normally be scrapped, even though there may be no visible evidence of damage to the inside or outside of the tire. The only exceptions would • use for teased be where it was known that the fuze plug leaked air because of its being defective.
Spot Repairs When considered economical, spot repairs can be made to take care of tread injuries such as cuts, snags, etc., which are through no more than 25 percent of the actual body plies of the tire, and not over 2" in length at the surface. Vulcanized spot repairs are also made at times to fill in tread gouges that do not go deeper than the tread rubber and do not penetrate the cord body.
Low speed tires (under 160 mph) with tread injuries that penetrate no more than 25 percent of the actual body (breaker strips not included), and have a maximum surface length of 2", may be repaired. If the injury penetrates beyond 25 percent of the actual body plies, it may still be repairable but the surface length of the injury should be no more than 1".
High speed tires (over 160 mph) with tread injuries that penetrate no more than 40 percent of the actual body plies (breaker strips and fabric reinforcement strips in the tread not included) and have a maximum surface length of 1½" with maximum width no more than ¼", may be repaired.
Injuries through the cord body in the tread area, measuring ⅛" or less at the largest point, are considered punctures and are easily repaired.
Shallow cuts in the sidewall or shoulder rubber only are repairable if cords are exposed but not damaged.
ing strip, or slight injuries caused by tire tools in the general bead area, are repairable if the injury does not extend into the plies of the tire, and provided there is no sign of separation in the bead area. If the finishing strips are loose or blistered, they can only be replaced by bead-to-bead retreading.
Liner blisters smaller than 4" x 8" may be repaired if there are no more than two in any one quarter section of the tire, and no more than five in the complete tire. Normally, however, it would be more economical to do this repair at the time the tire is recapped.
It is generally considered uneconomical to repair 4- and 6-ply aircraft tires.
OPERATING AND HANDLING TIPS Taxiing Needless tire damage or excessive wear can be prevented by proper handling of the aircraft during taxiing.
Most of the gross weight of any aircraft is on the main landing gear wheels-on two, four, eight or more tires.
to absorb shock of landing and will deflect (bulge at the sidewall) about two and one-half times more than a passenger car or truck tire. The greater deflection causes more working of the tread, produces a scuffing action along the outer edges of the tread and results in more rapid wear.
Also, if an aircraft tire strikes a chuck hole, a stone, or some foreign objects lying on the runway, taxi strip, or ramp, there is more possibility of its being cut, snagged or bruised because of the percentage of deflection. Or, one of the main landing gear wheels, when making a turn, may drop off the edge of the paved surface causing severe sidewall or shoulder damage. The same type of damage may also occur when the wheel rolls back over the edge of the paved surface.
With dual main landing gear wheels, one tire might be forced to take a damaging impact (which two could withstand without damage) simply because all the weight on one side of the plane is concentrated on one tire instead of being divided between two.
As airports grow in size and taxi runs become longer, chances for tire damage and wear increase.
Taxi runs should be no longer than absolutely necessary and should be made at speeds no greater than 25 mph, particularly for aircraft not equipped with nose wheel steering.
For less damage in taxiing, all personnel should see that ramps, parking areas, taxi strips, runways and other paved areas are regularly cleaned and cleared of all objects that might cause tire damage.
Braking and Pivoting Increasing airport traffic, longer taxi runs and longer runs on takeoff and landing are subjecting tires to more abrasion resulting from braking, turning and pivoting.
Severe use of brakes can wear flat spots on tires and cause them to be out of balance, making premature recapping or replacement necessary.
Severe or prolonged application of the brakes can be avoided when ground speed is reduced.
Careful pivoting of aircraft also helps to prolong tire tread life. If an aircraft turned as an automobile or truck does--in a rather wide radiusthe wear on the tire tread would be materially reduced. However, when an aircraft is turned by locking one wheel (or wheels), the tire on the locked wheel is twisted with great force against the pavement. A small piece of rock or stone that would ordinarily cause no damage; can, in such a case, be literally screwed into the tire. This scuffing or grinding action takes off tread rubber and places a very severe strain on the sidewalls and beads of the tire at the same time.
To keep this action at a minimum, it is recommended that whenever a turn is made, the inside wheel (wheels) be allowed to roll on a radius of 20 to 25 feet and up to 40 feet for aircraft with bogies.
Takeoffs and Landings Aircraft tire assemblies are always under severe strain on takeoff or landing. But under normal conditions, with proper control and maintenance of tires, they are able to withstand many such stresses without damage.
Tire damage on takeoff, up to the point of being airborne, is generally the result of running over some foreign object. Flat spots or cuts incurred in pivoting can also be a cause of damage during takeoff or landing.
Tire damage at the time of landing can be traced to errors in judgment or unforeseen circumstances.
Smooth landings result in longer tread wear and eliminate much of the excessive strain on tires at the moment of impact.
Landings with brakes locked, while almost a thing of the past, can result in flat spotting. Removal of the tire for recapping or replacement is almost invariably indicated. Brakes-on landings also cause very severe heat at the point of contact on the tire tread and may even melt the tread rubber (skid burn). Heat has a tendency to weaken the cord body and places severe strain on the beads. In addition, heat buildup in the brakes may literally devulcanize the tire in the bead area.
Under these circumstances, blowouts are not uncommon because air under compression must expand when heated.
On aircraft equipped with tail wheels, a two point landing is usually somewhat smoother than a three point landing, but is ordinarily made at considerably higher speed. As a result, more braking may be required to bring the aircraft to a stop If tires are skidded on the runway at high speed, the action is similar to tires being ground against a fast turning emery wheel.
Sometimes an aircraft will be brought in so fast that full advantage will not be taken of runway length and brakes are applied so severely that flat spots are produced on the tires. Or, if brakes are applied when the plane is still traveling at high speed and still has considerable lift, tires may skid on the runway and become damaged beyond further use or reconditioning.
The same thing may occur during a rough landing if brakes are applied after the first bounce.
For maximum tire service, delay brake application until the plane is definitely settled into its final roll.
More tires fail on takeoff than on landing and such failures on takeoff can be extremely dangerous. For that reason, emphasis must be placed on proper preflight inspection of tires and wheels.
Condition of Landing Field Regardless of the preventive maintenance and extreme care taken by the pilot and ground crew in handling the aircraft, tire damage is almost sure to result if runways, taxi strips, ramps and other paved field areas are in a bad condition or poorly maintained.
Chuck holes, pavement cracks or step-offs from these areas to the ground, all can cause tire damage. In cold climates, especially during the winter, all pavement breaks should be repaired immediately.
Another hazardous condition often overlooked is accumulated loose material on paved areas and hangar floors. Stones and other foreign materials should also be swept off all the paved areas. In addition, tools, bolts, rivets and other repair materials are sometimes left lying on the aircraft and when the aircraft is moved, these materials drop off. These objects picked up by the tires of another aircraft can cause punctures, cuts, or complete failure of the tire, tube and even the wheel.
With jet aircraft, it is even more important that foreign material be kept off areas used by aircraft.
Hydroplaning This is a condition whereby on wet runways, a wave of water can build up in front of spinning tires and when over-run, tires will no longer make contact with the runway. This results in the complete loss of steering capability and braking action.
Hydroplaning can also be caused by a thin film of 395
water on the runway mixing with the contaminants present. (See figure 9-63.)
Cross cutting of runways has been completed at some of the major airports and reportedly has greatly reduced the danger of hydroplaning. However, the ridges of concrete created by this crosscutting can cause a chevron type of cutting of tread ribs, particularly with the higher pressure tires used on jet aircraft. These cuts are at right angles to the ribs and rarely penetrate to the fabric tread reinforcing strip. Such damage would not be considered cause for remova, unless fabric was exposed due to a piece of tread rib being torn out.
Replacement of this valve can be made by most any gasoline service station or garage, providing they have the proper valve for replacement.
2. Metal valve with rubber base. These are easily recognized since the rubber base is similar to the one used on the all-rubber valve just described. Generally, replacement is made by the same method but it is absolutely necessary that the replacement valve have the same dimensions as the original.
3. Metal valve with a fabric-reinforced rubber base. The base may be cured on top of the tube, or it may be cured into the tube. The metal valve may have to be bent to the proper angle, or angles. Repairing of this type of tube is more difficult because it is necessary to replace the valve pad.
perienced personnel and special equipment are necessary to effect the proper cure of the replacement valve pad to the inner tube.
Repair type valves are also available tor valves mentioned in 2 and 3 above. These are applied by cutting off the original valve and screwing on a replacement repair valve to the spud of the original valve. Follow instructions provided by the manufacturer of these repair type valves.
FiGurE: 9-63. Cross cutting runways reduces danger of hydroplaning.
TUBE REPAIR Most tube repairs are necessary because of valves being broken off or otherwise damaged. However, occasionally, a tube might be cut, punctured, or damaged by the tire tools during mounting or de- Injuries larger than one inch can be repaired by using a reinforcement patch inside the This reinforcement would be the same material that is used for repairing the tube on the outside. An injury smaller than one inch should not need a reinforcement piece.
Chaffing damage caused by the toe of the tire bead, or thinning out of the tube from brake heat should be cause for immediate scrapping of the
SIDEWALL-INFLATED AIRCRAFT TIRES Some tires for small aircraft are manufactured with a valve in the sidewall, thus eliminating the need for machining the wheel to take a conventional valve (see figure 9-04).
There are three general types of valves used in aircraft inner tubes:
1. The rubber valve, which has a rubber stem and a rubber base cured to the outside surface of the inner tube. This valve is similar to those used on inner tubes in passenger car service. FIGURE 9-64. Sidewall-inflated tire.