The reliability of aircraft tires is often taken for granted simply because they are not so much of a flight concern as being solely counted upon for safe take-offs and landings. The most common tires used in general aviation look pretty simple but are actually designed to ensure that they meet the demands of infrequent high-impact use. Aircraft owners should remember that the tires on their aircraft are subject to wear and tear as a result of both active and inactive conditions. In this article, we will review some basic operational stressors and how they are addressed by specific tire construction methods, and then move on to a few more guidelines for tire selection and servicing.
Tire Design
Tire construction starts with the basic components of rubber, fabric and steel which form the casing of the tire while also giving it sufficient form and strength. The intermittent high load stressors impacting tire performance include centrifugal force of the rotating wheel, continual flexing of the formed rubber sidewall from taxiing to take-offs and landings, and the resulting heat generated from these active forces. Aircraft tires can be either tube type or tubeless design with tube tires being far more common on the smaller certified aircraft. Tube type tires are usually cheaper than tubeless, easier to assemble and can be quickly replaced during repair or maintenance. Standard tire construction is based upon either bias-ply or radial design with the bias-ply also being more common to small aircraft. The bias-ply theory incorporates layers of nylon fabric applied on an angle to the tire center line to strengthen the rubber composition. The radials used for larger aircraft utilize a common steel belted design just like regular automobile tires.
Tensile strength for the tire bead is provided by tightly wound hoops of steel wire to anchor the rubber casing and form the round shape of the tire. The bead section will also be instrumental for mounting the tire securely onto the wheel rim. The ply rating of the tire will be determined by the strength of these fabric layers. Common ply ratings on small aircraft tires include 4, 6 or 8 ply options. This ply rating references the tires ability to adequately support a certain weight of aircraft and is not merely an indication of being a more durable tire. This factor is sometimes misinterpreted by owners when selecting the best tire for their aircraft. The aircraft manual will require a specific tire design or perhaps include other specific options.
This clean Cessna wheel assembly reveals torqued bolts, axle nut cotter pin, and valve stem cap.
The sidewall section is what you see when viewing either side of the tire as it fills the space between the bead area and the tread. The sidewall is thinner than the other areas of the tire and is specifically crafted to provide some flexibility and weather resistance. Now that we have a completely formed casing, it’s time to look at the tread section where all of the impact occurs through normal operation. The tread gets the most attention from pilots when determining the general condition of the tire for continued service. Once the ply layers have been established for a particular tire design, the outer most cushion layer provides a reliable foundation to secure and stabilize the outermost tread section. This is the same base layer used as a foundation for re-treading as well. The tread section itself is made from a tough rubber with the recognizable grooves which help with directional control on the ground, they minimize hydroplaning in wet conditions and serve as an indicator for operational wear and tear.
Tire Selection
Tire selection for your aircraft is based upon which tire sizes are approved for the aircraft type. Some aircraft have only one approved installation whereas others may offer options depending on preference to suit various operating conditions. There are three classes of tires manufactured for general aviation starting with Type I – which covers smaller tailwheel designs up to 14.50 inches outside diameter. Type III tires are the most common and are used on most small certified aircraft. Type VII are dedicated to medium and larger aircraft. Current tire manufacturers will promote their own improved designs and boast comparisons to competitor brands but, for the most part, they are all built to similar specifications. The more expensive brands are usually built upon proven experience for exceeding the general manufacturing standards. This may translate to greater product reliability and favorable speed ratings according to the published manufacturer data.
You can see the built-up tread rubber beyond the basic tire casing shape on this Air Hawk design.
Tire selection is often tailored to owner/operator spending habits and an awareness for the aircraft operating environment. Flight schools will often use more economical brands as they are primarily affected by high utilization and the various stages of taxi and landing control by active students. Flight schools also tend to lean more towards purchasing in bulk and would likely consider Specialty brand Airhawk or Airtrac tires, Condor brands, or various retread tire deals. These tires are also very compatible with many small private aircraft installations where cost is more of a concern, whereas others may lean more towards the higher end Goodyear Flight Special or Flight Custom options. Sample prices for a typical 6.00x6 main wheel tire range from $150 up to $350 or more for similar competing brands. On a typical 6.00 x 6 – 6 PLY tire, the 6.00 indicates a 6 inch cross section of the tire, on a 6 inch diameter rim, with a 6 ply rating. This is commonly referred to as a “Six hundred by six, 6 ply”
Harmful Effects on Rubber
Aircraft tires always remain vulnerable to external factors whether static or during normal operation. Aircraft that are regularly parked outdoors are subject to potentially harmful weather/UV conditions. The competing effects of the hot sun and bitter cold on the rubber leads to sidewall cracking also known as “weather checking”. This condition is not normally an issue for the continued serviceability of the tire but can become severe if neglected for too long. Owners can help preserve the condition of the tires by simply installing some form of covers to keep tires protected from these naturally harsh elements. Aircraft stored in closed hangars do not share this concern during times of storage. Closed hangar storage is the overall best protection for tires, acrylic windows, interior plastics and painted surfaces as well.
Notice the sidewall weather checking which may be accelerated when aircraft is parked outdoors.
Tires are also prime targets for FOD (Foreign Object Damage) on or around the apron and taxi areas. It would not be highly unusual to find bits of hardware and other debris embedded in the tread or even in the sidewall. Make note of the housekeeping habits in your regular maintenance or storage hangar for their dedication to preventing FOD on or around the hangar floor. It’s also a good idea to take notice of any fluid contaminants on the tires. Engine/Hydraulic oil, avgas or commercial solvents are quite aggressive when coming into contact with the rubber tires. The best remedy to remove these contaminants is to wipe the area as soon as possible and wash the tire with mild soap and water.
Wear Patterns
The most common tire wear occurs through regular operation and affects the condition of the tread. Most pilots will make direct reference to tread condition as part of their pre-flight inspection. Keep in mind that aircraft equipped with wheel pants are more difficult to inspect for tire condition. Taking the time to roll the aircraft for a better look is usually necessary to complete this pre-flight task properly. If using wheel pants, consider removing them for winter operations as snow and ice will otherwise tend to build up and freeze inside causing wheel and brake issues. The generally accepted minimum limit concerning tread wear is to the bottom of the tread grooves provided that the wear is fairly even. Once the fabric cord pattern is visible through the bottom tread layer, then the tire is considered unserviceable in both private and commercial aircraft. Even an obvious flat spot may be considered acceptable unless it promotes a noticeable out-of-balance condition or exposes the cord fabric in any way.
This tire has good tread with the exception of one bad flat spot worn deep into the cord fabric
Observing tread wear patterns may also reveal other factors pertaining to tire condition. For example, tread wear primarily in the center tread grooves indicates over-inflation. Tread wear primarily on the inner and/or outer tread grooves indicates under-inflation. Wheel assemblies mounted on oleo type landing gear tend to wear fairly evenly provided that proper tire pressure is maintained. Wheel assemblies mounted on Cessna spring steel main landing gear will tend to wear more on the outboard tread even with proper inflation due to the natural lie of the gear legs upon landing. These spring steel gear legs are relaxed during flight so that the first point of contact with the runway will immediately affect the outboard tread of the tire only. Savvy flight schools or private operators may monitor this accumulated outboard wear closely to strategically plan an opportune time to flip the tire around on the rim to facilitate extended tire life. This is a fairly common practice where saving a few bucks is concerned.
The tread wear is clearly more noticeable on the center tread grooves of this C-172 nose tire.
Signs of internal blistering or bulging of the tire rubber represents delamination of the tire structure and is cause for replacement. Sidewall damage and very deep weather checking/cracking down to the cord fabric would also necessitate timely replacement. Also keep an eye on the only visible part of the tube which is the valve stem where it is accessible through the wheel rim. Too much bending and twisting of the stem to accommodate the air filler may lead to premature cracking near the base of the stem. Noticeable cracks in the rubber stem is also cause for tube replacement.
Tire Pressures
Monitoring proper tire inflation is the single most important factor in servicing the tire/wheel assembly. The approved tire pressure figures are included in the aircraft Owner’s Handbook or POH. Tire pressure is usually applicable to unloaded tires (no weight on wheels) and may differ up to 4 % from the loaded position (with tires on the ground and supporting the weight of the aircraft). The ideal tire pressure may vary with temperature but an allowance of +/- 5% is permitted. Keep in mind that a slightly over-inflated tire is better than an under-inflated tire. Not monitoring the tire pressure on a regular basis could eventually lead to a severely under-inflated condition and the possibility of extended damage. Under-inflation may cause uneven tread wear, damage to the valve stem, and even tire creep on the rim. This condition also promotes shimmy on nose wheel installations and difficulty steering during taxi operations. Always remember that the aircraft manufacturer determines the required size, ply rating, and tire pressure according to the size and weight of aircraft so following the manufacturer’s data is necessary to safety and continued airworthiness.
When checking tire pressure and paying attention to published data, it also makes sense to use a reliable tire pressure gauge. Cheap tire gauges are notoriously inaccurate and many shops actually maintain a calibrated gauge that is of good quality and not a cheap imitation found in the bottom of a junk drawer or old tool box. Many of the medium to larger sized aircraft require the use of clean dry nitrogen instead of shop air. This protects the integrity of the wheel assembly on tubeless designs and prevents impurities from entering the tire/ tube installation as well. Most small aircraft rely on shop air so it is helpful to maintain a clean compressor that is drained regularly as well as any portable air tanks (aka an “air pig” around the airport). Another small yet critical detail for checking tire pressures is to keep the valve stem capped with a secure metal valve cap. These caps are usually yellow in color or simply have a brushed metal finish. This serves as a back up to the valve core and also keeps the core and inner stem clean of contaminants.
A decent quality pressure gauge will provide accurate tire pressure readings.
Tire Replacement
Tires and tubes are closely inspected by maintenance personnel at tire change and/or during routine inspection. Rubber manufacturers discourage re-using these tubes over again at tire replacement since they tend to stretch in service and remain susceptible to wrinkling or creasing under extended use which ultimately weakens the integrity of the tube. Maintainers are also reminded to use the proper size tires and tubes specific to the aircraft as indicated in the appropriate reference manuals. The first step to tire replacement is to always deflate the tire before working on it, then break the bead away from the rim.
When replacing the tire and tube on a wheel assembly, notice that the tire sidewall has a painted red dot indicating the light side of the tire after manufacture. This red dot indicator should be aligned with a yellow stripe on the tube representing its heavy side. Tubes without the yellow stripe indicator may be aligned with the tire using the valve stem instead. Care must be taken to ensure that the tube is not pinched between the wheel halves during re-assembly. Technicians also use talcum powder inside the tire so that the new tube does not stick or fold during installation or in service.
The wheel half assemblies, bearing cones and bearing races are all cleaned and inspected to ensure that they are acceptable for continued service. The bearings are then lubricated with new grease and installed with the appropriate spacers, felt and locking ring. Brake discs for main wheel assemblies are also inspected and measured for minimum service limits before re-assembly. Brake discs which have been measured below minimum thickness limits should also be replaced.
Conclusion
As a pilot, being mindful of not riding the brakes during taxi or landing can surely go a long way in helping to preserve the serviceability of the tire. This overview of tires and tubes used in general aviation should inspire pilots and maintainers alike to better monitor the condition of these critical wheel assemblies. Consultation with your AME about tire condition may be best served at the annual inspection when there is time to review the effects of normal wear and tear since previous inspection. The aircraft tires are merely along for the ride during flight but nevertheless play a crucial role when landing to ensure safety of the aircraft and the crew.
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