October 11, 2006
An aircraft tire is a sophisticated, computer-designed, multi-component product consisting of three major materials: steel, rubber and fabric. Taking this down a level, there are multiple types of nylon and rubber compounds in tire construction, each with its own special properties designed to complete the task assigned. The only thing they have in common with auto tires is that they are round.
Tires are available in tubeless and radial construction for the heavy iron, but by and large light twins and single-engine, piston-prop aircraft have a choice limited to tube-type, bias-ply tire brands and subsets within brands. Your choice is nominally an economical model, a mid-price version, a high-end model or a retread. Retreads are particularly popular with flying schools.
Aircraft tires are approved under the FAA's Technical Standard Order system (TSO). All TSO-C62b-qualified tires with a speed rating of 160 mph or less -- as well as TSO-C62c-qualified tires -- do not require re-qualification to TSO-C62d unless the tire is changed.
|Tires are marked with lots of information you need when you're ready to replace them. (Click here for larger version.)
When selecting a tire, check your POH for the requirements for your aircraft. It will include both a size, such as 6.00-6, and a ply rating, and sometimes a brand and model recommendation. The ply rating determines the load-carrying capability of the tire. Stick to the POH on both size and ply, because you can end up with unexpected results if you elect to experiment with unapproved tires or ply ratings. If there is an STC for specialty tires, such as flotation types as may be used on a bush plane, that's fine. You, of course, will pay an airspeed penalty in that case.
Note that the term "ply" is used to identify the maximum-rated, static-load capability and corresponding inflation pressure applicable to specific operational requirements. The ply rating is an indication of tire strength and does not specify the actual number of carcass plies within that tire.
Only a small number of piston GA type aircraft come with tubeless tires and include the Beech Duke, Queen Air, Twin Beech and Cessna 404 Titan. Note that the same tire may have different inflation pressures (especially nose wheels) due to airplane loading differences. Be sure to check your aircraft's maintenance manual for the correct pressure for your airplane. There is also a good chance that the nose-wheel tire is a different size and different inflation value than the mains.
The valve stem should be aligned with the red dot or triangle on the tire, which indicates the circumferential location of the light spot on the tire.
Goodyear tubes are marked with a yellow stripe to indicate the heavy spot; if there is no yellow spot, then the valve is considered the heavy spot.
Due to the properties of the materials from which they are constructed, aircraft tires will expand for up to 12 hours after initial inflation. As the volume of the tire increase, the effective pressure will decrease.
Therefore the inflation pressure of newly mounted tires should be checked after a minimum of 12 hours and re-inflated to the required pressure. Consequently, tires should not be placed in service until they have been inflated a minimum of 12 hours, pressure rechecked, and tires re-inflated if necessary. Within the next 24 hours, if the pressure decreases more than five percent, it could be caused by trapped air between the tire and tube, valve-core leakage, or a damaged tube.
It's really important to maintain proper inflation, as this affects the tire life (from wear and heat) as well as performance, from hydroplaning to wheel shimmy. (Of course improper maintenance and wear can also cause similar performance problems.)
|This tire suffered from both overinflation and an alignment problem. Aircraft with tire wear problems, not attributable to tire balance, need to have the alignment checked. Check your maintenance manual for applicability and specific procedures. Some aircraft have very limited adjustments. Inflation should also reflect the aircraft load and temperature profile on heavier aircraft.
Check pressure and tire condition daily with multiple flights or before flight with cool tires. A temperature change of 5°F produces approximately one-percent pressure change. It can take up to three hours after a flight for tire temperatures to return to ambient. This means that tire pressures change on a daily basis.
These are all reasons why it is important to check pressure daily or before each flight. Excess inflation pressure should never be bled off from hot tires. All adjustments to inflation pressure should be performed on tires cooled to ambient temperature.
When tires are going to be subjected to ground temperature changes in excess of 50°F because of flight to a different climate, inflation pressures should be adjusted to worst case prior to takeoff. The minimum required inflation must be maintained for the cooler climate; pressure can be readjusted in the warmer climate. Before returning to the cooler climate, adjust inflation pressure for the lower temperature.
Aircraft Spruce & Specialty has an excellent pressure gauge that reads in one-pound increments, as well as a separate tool that has a specialized adapter for fitting through wheel pants. And while you're at it, always use the MS valve caps with the o-ring core to keep the valve stem clean and the loss of any air through the valve core -- Spruce has them, too.
Given a choice, use nitrogen gas in lieu of shop air. Shop air may have excessive moisture, which makes it even less desirable than ambient air for use in tires. Nitrogen will not sustain combustion and will reduce degradation of the tube or liner material, casing plies and wheel due to oxidation.
If you find any signs of damaged wheels, be sure to replace them as they are under terrific stress. Aircraft Spruce has an excellent selection.
Making Tires Last
Besides maintaining proper inflation, tires should be kept clean and free of contaminants such as oil, hydraulic fluids, grease, tar, and degreasing agents, which deteriorate rubber. Contaminants should be wiped off with denatured alcohol and then the tire should be washed immediately with soap and water. When aircraft are washed, ideally, tires should be covered with a waterproof barrier.
Goodyear adds antioxidants and antiozonants to the sidewall and tread to help prevent premature aging from ozone in the air and weather exposure. The use of aftermarket tire dressings (especially any automotive type) and wipe-on or spray-on protectants is generally not recommended since they may actually accelerate the loss of factory protectants.
Aircraft tires, like other rubber products, are affected to some degree by sunlight and extremes of weather.
While weather checking does not impair performance (as long as no carcass is showing), it can be reduced by protective covers when the plane is tied-down outside, especially in hot, dry, sunny climates.
If you have spare tires on hand in your hangar, be sure to store them away from fluorescent lights, electric motors, battery chargers, electric welding equipment and electric generators, since these products create ozone, which is harmful to rubber.
According to Goodyear, there is no specific shelf life for aircraft tires; it depends on storage conditions.
|You can see that the room for error in some retractables is very tight. While sizes are reasonably consistent between brands, a retract check is a good idea. Also, a new tire will grow several percent over the first 12 hours of inflation, so it should be rechecked and inflated as required.
Retractable aircraft represent a special case for conservatism. If you are switching tire brands or even tire types from what is currently installed, a gear retraction test is a good idea to be sure that the tire will not interfere in any way with the gear wells or the retraction mechanism.
Some aircraft have quite limited space around the tires and the retraction mechanisms. And remember the tire will grow a bit in service, so if it's a close call when brand-new, chances are you may have a problem later on.
I also recommend against retreads on a retractable. While it's true airlines use retreads, it's a different ballgame there. The airline tires are serial-number tracked, so the carcass history is known from manufacture to retirement.
When to Change the Tube
When changing tires, it may be hard to dump a tube that appears serviceable, , but it's best to use a new tube with a new tire, because the old tube may have grown as much as 25 percent in service. Thus, reusing the tube may be difficult or you may end up with a fold or binding spot in the tire that will fail prematurely. There is no reason why a tube cannot be patched with an in-service tire, providing the tire passes a safety inspection, e.g., no sidewall punctures allowed.
When to Replace a Tire
|This tire had to be re-tired after just one landing when the student landed with the brakes on.
Tires should be examined and possibly replaced when they have certain types of cuts, sidewall damage, bulges, fabric fraying, groove cracking or flat spots. The specific criteria for each type of damage is listed below.
Tread Cuts: Inspect tread for cuts and other foreign object damage and mark with crayon or chalk. Follow specific cut-removal criteria from aircraft maintenance manuals, operation manuals or tire-cut limits on the tire sidewall when available. When specific cut-removal criteria are not available, use the following Goodyear removal criteria: any cut into the casing plies on bias tires, any cut into the belt package on radial tires, any cut which extends across one or more rubber tread ribs to the fabric, or rib undercutting at the base of any cut. Warning: Do not probe cracks, cuts or embedded foreign objects while tire is inflated.
Sidewall Damage: Remove tire from service if weatherchecking, cracking, cuts or snags extend down to the casing ply in the sidewall and bead areas. Cuts and cracks deeper than one ply require the tire to be scrapped.
Bulges: Bulges in any part of tire tread, sidewall or bead area indicate a separation or damaged tire. Mark with crayon and remove from service immediately.
Fabric Fraying/Groove Cracking: Tires should be removed from service if groove cracking exposes fabric or if cracking undercuts tread ribs.
Flat Spots: Generally speaking, tires need not be removed because of flat spots due to touchdown and braking or hydroplaning skids unless fabric is exposed. If objectionable unbalance results, however, rebalance the assembly or remove the tire from service.
To help with removing old tires, Aircraft Spruce & Specialty sells a bead breaker that is much easier than stomping on the old tires.
Aircraft Spruce also has a wealth of jacks and jacking aids to help cradle the main gear legs in many Cessnas and provide a non-slip, safe jacking point.
Problems with New Tires
While quality control of tires continues to be high, it is not infallible. New tires should be balanced and the proper weights applied. Ninety-nine percent of the time that is all there is to it. However, on occasion, a tire may be out-of-round as a problem with the construction process and slip through the QC screen.
It doesn't take a lot to cause a balance problem. It will not generally be obvious to the naked eye without some form of a reference to continuous diameter checks. So if the tire seems to not want to be balanced, don't dismiss the possibility that the "new" tire needs to have some rubber shaved off to make it round.
|Here is an example of a pure alignment problem with the aircraft. Such wear issues are sometimes difficult to avoid, so one strategy is to reverse the tires before wear on one side becomes too significant. This will effectively double the life of the tire, notwithstanding other disqualifying issues.
Let's look at retread tires. Are they right for you? Generally they are less expensive than new tires of similar quality construction. Goodyear for one has an extensive rejuvenation and testing procedure. As said earlier, some flight schools use them where they get a lot of use over a short time in trainer-type aircraft. That said, the price differential over new is not so significant that I would use them on my plane. Unlike the airlines, you do not know the history of the retread you buy.
Top Tire Recommendations
For a retractable, I recommend a top-level tire such as a Goodyear FC II or III, or whatever is recommended by the POH. With higher takeoff and landing speeds as well as more likelihood of foul weather use, the best tires are the only choice in my opinion. The Goodyear FC III has the greatest cross-section of tread in the 600x6 compared to other popular brands.
Compared to the FC II, the FC III also has Kevlar belts, improved tread design for wet performance, deeper tread, additional resistance to sunlight, and an improved tread compound. These factors add up to about 20 percent more potential landings than the FC II.
For most use, however, the question is not the number of landings but how long it stands up to time. If you fly infrequently in, say, a Skyhawk, you may want to save some money with a more economical choice. Here offerings from Specialty Tire Co. (formerly McCreary Tire) may meet your budget. In increasing costs are the Air Trac, Air Hawk and Super Hawk. Mechanics I deal with recommend the middle-line tires as the most cost-effective.
General Mounting Guidelines
|Be sure to keep the paper sticker on a new tire. It has a set of basic mounting instructions for that tire.
Use a clip-on chuck, an extension hose, and a safety cage for inflation. Use a direct reading or dial-type pressure gauge with 5 psi or finer increments that is calibrated on a regular basis. When inflating a tire/wheel assembly, regulate the supply line to a pressure no more than 50 percent higher than the tire service pressure. Do not inflate a tire above rated pressure to seat beads. Make certain that every valve has a cap to retain inflation and prevent dirt, oil and moisture from damaging the core.
Tube Type Mounting
- Use the correct tire and tube for the wheel assembly.
- Clean inside of tire, then lubricate lightly with tire talc, not baby powder. Aircraft Spruce sells the proper talc.)
- Inflate tube to slightly round, and insert in tire.
- Align yellow stripe on tube with red balance dot on tire; align red dot with valve if no stripe on tube.
- When mounting tire and tube on wheel, be sure that wheel bolts are torqued to wheel-manufacturer's instructions before inflating.
- Inflate tire in a safety cage to rated pressure.
- Deflate assembly to equalize stretch.
- Reinflate to rated pressure.
- After 12-hour stretch period, re-inflate to rated inflation pressure.
Tire Balancing and Landing-Gear Vibration
|When jacking, be sure to do it properly. While a wing jack is not necessarily required, be sure any method that you use is safe and solid in case the wind blows while the wheel is off the plane. Check your pilot manual for proper jacking methods for your particular aircraft. A couple of 4x4s stacked under the bare axle is a good safety practice.
It is important that aircraft wheels and tires be as well-balanced as possible. Vibration, shimmy, or out-of-balance is a major complaint. However, in most cases, tire balance is not the cause. Other factors affecting balance and vibration are:
- Flat-spotted tire due to wear and braking
- Out-of-balance wheel halves
- Installation of wheel assembly before full tire growth
- Improperly torqued axle nut
- Improperly installed tube
- Use of non-aircraft tubes
- Poor gear alignment
- Bent wheel
- Worn or loose gear components
- Incorrect balancing at wheel assembly
In addition, pressure differences in dual-mounted tires and incorrectly matched diameters of tires mounted on the same axle may cause vibrations or shimmy. With some split wheels, the light spot of the wheel halves is indicated with an "L" stamped on the flange. In assembling these wheels, position the "L's" 180 degrees apart. If additional static balancing is required after tire mounting, many wheels have provisions for attaching accessory balance weights around the circumference of the flange.
Where to Find
Aircraft Spruce & Specialty has a convenient chart for all popular aircraft by aircraft model, including tire sizes and ply ratings. They also have a good brand selection.
More information can also be found from the Goodyear Web site.
For more about maintaining general aviation aircraft, check out AVweb's Maintenance section.