Would you like to guess the approximate weight of an aircraft (eg A320) ?
It is approximately 70000kgs to 90000kgs!!
The number of tyres required for aircraft increases with the weight of the aircraft, as the weight of the airplane needs to be distributed more evenly. Aircraft tyre tread patterns are designed to facilitate stability in high crosswind conditions, to channel water away to prevent hydroplaning, and for braking effect.
And the entire aircraft weight is taken up by tyres during landing, take off and on ground. Aircraft wheels are subjected to the daily punishment of multiple take offs and landings. Tyres are exposed to temperatures below -40°C during cruise and as low as high as 200°C during touchdown. Thus the tyres withstand extreme heat and stress throughout its journey.
This Article has been divided into the following 12 parts:
- Tyre Construction
- Why Tyres are filled with Nitrogen?
- Tyre Pressure
- Fusible Plug
- Over Pressure Relief Valve(OPRV)
- Brake Pressure Monitoring System
- Thread Pattern
- Types of Tyres
- Rated Speed
- Hazards to Tyre
- Cost of Tyres
- Tyre Handling
1. Tyre Construction
Unlike automobile tyres which are closed into a single rim, the aircraft tyres are too rigid to be forced into a single rim structure so aircraft wheel hubs come in two parts. The inboard and outboard hubs are bolted together with the tyre in the centre, then pressurized with nitrogen.
2. Why tyres are filled with Nitrogen ?
- The tyres of airliners are filled with inert dry gas like nitrogen because they are inexpensive and perfect for the purpose. Let us know how!
- Nitrogen does not support combustion, greatly reducing the risk of a tyre fire or explosion.
- Dry nitrogen contains no water vapor. The lack of moisture reduces tyre pressure variations at temperature extremes (water density varies significantly at different temperatures). Thus with the effects of moisture eliminated, change in tyre pressure due to temperature is linear and predictable.
- Oxygen in place of Nitrogen cannot be used as oxygen and moisture in atmospheric air cause corrosion to Aluminum and steel wheels. Dry nitrogen eliminates this problem. Also air and moisture may cause oxidation of a tyre’s inner liner. Nitrogen does not degrade the rubber.
3. Tyre Pressure
Automobile tyres are pressurized to around 30-40 psi. If large aircraft tyres were filled with 35 psi, they would be flat under the weight!!
Large aircraft tyre pressures are ridiculously high. A Boeing 767-300 main wheel is inflated to 205 psi.
The high pressure supports the tyre’s maximum rated load of 51,000 lbs (approx 24000 kgs).
4. Fusible Plugs
- Fusible plugs protect tyres and wheels from exploding when the brakes get too hot. A fusible plug is a small hollow bolt filled with low melting-point metal (like solder used for electronics or plumbing).
- In the event when a wheel becomes too hot, the soft metal in the plug melts at a predetermined temperature to allow the tyre to safely deflate.
- Fusible plugs often come into play after heavy braking, as would happen during a high-speed rejected take off. After the aircraft stops, the hot brake assemblies continue to heat the wheels until the fuse cores reach their melting temperature and deflate the tyres. Fusible plugs are mounted inside the wheel hub. When the plugs deflate the tyre, nitrogen is directed over the brakes to aid in cooling.
5. Over Pressure Relief Valve(OPRV)
As the name the indicates an over pressure relief valve is a hollow bolt with a rupture disk inside, this disk blows off when pressure of the filled nitrogen due to some reasons exceeds the design limit and prevent the explosion of the tyre.
6. Brake Pressure Monitoring System
An electronic system is installed in the aircraft to measure the pressure of the aircraft tyres throughout the journey.
7. Tread Patterns
Generally Aircraft tyre treads have simple grooves rather than complex ones in cars and bike because wheels of aircraft wheels rotate freely and on straight path mostly. These have several circumferential grooves tread that help channel water away from the tyre surface.
With more grooves and patterns in an aircraft tyre the trouble increases as the amount of rubber contact with the runway increases, increasing landing distances and hurting rejected take off braking performance.
8. Type of tyres
Based on the type of plies used the tyres can be:
- Radial ply tyres – 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 tyres – 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.
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.).
9. Rated Speed
The general Speed of an aircraft while Take-off and landing is between 150-180 mph whereas the aircraft tyres can easily bear the speed from 120 – 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).
10. Hazards to tyres
There are 2 primary hazards associated with tyres:
- 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.
These hazards are mainly associated with 4 distinct periods of aircraft operation:
- Ground operations e.g. taxiing – Long taxi patterns at heavy weights with tight turns will generate a lot of heat in the tyres, even if the brakes are used sparingly. This could cause the heat fuses to melt, resulting in a controlled deflation of the tyre.
- Take-off (up to gear retraction)- High speed aborts generate a great deal of heat in both the brakes and the tyres and restrictions may need to be placed on the degree of ground movement that can be undertaken after an abort.
- Post take-off (gear stowed)-Rapid retraction of the undercarriage following a long, high speed, heavy weight taxi with immediate take-off, or multiple touch-and-go during crew training, can lead to the tyres overheating in the wheel well. The heat fuses should prevent an explosive deflation, but it is not guaranteed.
- Landing (to the end of the roll-out)-Landing with a deflated tyre will put additional strain on the remaining tyres, with an increased potential for one or more to subsequently suffer an explosive deflation due to overstress.
Defences to such Hazards:
- Aircraft Maintenance & Ground Maneuver: operating with the correct tyre pressures and maximizing turn radii during ground maneuvers can mitigate against overheating and wear issues.
- Procedures: managing taxi patterns, reduced taxi speeds and allowing sufficient cooling time when necessary.
- Inspections: aircraft tyre inspections can identify a worn or damaged tyre that can subsequently be changed before it may explosively deflate or breaks-up, whilst airfield inspections can reduce the possibility of undetected damage occurring due to FOD.
The aircraft tyre manufacturing industry is dominated by a four firm oligopoly that controls 85% of market share.
The four major manufacturers in aircraft tyre manufacturing are the following according to a report by Pelmar Engineering in 2013:
- Goodyear (US)
- Michelin (France)
- Dunlop Aircraft Tyres (UK)
- Bridgestone (Japan)
12. Cost of Tyres
Tyre prices range from a few hundred dollars for regional aircraft to as much as $5000 (approx 4 lacks) for a wide-body main tyre. Airlines negotiate purchase prices or service contracts with tyre manufacturers and distributors.
13. Tyre Handling
Points to be noted during storage
- Tyres and tubes should be stored in a cool, dry place out of direct sunlight. Temperatures should be between 0°C and 30°C.
- Particular care should be taken to store tyres away from battery chargers, electric welding equipment, fluorescent lights, electric motors, electric generators and similar equipment. These items create ozone, which has a deteriorating effect on rubber.
- Local aviation authority regulations may address limits to tyre and tube storage humidity limits.
- Care should be taken that tyres do not come in contact with oil, gasoline, jet fuel, hydraulic fluids or similar hydrocarbons. Rubber is attacked by these in varying degrees.
- All tyres and tubes should be inspected immediately upon receipt for shipping and handling damage.
- Whenever possible, tyres should be stored vertically on tyre racks. The surface of the tyre rack against which the weight of the tyre rests should be flat and wide to minimize distortion.
- Tubes should be stored in their original cartons whenever possible. If stored without their cartons, they should be lightly lubricated with talc powder and wrapped in heavy paper.
- Under no circumstances should tubes be hung over nails, pegs or any object that might form a crease in the tube. Such a crease will eventually produce a crack in the rubber.
- If the tyre does not meet the inspection and inflation criteria, the tyre should either be scrapped or returned for retreading, depending on the tyre’s condition.
- Age is generally not an indicator of tyre serviceability as long as all service criteria, visual criteria, or individual customer-imposed restrictions are met.