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Dunlop's Really Useful Guide to Tire Maintenance



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This is a simple guide giving practical hints and tips to getting the most out of the tires on your airplane or helicopter.

Dunlop's Really Useful Guide to Tire Maintenance

  2. 2. All items on commercial airliners suffer wear and tear. Tyres are no different. There is no way to stop a tyre wearing but there are practical things that airplane operators can do to increase their service life. Thisreallyusefulguideintroducesapractical perspective to tyre wear which is intended to help aircraft operators, maintenance staff and purchasers to understand a little more about this critical component. • General Information • When and How Tyres Wear • Identifying Wear Profiles • Suggestions for Reducing Wear Really Useful Guide to Optimising Tyre Service Life In This Guide
  3. 3. An aircraft tyre is a pressure vessel. Its role is to hold the gas that supports the load of the aircraft. No other tyre has to deal with such high loads and such high speeds as an airplane tyre. For the purpose of this guide, we can consider the tyres as being made up of three principle elements, the tread, the beads and the casing. This guide focuses on the tread and the casing. General Information Terms used in this Guide: Tread The part of the tyre in contact with the ground. Designed to resist abrasion. Circumferential grooves displace water. Bead Metal wire used to anchor the tyre to the wheel and anchor the casing plies. Shown in image one and two as blue circles. Casing For the purpose of this guide the casing is considered as everything else that makes a tyre. It is principally comprised of fabric cords (ply cords) encapsulated inside a rubber sheet (known as a ply) and a number of other components combined during final assembly. Multiple plies are combined to form a pocket. The ply cords provide strength. The angle of the cords determine if the tire is considered bias or radial. Tread Life This refers to the landings per tread - the metric with which operators are most concerned. Casing Life This refers to the number of tread lives (retreads) that a casing can have relative to its design goal or total fatigue life. FOD Acronym for Foreign Object Damage. This is a significant cause of premature tyre removal. FOD is not specifically discussed in this guide. Beads Tread Plies Beads Tread Plies Images one and two depict a rough representation of an aircraft tyre with no load and an aircraft tyre with load. Images three and four show the components of the tyre - the bit that nobody sees - under the black. For more details see DM1172. Unloaded Tyre (Image 1) Bias Tyre (Image 3) Radial Tyre (Image 4)Loaded Tyre Showing Deflection (Image 2) The full load and high speed is applied in a fraction of a second during landing. During take off, the speed builds and the aircraft is typically heavier than it is when it lands.
  4. 4. When Tyres Wear Landing. A puff of smoke; the screech of tortured rubber. It’s always dramatic. But that’s not where most of the wear occurs. The graphic below gives a rough indication of when, in the life-cycle, a tyre wears. For both casing and tread wear, heat is one of the most significant causes of wear. Local factors and operating conditions or practices will determine the precise ratio between the various stages of the life-cycle. Rolling/ Accelerating ~20% Flying 0%! Landing ~15% Braking ~ 65%
  5. 5. How Aircraft Tyres Wear - Tread Life The wear of the tread is caused by the abrasion of the tyre against the runway as the tyre progresses through its service life. Loading rubber with a static and vertical force alone will not cause wear. Adding a dynamic force (the tyre rolling along the runway) certainly will! Factors that increase heat such as hard braking and fast cornering will wear the tread of an aircraft tyre, just like it does the tread of a car tyre. Load & Pressure: If the longitudinal force remains constant a higher aircraft load and higher inflation pressure will increase wear. Tyre Shape: Certain profiles of tyre will wear quicker due to the relationship of centre size and shoulder size. Under Inflation: The footprint of the tyre on the ground and the deflection on landing will be higher than intended which means more wear. Uneven Inflation: (On Multi Tyre Struts): The tyre with correct inflation pressure will carry more of the load which will increase wear of the tyre. Wear on the under-inflated tyre will increase because of the larger footprint and higher operating temperatures. High Altitude Airports: The higher the airports served the greater the tyre wear will be. This is because air, at high altitude is less dense and generates less lift. The aircraft needs to travel both faster and further to take off. Ambient Temperature: High temperatures cause warm tyres. Heat is the enemy of all tyres and is generated during ground operations. Furthermore, as temperature increases the distance covered before take-off increases because the air is less dense which generates less lift. Again, increasing wear. Position of tyres on the airframe: Tyres fitted to different positions of the aircraft will experience different wear characteristics due to the interaction of the tyre with the aircraft and the ground. Camber: For landing gear or conditions which induce a camber the footprint of the shoulder of one side of the tyre will be greater than that on the other side of the tyre. More camber causes more wear. Too much negative camber will cause wear to the inner shoulder. Too much positive camber will cause wear of the outside shoulder. Toe-In/ Toe-Out: If the tyre does not run straight, i.e. there is toe-in/ toe-out, the tread will be dragged down the runway at an uncomfortable angle which will cause wear. Too much toe-in will cause wear to the outside shoulder, too much toe- out will cause wear to the inside shoulder. Both tyres might look to be the same but their inflation pressure can be quite different. The only way to be sure is to measure with an approved pressure gauge. Flying from hot to cold? Increase pressure before take off in accordance with the AMM. If flying from cold to hot decrease the pressure in accordance with the AMM. See DM1172 for more information. Positive Camber Negative Camber
  6. 6. How Aircraft Tyres Wear - Casing Life The fatigue (wear) of the casing is invisible and is mostly concerned with the impact on retreadability. A higher scrap rate at each retread level will increase costs for the operator of the aircraft or, under certain conditions, the supplier of the tyre. Even so, conditions can lead to premature scrapping in the interests of safety. Heavy Landings will increase the deflection of the tyre at the point of touch-down which can damage the internal components of the tyre. Fast and tight turns increase heat in the tyre and combined with the high forces of fast and tight cornering might cause separation of the internal components. Such use increases tread wear, Very slow and very tight turns where a tyre remains broadly static or even moves backwards can cause tearing of internal components. Single engine operation induces a turning force which is corrected with steering input. The effect of the thrust on the ‘engine-off’ side makes the tyre want to ‘crab’ which induces wear of the internal components. The decision for the airline is based upon the value of fuel savings compared with increased tyre consumption. Fast turnaround times leave heat in the tyre for longer. Heat induces fatigue as it causes the bonds between rubber components to break down. This can cause voids to form in the tyre which will be identified during the retread inspection process. 10% Under-inflated: ~12 cycles till failure. 20% Under-inflated: ~8 cycles till failure. 40% Under-inflated: ~2 cycles till failure. Under-inflation is a significant cause of casing damage which prevents a tyre reaching its full retread potential. At worst, a catastrophic failure might occur as a result of under-inflation. Under-inflation causes extra heat build up which can make components separate from each other. Each phenomena on this page is magnified by low pressure. Impact of Under-Inflation
  7. 7. Identifying Wear The wear of the casing is invisible and is mostly concerned with the impact on retreadability. A higher scrap rate at each retread level will increase costs for the operator of the aircraft or, under certain conditions, the supplier of the tyre. Foreign Object Damage can force the premature removal of the tyre in the interests of safety. See DM1172 or the Dunlop Pocket Guide for more information. Wear of the Tread Wear of the Casing When Dunlop receive a tyre to retread the external characteristics of the tyre are visually inspected. But it is the condition inside the tyre which is critical. To look inside the tyre to see if the casing is damaged Dunlop use shearography. A laser camera takes a photograph of a tyre in a relaxed state and then another photograph of the tyre is taken whilst a vacuum is induced. The two images are combined and any areas where the inside of the tyre is damaged are clearly shown. Perfect - even wear across the tread and a little life left to wear down to the bottom of the grooves. This has stayed on wing a bit too long and has worn below the grooves. It’s time to change if fabric is visible. This tyre has worn too much on one side of the tyre. This wear profile is caused by camber or uneven loading. Stepped wear is most prevalent on certain types of tyre and is made worse by under-inflation. This tyre has had a rough life. The highlighted area indicates separation within the tread. There is only one separation - the software indicates a double bulls-eye where the two images are layered over each-other. Identifying Wear Even Wear across the tread. Excessively worn tyre Asymmetric Wear Stepped Wear
  8. 8. Suggestions for Reducing Tyre Wear As we have seen in this Really Useful Guide there are many different causes of tyre wear which affect different elements of the tyre. Friction will damage the tread but certain operational practices may impact the integrity of the casing. So in other words, the tyre will wear and that is just the way it is! But there are some things that operators can do. Turn slow and wide: This will ensure that all wheels rotate as the aircraft manoeuvres which prevents the tyres being scrunched into the asphalt. Starting all manoeuvres with wheels straight ahead also helps. Be cool: Heat is the enemy of all tyres. Slower turn around times will reduce heat and thus prolong the life of the tyre. Excessive heat can cause the joins in the casing to breakdown. Slow down the taxi: Fast taxiing causes more heat to build up in the tyre which increases wear of the tread and fatigue of the casing. Maintain the Gear: Sometimes wear on landing gear components can cause extra wear on the tyre. Maintaining the gear well keeps everything running in the correct tolerances. Check out the tyre care data: Dunlop publication DM1172 provides lots of information about tyre phenomena and should be read with the AMM. And Dunlop User Support Specialists are available for you to provide bespoke training sessions at a venue of your choosing. Pump Up The “If there is a silver bullet for increasing both tread life and casing life then correct inflation is it. Dunlop recommends 24hr checks.” An over-inflated tyre wears more quickly on the crown (top) of the tread and will be more susceptible to FOD damage in this area. It will be worn on the top but tread will remain on the shoulders. An under-inflated tyre deflects more than a correctly inflated tyre for a given load. This can lead to increased casing fatigue, reduced retreadability, creep on the wheel or induce the wheel to pinch the tyre or the wheel to strike the runway. The tyre will also run hotter and will wear faster on the shoulder than on the tread. And finally, FOD free runways and ramps will guard against costly early removals. Under-inflated tyre: As the load increases the deflection of the tyre increases more than intended. Load will not be shared evenly. Correctly inflated tyre. The tyre deflects in line with the design and the load is evenly shared. Deflection Load % 100% Tyres
  9. 9. Dunlop Europe, Middle East & Africa (44) 121 384 8800 Dunlop The Americas (1) 336-252-2801 Dunlop Asia Pacific (86) 595 85931007