Aircraft Wheel and breaks

About aircraft wheels and brake systems

1. AIRCRAFT WHEELS AND TYRES

2. AIRCRAFT WHEELS Aircraft wheels are an important component of a landing gear system. With tires mounted upon them, they support the entire weight of the aircraft during taxi, takeoff, and landing. The typical aircraft wheel is lightweight, strong, and made from aluminum alloy. Some magnesium alloy wheels also exist . Early aircraft wheels were of single piece construction, much the same as the modern automobile wheel.

3. FIXED FLANGE WHEEL

4. AIRCRAFT WHEELS As aircraft tires were improved for the purpose they serve, they were made stiffer to better absorb the forces of landing without blowing out or separating from the rim. Stretching such a tire over a single piece wheel rim was not possible. A two-piece wheel was developed.

5. Removable flange wheels found on older aircraft are either drop center or flat base types.

6. AIRCRAFT WHEELS Early two-piece aircraft wheels were essentially one-piece wheels with a removable rim to allow mounting access for the tire . These are still found on older aircraft. Later , wheels with two nearly symmetrical halves were developed . Nearly all modern aircraft wheels are of this two piece construction.

7. Two-piece split-wheel aircraft wheels found on modern light aircraft

8. Features of a two piece aircraft wheel found on a modern airliner

9. WHEEL CONSTRUCTION The typical modern two-piece aircraft wheel is cast or forged from aluminum or magnesium alloy . The halves are bolted together and contain a groove at the mating surface for an o-ring , which seals the rim since most modern aircraft utilize tubeless tires. The bead seat area of a wheel is where the tire actually contacts the wheel.

10. WHEEL CONSTRUCTION It is the critical area that accepts the significant tensile loads from the tire during landing. To strengthen this area during manufacturing, the bead seat area is typically rolled to pre stress it with a compressive stress load.

11. WHEEL CONSTRUCTION Modern tyres are much more rigid, due to the load bearing requirements, which results in the wheels having to be of two piece construction . The two piece wheel construction, are of 2 types, removable rim or split wheel.

12. INBOARD WHEEL HALF Wheel halves are not identical . The primary reason for this is that the inboard wheel half must have a means for accepting and driving the rotor(s) of the aircraft brakes that are mounted on both main wheels. Tangs on the rotor are fitted into steel reinforced keyways on many wheels . Other wheels have steel keys bolted to the inner wheel halves.

13. INBOARD WHEEL HALF These are made to fit slots in the perimeter of the brake rotor . Some small aircraft wheels have provisions for bolting the brake rotor to the inner wheel half. Regardless , the inner wheel half is distinguishable from the outer wheel half by its brake mounting feature.

14. Keys on the inner wheel half of an aircraft wheel used to engage and rotate the rotors of a disc brake

15. Both wheel halves contain a bearing cavity formed into the center that accepts the polished steel bearing cup, tapered roller bearing, and grease retainer of a typical wheel bearing set-up . A groove may also be machined to accept a retaining clip to hold the bearing assembly in place when the wheel assembly is removed. The wheel bearings are a very important part of the wheel assembly. The inner wheel half of a wheel used on a high performance aircraft is likely to have one or more thermal plugs.

16. Heavy use of the aircraft brakes can cause tire air temperature and pressure to rise to a level resulting in explosion of the wheel assembly. To alleviate this, thermal plug(s) mounted in the inner wheel half of a high performance aircraft wheels are made with a fusible core that melts and releases the air from the tire before explosion.

17. During heavy braking, temperatures can become so great that tire temperature and pressure rise to a level resulting in explosion of the wheel and tire assembly . The thermal plug core is filled with a low melting point alloy. Before tire and wheel temperatures reach the point of explosion , the core melts and deflates the tire. The tire must be removed from service and the wheel must be inspected in accordance with the wheel manufacturer’s instructions before return to service if a thermal plug melts.

18. Adjacent wheel assemblies should also be inspected for signs of damage . A heat shield is commonly installed under the inserts designed to engage the brake rotor to assist in protecting the wheel and tire assembly from overheating . An over inflation safety plug may also be installed in the inner wheel half .

19. This is designed to rupture and release all of the air in the tire should it be over inflated. The fill valve is also often installed in the inner wheel half with the stem extending through holes in the outer wheel half to permit access for inflation and deflation.

20. OUTBOARD WHEEL HALF The outboard wheel half bolts to the inboard wheel half to make up the wheel assembly upon which the tire is mounted . The center boss is constructed to receive a bearing cup and bearing assembly as it does on the inboard wheel half . The outer bearing and end of the axle is capped to prevent contaminants from entering this area.

21. Features of a two piece aircraft wheel found on a modern airliner

22. OUTBOARD WHEEL HALF The outboard wheel half provides a convenient location of the valve stem used to inflate and deflate tubeless tires . Alternately, it may contain a hole through which a valve stem extension may pass from the inner wheel half or the valve stem itself may fit through such a hole if a tube-type tire is used.

23. WHEEL INSPECTION An aircraft wheel assembly is inspected while on the aircraft as often as possible . A more detailed inspection and any testing or repairs may be accomplished with the wheel assembly removed from the aircraft.

24. ON AIRCRAFT INSPECTION The general condition of the aircraft wheel assemblies can be inspected while on the aircraft . Any signs of suspected damage that may require removal of the wheel assembly from the aircraft should be investigated.

25. PROPER INSTALLATION The landing gear area is such a hostile environment that the technician should inspect the landing gear including the wheels, tires and brakes whenever possible . Proper installation of the wheels should not be taken for granted . All wheel tie bolts and nuts must be in place and secure . A missing bolt is grounds for removal and a thorough inspection of the wheel halves in accordance with the wheel manufacturer’s procedures must be performed due to the stresses that may have occurred.

26. PROPER INSTALLATION The wheel hub dust cap and anti-skid sensor should also be secure. The inboard wheel half should interface with the brake rotor with no signs of chafing or excessive movement. All brake keys on the wheel must be present and secure. Examine the wheels for cracks, flaked paint and any evidence of overheating. Inspect thermal plugs to ensure no sign of the fusible alloy having been melted.

27. PROPER INSTALLATION Thermal plugs that have permitted pressure loss in the tire require that the wheel assembly be removed for inspection. All other wheels with brakes and thermal plugs should be inspected closely while on the aircraft to determine if they too have overheated . Each wheel should be observed overall to ensure it is not abnormally tilted. Flanges should not be missing any pieces , and there should be no areas on the wheel that show significant impact damage.

28. PROPER INSTALLATION The removable rim wheel has an inner tube where as the split wheel is tubeless and requires a perfect seal between the halves. An O ring is located between the mating surfaces. To be as light and strong as possible they are usually constructed from alluminium or magnesium alloys and may be cast or forged.

29. The inboard wheel section is fitted with key ways that allows the brake discs to slot into. These key ways drive the brake discs with the wheels. Larger aircraft wheels have one or more fusible plugs fitted. These plugs have a centre hole which is filled with a low melting point alloy. In the event of the tyre overheating, when a temperature limit is reached the low melting point alloy melts and allows the tyre to safely deflate.

30. AXLE NUT TORQUE Axle nut torque is of extreme importance on an aircraft wheel installation . If the nut is too loose, the bearing and wheel assembly may have excessive movement. The bearing cup(s) could loosen and spin, which could damage the wheel. There could also be impact damage from the bearing rollers which leads to bearing failure .

31. Improper loose torque on the axle nut can cause excessive end play leading to bearing race damage known as scalloping . Eventually, this leads to bearing failure.

32. AXLE NUT TORQUE An over-torqued axle nut prevents the bearing from properly accepting the weight load of the aircraft. The bearing spins without sufficient lubrication to absorb the heat caused by the higher friction level. This too leads to bearing failure. All aircraft axle nuts must be installed and torqued in accordance with the airframe manufacturer’s maintenance procedures.

33. TYPES OF WHEELS There are three basic types of wheel used for aircraft : Well-based Divided (or Split) Loose and Detachable Flange

34. Well-based This type is limited to smaller light aircraft and is similar to those found on a typical family car.

35. Divided (or split) This type is used on most modern commercial airliners. It consists two half assemblies matched up and bolted together to form the complete wheel. Each half is more or less identical and has its own tapered bearing assembly.

36. Inner Bearing DIVIDED (SPLIT) WHEEL

37. Divided (or split) A sealing ring is incorporated between the two halves, to provide an airtight joint when the wheel is used with a tubeless tyre. Additionally , the inner half will carry the brake rotor drive blocks and the outer half may be fitted with fusible plugs.

38. Loose and Detachable Flange This type of wheel has a main hub, which carries both bearings, brake rotor drive blocks and fusible plugs. To facilitate tyre replacement, one of the two wheel flanges can be removed. The flange when refitted to the wheel hub is retained by a locking ring (loose flange) or by means of a series of nuts and bolts (detachable flange). As with the divided wheel a sealing ring is incorporated in the flange recess to provide the airtight joint when used with tubeless tyres.

39. LOOSE FLANGED SEALING

40. THREE PIECE LOOSE FLANGED WHEEL

41. 11-Nov-15 52 AIR CRAFT TYRE TECHNOLOGY

42. An Aircraft tyre carries 250 to 1,000 times its mass compared to 50 times for a passenger car tyre , while aircraft tyre temperatures can fall to -50 ˚ during flight and reach + 60 ˚ on the runway

43. ROLLING ON TIRES 1736 - natural rubber is developed from South American threes, presenting good rubbing of pencil marks (hence “rubber”) 1887 - pneumatic tire developed by John Boyd Dunlop for son bicycle Vulcanization of natural rubber is credited to Charles Goodyear & Robert William Thomson 1920 - Synthetic rubbers by Bayer Today - over 1 billion produced over 400 tire factories - Many A/C tires and inner tubes are still made of natural rubber due to the high cost of certification for A/C use of synthetic replacements - rubber delivered from petroleum Latex being collected from a tapped rubber tree Airless tire A/C tire maintenance at sea aboard USS Abraham Lincoln A blown-up tire FE analysis of A/C tire

44. AIRCRAFT TIRE OPERATING CHARACTERISTICS

45. Dunlop Goodyear Bridgestone Michelin Condor Specialty Tires INTERNATIONAL AIRCRAFT TYRE MAKERS

46. tyres Tyres with patterned tread became important when aircraft got effective brakes that could be used for slowing the aircraft during landing . At first the treads were a diamond pattern that provided good braking on wet grass but the ribbed tread proved to be more suitable for operation on hard surface runways . Today almost all aircraft tyres have a ribbed tread that consists of straight grooves, which run around the tyres’ circumference.

47. Bogie type design for runway load reduction Bomber XB-36 – 9’ DIA circa 1946 Spoke wheel type tire – WWI era Cargo type A/C Antonov 225 landing gear

48. heavy loads, high speeds and high deflections HEAT RUBBER (good insulator ) reduce the tire life Traction waves groove cracing rib undercutting tread separation CENTRIFUGAL FORCES taxi speed – inflation pressure – taxi distance Internal tensile forces on each layer carcass separation TENSILE COMPRESSION SHEAR FORCES

49. Automobile and truck tyre Long operation Relatively large load Reasonably high speed Deflection 12-14% Aircraft tyre Tremendous load Very high speeds Deflection 32-35% TYRE CONSTRUCTION

50. TYRE INFLATION AND DEFLATION The tyres are inflated with nitrogen from a ground cart. The required pressure will be laid down in the AMM and a tyre inflation box is used to regulate the charge rate and pressure. A deflation tool is used to release the pressure and any ice that forms must be allowed to thaw before the valve core is removed.

51. WHAT IS NITROGEN? Nitrogen is a dry, inert gas used to inflate airplane tires, off-road truck tires, military vehicle tires, and race car tires for improved performance, more tire mileage and better fuel economy . Why use Nitrogen? Less inflation pressure loss Reduced wheel corrosion Prevents inner-liner rubber deterioration by oxidation Tires run cooler Increases tread life Increases fuel mileage Helps prevent uneven wear

52. Oxygen in compressed air permeates through the wall of the tire, thus reducing the tire's inflation pressure. During it's journey through the tire wall, oxygen oxidizes the rubber compounds in the tire, causing under-inflation and deterioration of the rubber . Dry nitrogen will maintain proper inflation pressure and will prevent auto-ignition, will not corrode rims, extends valve core life and will help the tire to run cooler.

53. The biggest advantages - improved tire life Experts in the tire industry indicate that oxidative aging is one of the primary causes of decreased tire life. Oxidative aging is caused by the diffusion of oxygen from the pressurized air cavity of the tire to the outside atmosphere. Tests have shown that if tires are inflated with nitrogen, there is a significant reduction in tire failure.

54. Oxygen and moisture corrodes aluminum and steel wheels. Oxygen also reacts with rubber, another type of "corrosion". When this corrosion starts, the small particles break off and form rust and dust, which can clog vavle cores, causing them to leak. The rough surfaces created from the corrosive action on the wheels leads to tire beads that don't seal properly, causing additional leaks. Oxygen also ages the inner liner, the thin layer of rubber inside the tire whose function is to keep air away from the carcass. As the inner liner ages, more and more air molecules can pass through it, causing more pressure losses. These pressure losses in a truck tire can average 2 psi a month as a result of the air passing through the sidewalls. As it passes through the rubber, the oxygen can also corrode the steel cords, causing them to rust too.

55. TYRE CONSTRUCTION

56. THE BEAD The bead gives the tyre its strength and stiffness to assure a firm mounting on the wheel. The bead is made up of bundles of high strength carbon steel wire with two or three bead bundles on each side of the tyre.

57. THE BEAD Rubber strips streamline the round bead bundles to allow the fabric to fit smoothly around them without any gaps. The bead bundles are enclosed in layers of rubberised fabric, to insulate the carcass plies from the heat absorbed in the bead wires.

58. THE CARCASS The carcass (or chord body) is the body of the tyre that is made up of layers of rubberised fabric cut in strips with the threads running at an angle of about 45 degrees to the length of the strip. These strips extend completely across the tyre around the bead and partially up the side.

59. THE CARCASS Each ply is put on in such a way that the threads cross each other at about 90 degrees to that of the adjacent ply. This type of construction is known as bias ply . The cords of the ply fabric were originally cotton, then nylon and now aramid fibres ( kevlar ) are used.

60. THE CARCASS This is stronger than nylon, polyester or fibreglass and even strong pound for pound than steel . Chafing strips are rubberised strips of fabric that wrap around the edges of the carcass plies and enclose the bead area . The chafing strips provide a smooth chafe resistant surface between the tyre and the bead seat of the wheel.

61. THE CARCASS The under tread is a layer of compound rubber between the plies and the tread rubber that provides good adhesion between the tread and the carcass. On top of the under tread are more plies of strong fabric that strengthen the tread and oppose centrifugal forces that try to pull the tread from the carcass during high speed rotation.

62. THE CARCASS The inner liner is a thin coating of rubber over the inside plies . For tubeless tyres it is made from a compound which is less permeable than other rubbers used . It seals the tyre and reduces the amount of leakage. On tyres with inner tubes the liner is very smooth to help prevent chafing.

63. Aircraft Tyre Construction

64. THE TREAD The tread is the thick layered rubber around the outer circumference of the tyre that serves as a wearing surface. The tread has a series of moulded grooves moulded into its surface to give optimum traction with the runway surface.

65. A/C TIRE MAINTENANCE/OPERATION PPE equipment Storage/Protection of tires - Bright sunlight - Excessive heat - temperature: cold (flat spot) - Oil, fuel, hydraulic fluid, etc. - ozone: electric motor, welding sparks Inspection of tires Special: FOD, hard landing, under inflation (-95%), wear Tire defects - ozone cracking (degradation) - break burns - skid burns - hydroplaning

66. TYRE WEAR ASSESSMENT The manner in which tread wear of a tyre is established, is dependent upon which of a number of methods of indicating wear has been incorporated into the tyre by the manufacturer . Tyres used on modern aircraft have a series of circumferential grooves in the tread, primarily to displace water on the runway and so help to prevent the tyre from aquaplaning.

67. TYRE WEAR ASSESSMENT These grooves can be also used as a means of establishing tyre wear. If this method is adopted, then wear which results in any groove being less than 2mm in depth, for more than 25% of the tread circumference, requires the tyre to be replaced.

68. TYRE WEAR ASSESSMENT Other ways of establishing wear assessment are by the use of : Tie Bars Wear indicator Grooves Sipes

69. Tie Bars These are small transverse bars of rubber, moulded at intervals in the circumferential grooves around the tyre as described above. They are set at a depth of 2mm, or as required by the particular manufacture and thus provide an easy visual means of establishing wear limits. Limits – tyre worn to the top of the tie bar.

70. Tie Bars

71. Wear Indicator Grooves These are dedicated grooves set in the tread pattern and have a depth graduated by the manufacturer, but typically 2mm shallower than the water-displacing grooves . Limits – tyre worn to the bottom of the indicator groove anywhere on the circumference of the tyre.

72. Sipes Certain tyres, normally those having a zigzag tread pattern have an axial slit in the tread rubber at some of the zigzag corners. The slit does not extend into the depth of the tread and is called a sipe . Limits – Tyre worn to the bottom of the sipe .

73. TYRE DAMAGE The amount of tyre damage a tyre can suffer without becoming unserviceable is very small . Damage in the vicinity of the bead is rarely tolerated . While cuts in the casing plies must be assessed very carefully in accordance with the manufacturer’s requirements before deciding on the degree of serviceability.

74. TYRE DAMAGE Normally if the chords are exposed due any form of damage, including splits or crazing, then the tyre will be classed as unserviceable . NOTE: Always consult the Aircraft/Component Maintenance Manual.

75. LEAK HOLES (AWL HOLES) During inflation of a tyre/tube assembly, air may become trapped between the tube and the inside surface of the tyre, giving an incorrectly inflated assembly . The risk is reduced by allowing the air to escape through Leak Holes, pierced completely through the sidewall of the tyre, during manufacture.

76. LEAK HOLES (AWL HOLES) The holes are often made with a pointed tool called an Awl . Because of this, the holes are sometimes referred to as Awl Holes . The position of these holes is indicated by a series of 6mm diameter spots of grey or green litho ink, usually grey.

77. VENT HOLES During the manufacture of tubeless tyres, air that gets trapped between layers in the casing is permitted to escape to atmosphere through vent holes pierced in the sidewall. The vent holes do not penetrate right through the sidewall in this case and are identified, as with leak holes, by 6mm diameter spots of grey or green litho ink, usually green.

78. BALANCE MARKS A red spot (sometimes triangular) on either side of the tyre indicates its lightest point around the circumference as ascertained during the manufacturer’s balancing procedure . During assembly with the wheel the red spot should be aligned with the inflation valve on a tubeless assembly . On a tubed assembly, the spot should be aligned with a red line (heavy point) on the tube. If it has no red line, align with the inflation valve of the tube.

79. ELECTRICALLY CONDUCTING TYRES Some wheel assemblies are fitted with tyres that are designed to conduct electrical charges to earth as the aircraft touches down. Such tyres are identified with the word CONDUCTIVE or the letters ECTA (electrically conducting tyre assembly) on the sidewall.

80. AQUAPLANING Aquaplaning is a condition that occurs on wet runways when a wave of water builds up in front of a spinning wheel. This could result in the tyre being lifted from the runway surface and to float on the thin layer of water. This is dangerous, as a complete loss of braking efficiency will occur.

81. AQUAPLANING Although it appears only to be an aircrew problem, there is a significant factor that affects the maintenance engineer . Mathematically there is a formula for Aquaplaning speed - Aquaplaning Speed (Kt.) = 9 (approx.) x Square Root of the Tyre Pressure .

82. AQUA PLANING This speed will be placard for the crew, so that in wet conditions they will quickly traverse through it on landing . However, if the tyre pressures are incorrect, the placard speed will be useless and aquaplaning will occur at a different speed. Take care, therefore, to maintain tyre pressures at their correct value at all times.