Understanding the basics of aircraft tire construction and maintenance
By Joe Escobar
Aircraft tires can easily be taken for granted. Their simplistic appearance may lead some to a false feeling of complacency. Many technicians are not aware of the critical design factors that go into every aircraft tire in use today. Seemingly minor flaws can lead to disastrous results, and inadequate maintenance practices can cause shortened tire life and even cause unsafe operating conditions.
When one considers the forces that aircraft tires endure, it seems amazing that engineers were ever able to develop these special products. Some tires are subjected to speeds as fast as a race car while at the same time supporting more weight than the largest land moving machines. They go through cyclic exposure to varying temperatures and pressures. These factors make aircraft tire design a critical process and tire maintenance an ever important step in safe aircraft operation.
Designing aircraft tires is a very intense undertaking. Once an aircraft manufacturer goes to the tire manufacturer with the size and weight requirements, the tire manufacturer begins the process of designing a tire that meets the needs of the aircraft manufacturer as well as all regulatory requirements — including foreign regulations in many cases.
Engineers are challenged with designing tires with cool running, heat-resistant materials while simultaneously exceeding tire service requirements. Tire prototypes are put through rigorous tests that simulate cycles of landings, take-offs, and taxi operations. It is only after the tires pass all mandatory tests and meet the requirements of the aircraft manufacturer and airworthiness authorities that the tires are put into production.
Two types of tires are used in aircraft applications — bias-ply tires and radial tires. Although both types of tires share some similarities in design, there are differences that need to be noted.
Bias-ply tire construction
A cutaway of a typical bias-ply tire is shown in Figure 1. Bias-ply tires are popular choices for aircraft tires because of their durability and retreadability. The tire construction consists of the following components:
Tread: The tread is made of rubber mixed with other additives to obtain the desired level of toughness, durability, and resistance to wear. The tread pattern is designed to aircraft operational requirements, with the ribbed tread design used widely due to its good traction under varying runway conditions.
Sidewall: The sidewall is a protective layer of rubber that covers the outer casing ply. It extends from the tread edge to the bead area.
Tread Reinforcing Ply: One or more layers of fabric that strengthens and stabilizes the tread area for high-speed operation. It also serves as a reference for the buffing process when tires are retreaded.
Buff Line Cushion: The buff line cushion is made of rubber compound to enhance the adhesion between the tread reinforcing ply and the breakers or casing plies. It is of sufficient thickness to allow for the removal of the old tread when the tire is retreaded.
Breakers: Breakers are reinforcing plies of rubber-coated fabric placed under the buff line cushion to protect casing plies and strengthen and stabilize the tread area. They are considered an integral part of the casing construction.
Casing Plies: Alternate layers of rubber-coated fabric (running at opposite angles to one another) provide the strength of the tire.
Wire Beads: Hoops of high tensile strength steel wire that anchor the casing plies and provide a firm mounting surface on the wheel. The outer edge of the bead that fits against the wheel flange is called the bead heel. The inner bead edge is called the bead toe.
Apex Strip: A wedge of rubber affixed to the top of the bead bundle.
Flippers: Layers of rubberized fabric that help anchor the bead wires to the casing and improve the durability of the tire.
Ply Turnups: The casing plies are anchored by wrapping them around the wire beads, thus forming the ply turnups.
Chafer: A protective layer of rubber and/or fabric located between the casing plies and wheel to minimize chafing.
Liner: In tubeless tires, the liner is a layer of low permeability rubber that acts as a built-in tube and restricts gas from diffusing into the casing plies. In tube-type tires, a thinner liner is used to prevent tube chafing against the inside ply.
Radial-ply tire construction
A typical radial tire is illustrated in Figure 2. With their rigid belt, they provide increased landings and reduced rolling resistance. They have fewer components in their construction and are lighter than similarly sized bias ply tires. Components that differ from bias-ply construction as follows:
Overlay: A layer of reinforcing rubber coated fabric placed on top of the belts to aid in high speed operation.
Belt Plies: A composite structure that stiffens the tread area for increased landings. The belt plies increase the tire strength in the tread area.
Casing Plies: As in bias-ply tires, the casing plies are layers of rubber-coated fabric. However, unlike those in bias plies that run at opposite angles to one another, radial plies run radially from bead to bead.
Chippers: The chippers are layers of rubber-coated fabric applied at diagonal angles that improve the durability of the tire in the bead area.
In order to ensure safety and increase tire life, systematic inspections of mounted tires should be performed. A word of warning — Never probe cracks, cuts, or embedded foreign objects while the tire is inflated! Serious injuries could result. In addition, defects such as cuts, cracks, and bulges should be marked with a crayon or chalk prior to deflation since the defects may disappear when the tire is deflated. The following should be checked:
Tread wear: Treads should be visually inspected for wear. The tire should be replaced when tread has worn to the base of any groove at any spot or to a minimum depth specified in the aircraft maintenance manual. Tread wear can also be an indication of improper tire inflation. Refer to Figure 3 for tread wear indications.
Tread cuts: The tire should be inspected for cuts and other foreign damage. Follow the criteria from the applicable aircraft maintenance manual. If no specific criteria is outlined in the manual, the following can be used for guidance in determining tread cuts:
Even tread wear on this tire indicates that it has been properly maintained and run at correct inflation pressure.
Worn to the breaker/casing plies, the tire should not be left in service or retreaded.
Continuous overinflation accelerates center tread wear. It reduces traction while making tread more susceptible to cutting.
Excessive tread shoulder wear results from chronic underinflation. It increases the chance of damaging shoulders and sidewalls which shortens tire life because of excessive flex heating.
• Any cut that extends into the casing plies on bias tires.
• Any cut into the belt on radial tires.
• Any cut that extends across one or more rubber tread ribs to the fabric.
• Rib undercutting at the base of any
cut or tread splice warrants replacement.
Sidewall damage: Tires should be replaced if weatherchecking, cracking, cuts, or snags extend down to the casing ply in the sidewall and bead areas.
Bulges: Bulges in any part of the tire tread, sidewall, or bead area are indicative of a separation or damaged tire. The area should be marked and the tire should be removed from service immediately.
Fabric fraying/groove cracking: Tires should be removed from service if groove cracking exposes any fabric or if cracking undercuts the tread ribs.
Flat spots: Flat spots can be caused by braking during touchdown or hydroplaning skids. Tires with flat spots don’t necessarily need to be removed from service unless fabric is exposed. However, if there is an objectionable unbalance issue, the tire assembly can be re-balanced or removed from service.
Radial tire sidewall indentation: Remove tire from service with 3mm or greater sidewall indentation.
Beads: The tire bead areas should be inspected next to the wheel flanges for heat-induced damage, especially if brake drag or severe braking was experienced.
Tire clearance: The tires should be inspected for marks that might indicate rubbing the wheel area due to inadequate clearance.
Wheel assemblies: The wheel assemblies should be inspected for damage. Wheels that are cracked or damaged should be taken out of service for repair or replacement.
Tire inflation is the most critical issue when inspecting installed tires. Although overinflation can damage the tires by causing uneven tread wear, reduced traction, increased susceptibility to cutting, and increased stress on the wheel assemblies, underinflation is by far more damaging. Underinflation produces uneven tread wear and shortens tire life due to excessive flex heating. The bead area of an underinflated tire can be 50 percent hotter than that of a properly inflated one. Heat is damaging to tire rubber compounds and fabrics. It contributes to tread and carcass separations and bead failures.
Ideally, tire pressure should be checked with a calibrated gauge prior to each flight. At a minimum, they should be checked daily. Do not make the mistake of relying on your "calibrated eye" to determine tire inflation. Tire inflation is difficult to determine visually, especially in tandem configurations. Always use a gauge to check inflation. If servicing is required, dry nitrogen should be used due to its inability to sustain combustion. Also, degradation of the casing plies and liner due to oxidation occurs if regular shop air is used. Nitrogen also helps to deter corrosion on the wheel assembly.
It is a good practice to establish a method of recording tire servicing actions. This aids in identifying chronic leakage problems.
Another thing to remember pertaining to tire inflation is the change in pressure with temperature changes. For every five degrees Fahrenheit change in temperature, there is approximately a one-percent pressure change in the same direction. This should be taken into consideration if the aircraft is going to be subjected to extreme ground temperature changes. In that case, the tires should be inflated to ensure the minimum required pressure is maintained for the cooler climate. Remember to always check tire pressures when they are at ambient temperature. Checking pressures on hot tires could mask an underinflation condition.
One thing that needs to be noted on newly installed tires is that the tire pressure will drop initially due to tire expansion. The tire pressure should be checked closely until it stabilizes — usually after about 12 hours at rated inflation. If it still exhibits a significant pressure drop after that, the tire should be inspected for possible leaks and corrective action taken.
Tires should be kept clean and free from oil, hydraulic fluids, grease, tar, and solvents. All of these chemicals have a deteriorating effect on the rubber in the tires. Any contaminant should be wiped off with denatured alcohol followed by a soap and water wash.
Aircraft tires are affected by sunlight and weather extremes. Installation of protective covers can help protect tires on aircraft that are tied down outside.
Whenever taxiing or towing aircraft, the aircraft manufacturer’s recommended operational procedures should always be followed. It is always prudent to use large radius turns and low speeds to prevent shoulder damage, tread scrubbing, and overheating. Care should also be taken to avoid running over hazardous areas. Tire damage can result from potholes, cracks in the pavement, or stepoffs from pavement to ground.
The areas where the aircraft will be operated should be kept free from debris. In addition to the damage potential FOD (Foreign Object Debris) poses to engines, it can cause considerable damage to tires if it becomes embedded in the tire tread.
This article has briefly discussed some basics on tire composition as well as some inspection and preventive maintenance techniques. Always refer to the applicable aircraft maintenance manual when working with aircraft tires. In addition, the FAA requires tire manufacturers to provide a care and maintenance manual for their products. This is an additional resource that is available to assist in tire maintenance and inspection procedures. With proper attention during inspection and good preventive maintenance practices, your aircraft tires should provide trouble-free service for many landings.
AC 20-97A — High Speed Tire Maintenance and Operational Practices
The Goodyear Tire & Rubber Company
1144 East Market Street
Akron, OH 44316-0001
Michelin Aircraft Tire Corporation
1305 Perimeter Road
Greenville, SC 29605
Specialty Tires of America
1600 Washington Street
Indiana, PA 15701