Article By- Captain. Rajkumar P Kesarkar. Air veteran, with more than 8000 hrs of flying experience on military and civil transport aircraft. Presently, flying airbus 320 as a commander with a private airline.
INTRODUCTION
1. Aircraft tires/wheels work under extreme conditions, carrying up to 340 tons and accelerating at over 250km/hour at takeoff, in addition to enduring varied environmental stress when in flight and taxiing. Aircraft wheels are subjected to the daily punishment of multiple takeoffs and landings. Tires are exposed to temperatures below -40°C during the cruise and at touchdown; rubber temperatures can momentarily exceed 200°C. Wheels must handle the most extreme torture in aviation: a maximum weight, high-speed rejected takeoff: A fully loaded aircraft accelerates to takeoff speed, and then stops on the remaining runway. Tires withstand extreme heat and stress until the aircraft is safely stopped. Few aircraft components take more daily abuse than the tire and wheel assembly.
Tyre / Wheel Construction
2. Aircraft tires are too rigid to be forced on to a rim like automotive tires. Aircraft wheel hubs come in two parts. The inboard and outboard hubs are bolted together with the tire in the center and then pressurized with nitrogen.
TYRE DESIGN REQUIREMENTS:
3. Aircraft tyres are designed to withstand extremely heavy loads for short durations, with the number of tyres required increasing with the weight of the plane in order to better distribute the weight. Aircraft tyre tread patterns are designed to facilitate stability in high crosswind conditions, to channel water away to prevent aquaplaning, and for braking effect. Some types of nose wheel tyres include one (or two) chine moulded into the rubber on the shoulder buttresses that deflects water away during aircraft movement on a wet runway.
Tyre Types
Types of tyres Based on the method of tread pattern can be:
- The tyre with grooved guiding tread pattern (R – ribbed / parallel) which ensures stability of the aircraft and adhesion on a wet surface.
- The TC (twin contact) designed for steerable front wheels.
- The MX (mixed) a combination of both previous types.
The tundra tire: Ballon type tyre, Is a large low-pressure tire used on light aircraft to allow operations on rough terrain and at times on water. These tires include an integral inner tube with the valve manufactured into the side-wall, allowing the tire to operate at very low pressures without risking shearing-off the valve stem and causing a flat tire. Low-pressure tires provide greater cushioning and enable aircraft to land on rough surfaces, unsuitable for normal tires. Eg. Maule (M-7-235C) on bushwheel-style tundra tires.
(e) Grip or horizontal profile type: Aircraft’s which land on rough terrain with low speeds.
Eg. Hawker sea fury. These types are good while taxiing at high speeds and turning due it’s gripping tyre design similar to high-speed motorcar sport.
But these are not good for bigger aircraft’s due:
(i) Uneven damage to tyres due more landingsan making it sleek
(ii) Deterioration of small individual grips are higher and the possibility of flying off rubber from tyre leading to unbalanced roll.
(iii) FOD or small stones could get gripped in tyre profile and get flung off during high speed damaging aircraft fuselage,wing etc.
(f) Parallel groove tyres: Aircraft tire treads have several circumferential grooves molded into the tread that help channel water away from the tire surface. Complex patterns that improve traction on automobile tires are not necessary on aircraft because the wheels rotate freely. Large aircraft land on straight, well-prepared runways. Modern runways are “crowned” — the runway gently slopes away from the centerline – to drain water. To further improve drainage and tire traction, runways often have grooves cut perpendicular to the direction of travel.
Workers add grooves to a runway.
Since modern aeroplanes land on straight runway and there Taxy speeds are much lesser, they use parallel groove tyres for better handling on wet runway surfaces. This design prevents aquaplanning or hydroplanning. Some General aviation aircraft’s with engine’s mounted behind and above the fuselage, use parallel groves tyres but with chin (rubber protruding out at end). The edges of the tire sidewalls have a curved protrusion (chine) that deflects standing water outward to reduce water ingestion into the engines. Some of them also usage metal frame (Deflectors / Gravel kit) near tyre to deflect water away from getting inside engine during takeoff or landing on wet runway.
Eg. Boeing737-200.
(g) Based on the type of plies used, the tyres can be:
- Radial ply tires – these tyres feature a flexible casing which is constructed of rubber-coated ply cords which extend around the beads and are substantially at 90° to the centerline of the tread. The casing is stabilized by an essentially inextensible circumferential belt.
- Bias ply tires – these tyres feature a casing which is constructed of alternate layers of rubber-coated ply cords which extend around the beads and are at alternate angles substantially less than 90° to the center line of the tread.
(h) Based on the method for containing the inflation gas the tyres can be:
- Tube-type tyres – these require tubes for inflation retention.
- Tubeless tyres – these do not require tubes. They are constructed with an innerliner (integral rubber lining that is engineered to prevent the diffusion of the inflation gas into the casing.).
Hazards associated with tyres:
4. In the first moments after a plane touches down, the tires are skidding, not rolling. The airplane essentially drags them down the runway until their rotational velocity matches the velocity of the plane. That’s why they smoke upon landing, and why tyres uses grooves instead of the block patterns seen on your car’s rubber-blocks would simply break off. (Most tire wear comes from this moment of contact-where the rubber meets the runway.) The stoutest tires are rated for speeds of up to 288 mph. These hazards are associated with 4 distinct periods of aircraft operation:
- Ground operations e.g. taxiing
- Take-off (up to gear retraction)
- Post take-off (gear stowed)
- Landing (to the end of the roll-out)
The various combinations of hazard and flight periods can have markedly different influences, but all can affect operations to some degree.
The main hazards are as follows:
- Deflation: the tyre deflates in a controlled manner with minimal direct consequence to other systems.
- Explosive break-up: the tyre (and sometimes the wheel holding the tyre) deflates or breaks-up in an uncontrolled manner with a significant probability of secondary damage to other unrelated systems.
- FOD Damage: Tire destruction by foreign object damage. Not every cut leads to immediate or even eventual tire failure. However, there are two basic “worst case scenario” when an aircraft tire hits a foreign object. The tire bursts if the foreign object penetrates through the tread into or through the casing OR The tire tread peels off if the cut damage is limited to the tread area.
Factors Affecting tyre performance
5. The tires themselves aren’t terribly large- a Boeing 737 rides on 27×7.75 R15 rubber. In English, that means it is 27 inches in diameter, 7.75 inches wide, and wrapped around a 15-inch wheel. The sidewalls aren’t terribly thick, and the strength of the tire lies in the cords embedded below the tread. They’re typically nylon, and more recently a variety known as aramid. Each layer of the casing contributes to its load bearing and air pressure resisting capabilities. Of course, tires can fail, especially when under-inflated or overloaded. Treads can come off and casings can blow out.
There are three major factors that affect tyre performance:
- Centrifugal force. This is a combination of load and speed. Due to centrifugal force and inertia, the tread surface doesn’t stop at its normal periphery but overshoots, briefly distorting the tyre from its natural shape. This sets up a traction wave in the tread surface. Traction wave is mostly affected by two factors – speed and underinflation. The result of traction wave exposure can be damaged tyre (e.g. groove cracking or rib undercutting).
The mechanism of traction wave and a picture of a tyre subject to traction wave
- Heat generation. Heavy loads and high speeds cause heat generation in aircraft tyres to exceed that of all other tyres. The physical properties of rubber compounds are susceptible to degradation by high temperatures. Both strength and adhesion are lost when the rubber reverts to the uncured state. Brief exposure to high temperatures is not as damaging to the tyre as are prolonged exposures. Internal heat can cause tread and casing separations. External heat sources (e.g. brakes) can cause bead face damage.
An example of tread and casing separation
- Tensile, Compression and Shear Forces. An aircraft tyre is designed so that in the unloaded condition the internal tensile forces acting on each layer of fabric are uniform. Due to the high deflection of the tyre section under the load, the tensile forces on the outer plies will be higher than those on the inner plies. Due to the force gradient from outer to inner plies, shear forces are developed between the various layers of fabric. Underinflating or overloading a tyre will increase these shear forces, thus rapidly decreasing the life of an aircraft tyre. Common types of damage associated with these forces are shoulder separation and lower sidewall compression break.
Examples of shoulder separation and lower sidewall compression break
Manufacturers of Tyres
6. Airlines often purchase tires directly from the manufacturer and retain ownership for the life of the tire. When tires are sent back to the factory for retreading, the same tires are returned to the airline that owns them. There are also tire leasing and tire service contracts available. Each airline makes its own deal with tire distributors and manufacturers. The aircraft tire manufacturing industry is dominated by four firms. These firms control approximately 85% of the manufacturing market. An aircraft tire carcass/casing (tire without the tread) is constructed super-tough. A carcass that is eligible for retread is a desirable asset; it has demonstrated that it can stand up to the abuse of airline operations. Retreading a tire is less expensive than buying a new one. Some tires can be retread as many as 16 times. Airlines often retread tires less than the manufacturer’s limit as an added measure of safety. Another benefit: retreads have more plies than new tires so they can handle more takeoffs & landings. You would never dream of mixing new Goodyear and Michelin tires on a car. Aircraft tires are all manufactured to the same specifications so it’s common to see two different brands on the same landing gear bogie. The four major manufacturers in aircraft tire manufacturing are the following:
- Goodyear (United States)
- Michelin (France)
- Dunlop Aircraft Tyres (United Kingdom)
- Bridgestone (Japan)
7. One thing you almost never see when an airplane land is a blowout. Think about that: Again and again, the tires hit the tarmac at 170 miles per hour and bear the weight of a modest office building. And they nail it, every time. Aircraft tires are amazing when you think about it. The typical airliner tire can handle a 38-ton load. It can meet the ground 500 times before needing a re-tread, a refresh it can take on seven to sixteen times in its life. A Boeing 777 uses 14 tires, Airbus’ A380 carries 22, and the enormous Antonov An-225 has 32. Each of the twelve Boeing 777-300ER main tires is inflated to 220 psi (15 bar / 1500 kPa), weighs 120 kg (260 lb), has a diameter of 134 cm (53 in) and is changed every 300 cycles while the brakes are changed every 2000 cycles. Each tire is worth about $5,000. The key to their remarkable durability is maximizing the air pressure. The high-flying rubber is typically inflated to 200 psi, roughly six times what you put in an automobile tire, and the tires on an F-16 fighter are pumped to 320 psi. “It’s really pressurized air that’s so strong,”. Tests of airliner aircraft tires have shown that they are able to sustain pressures of maximum 800 psi (55 bar / 5,500 kPa) before bursting. Aircraft tires are usually inflated with nitrogen to minimize expansion and contraction from extreme changes in ambient temperature and pressure experienced during flight
Why Nitrogen Is Used In Aircraft Tyres?
8. Now, one might notice that every time a car or even a motorcycle drops to the ground after being airborne for even a few seconds, they create friction and the tires feel like they are about to burst. Sometimes they even do. But that does not happen with airbuses or Aeroplane wheels. The reason is, motorbike/car tires or wheels are filled with air. Airplane wheels are filled with Nitrogen because of the following:
(a) Less Prone to Combustion: We all know that Air is nothing but a mere combination of the various gas molecules that can explode under high temperatures or pressures. Nitrogen is an inert gas, meaning it shows very little reaction to other gases or surfaces. Hence, the nitrogen gas inside the tires of the airplanes doesn’t initiate chain chemical procedure with rubber. Thus, they are not prone to combustion, making them the best choice for lifting airplanes or airbuses.
(b) Does not Change Its State Easily: The humidity present in the air can turn into water drops or ice at relatively lower atmospheric temperatures. As aircraft fly between 31,000-38,000 feet height on average, liquidation or crystallization of water molecules can be very dangerous and cause accidents when aircraft tires land on the ground. Nitrogen contains very low moisture. And it has a very low melting point of minus 210 degree Celsius. As the height aircraft fly on doesn’t usually get cooler than minus 55 degree Celsius. There’s no chance of liquidation or crystallization.
(c) Low Maintenance Cost: Another huge plus point of using Nitrogen is, they have very less water vapor than the air. Thus, they can hold the tire pressure longer than the air. As a result, aircrews don’t need to change the nitrogen supply or even the tires frequently. Because Nitrogen is an inert gas, so the pressure stays pretty much unchanged.
Tyre safety Devices
9. Tires often overheat if maximum braking is applied during an aborted takeoff or an emergency landing. The fuses provide a safer failure mode that prevents tire explosions by deflating in a controlled manner, thus minimizing damage to aircraft and objects in the surrounding. A fusible plug is a small hollow bolt filled with low melting-point metal (like solder used for electronics or plumbing).
(a) Fusible Plug:
Fusible plugs often come into play after heavy braking, as would happen during a high-speed rejected takeoff. After the aircraft stops, the hot brake assemblies continue to heat the wheels until the fuse cores reach their melting temperature and deflate the tires. Fusible plugs are mounted inside the wheel hub. When the plugs deflate the tire, nitrogen is directed over the brakes to aid in cooling.
(b) Over Pressure Relief Valve (OPRV)
An over pressure relief valve is a hollow bolt with a rupture disk inside. The disk ruptures when nitrogen pressure exceeds the design limit. OPRVs are installed on most wheel rims to protect tires from over-pressure or explosion that can occur during nitrogen servicing.
(c) TPMS (Tire Pressure Monitoring System)
Some aircraft models have TPMS sensors in their wheels. The system is very similar to the TPMS in automobiles. Cockpit displays show tire pressures for all tires equipped with the sensors.
- Brake Temperature Monitoring System
Many large aircraft have brake temperature monitoring systems. Eg. Boeing 767-300F
Size of Airliner Tires
10. Like cars and trucks, aircraft tires come in many sizes. Tire size data is molded into the sidewall of every tire. A Boeing 757-200 uses H40x14.5-19 tires on the main landing gear. Decoded, the “H” means high deflection, 40 inch tire diameter, 14.5 inch tire width, and 19 inch wheel/rim diameter.
Main gear tire diameter and width for a few popular airliners:
| Aircraft | Diameter | Width |
| Boeing 737-700, 800, 900 | 44.5″ | 16.5″ |
| Boeing 747-8 | 52″ | 21″ |
| Airbus A320 | 46″ | 17″ |
| Airbus A330 | 54″ | 21″ |
| Airbus A350/A380 | 55″ | 21″ |
Rotation, Balance & Speed ratings of Airline tyres:
11. Airlines don’t rotate tires. A tire’s lifespan is too short to worry about uneven wear. Large aircraft wheels are not balanced. Tires take a lot of punishment and each landing leaves rubber on the runway. Keeping them balanced would be a losing battle as every landing changes the weight distribution of the tires. Most airliners have tires rated for around 220-235 mph. This is way faster than the aircraft is typically traveling on the runway. Takeoff and landing speeds vary between 140-200 mph, so there is a good margin of safety in the event an aircraft needs to land at a high speed (due to emergency or equipment malfunction). The tire load rating for Boeing 767-300 for single wheel is 51,100 lbs and it has 8 main wheels. 51,100 x 8 = 408,800 lbs. It has maximum takeoff weight of 408,000 lbs. Tires Have Speed Ratings indicated on them.
When are tires changed?
12. They’re designed to meet full performance specs, even with the first layer of reinforcement fabric showing. If you see a tire that looks bald on an aircraft, don’t be worried. Tires are inspected after every flight. They are replaced when they reach the manufacturer’s service limit. The grooves molded in the tread are used as wear indicators. Tires are replaced when the tread is worn to the base of a groove. Cuts, sidewall damage, or bulges may require an early tire change. If a replacement tire isn’t available, the tire can stay in service, even with the first layer of fabric (cord) visible, until it reaches a maintenance base.
The grooves are still visible on this tire. It’s safe for a few more landings.
How many takeoffs & landings / Are tires changed in sets?
12. Tire change cycles vary based on runway conditions, weather, aircraft operating weights and landing technique by pilot. A rough average is about 150-200 cycles for the main tire on a large aircraft (one takeoff and landing = one cycle). Nose wheel tires last a few more cycles than main tires. Main landing gear and nose tires on large aircraft are usually changed only when they reach wear limits. It’s common to see new and old tires next to each other. Certain types of wear or damage will require tires to be changed in sets. Tire changes are actually wheel changes. The whole wheel is removed and replaced, just like changing a flat tire on a car. Wheel changes can be accomplished quickly, often without delaying the next departure. When fabric appears, it’s time to change the tire. Remember, this is not an automobile tire. Aircraft tires are designed to be flown until the tread grooves are gone. Again, they can be safely flown with fabric showing in order to reach a maintenance base. From start to finish, the wheel change takes less than 20 minutes.
It’s common to see new and old tires together. The tire on the right side of the photo (left nosewheel) is close to its service limit and was changed shortly after the photo was taken.
CONCLUSION:
13. The Lockheed U2 only has 2 sets of landing gears-one at the front, one at the back-and therefore a total of 4 wheels as compared to An-225 has 16 sets of wheels (32 total). Generally, heavier aircraft will have more wheels so that they can spread out the weight over a large surface area, resulting in lower pressure on the runway surfaces.
Tires are so abused and yet also underappreciated by both the general public and those who fly them. We could have written more about technical details, tyre assembly, inspection, manufacturing & storage etc. but thought of sticking to the article from the prospective of Pilots instead of technicians.
