When performing a preflight inspection, there’s usually little more than a cursory glance given to the tire treads. I think it’s worth your time to give the rest of your tires a closer look.
Tires are designed to take abuse from literally every angle. They must be rigid in one area while simultaneously being flexible in others, under heavy loads at high speeds, all while enduring dramatic temperature changes over a relatively short time. At first glance, it may look like a tire is nothing more than the product of pouring some rubber into a mold, but a tire’s construction is almost as complicated as its mission.
The part of the tire that comes in contact with the wheel is called the “bead.” Only a few dozen square inches of tire bead connect your airplane to the earth, so the rubber is thick and reinforcing wire is used to ensure an airtight seal, as well as to prevent the tire from slipping on the wheel. The last thing you’d want is for your tires to spin on the wheels when you grease that landing. You may be surprised by how much heat and smoke can be generated instantly when you spin a tire.
The part of the tire that takes the most abuse is the sidewall. Any “cushion” that comes from the tire is provided solely by the sidewall. The tire bead retains most of its shape as it goes through a rotation, but the sidewall is constantly in flex. Let’s say that the valve stem of a main landing gear tire is at the six o’clock position, where the tire is at its maximum compression. If you roll the tire 180 degrees where the valve stem is at the 12 o’clock position, the sidewall is at its least compressed. Now imagine that spot making a full revolution 27 times every second, which is what the main tires on our Falcon 900 are turning when we rotate. If I were better at math, I’d calculate approximately how many revolutions our tires make on a typical takeoff roll. As a placeholder, I’ll just say a bunch.
The tread is where the rubber meets the runway and comprises the sacrificial part of the tire. Keeping an eye on the tire tread will give you good insight into the health of your entire landing gear system. If your main tires are wearing evenly, you’re keeping the tire pressures where they need to be, and your camber and toe are as they should be. If your tires are wearing unevenly, you may have a problem somewhere in the landing gear that is starting to manifest. Odd tire wear is a great early indication of problems to come, and prompt attention may save you a few dollars or maybe even a trip off the side of the runway.
Those are all parts of the tire that you can see, but the real magic happens inside the tire in the form of plies. “Radial” and “Bias” are the two ply options, and those terms describe the way in which the reinforcing cords are oriented within the tire. The cords in a radial tire are perpendicular to the direction of the tire tread. Bias tire plies run at about a 40-degree angle to the direction of the tire tread and make a crosshatch with every ply.
I also think it’s important to note that just because a tire is “six-ply” doesn’t mean that it contains six plies. “Six-ply” means that the tire has a ply rating of six, which indicates the load capability of the tire, not the number of plies. Win a bar bet at the next hangar social with that one.
If your airplane has a max gross weight of less than 12,500 pounds and is not turbine powered, the POH should specify the approved tire size and ply rating. You can also find it in the aircraft’s Type Certificate Data Sheet. As long as the tire is the proper size, ply rating, and is FAA TSO certified, you can use any manufacturer you’d like.
If you operate a turbine airplane or an airplane with a gross weight over 12,500 pounds, your tire options are more limited. The certification standards of these airplanes are different than their lighter, gas-burning cousins, so manufacturers don’t specify tire sizes, they specify approved tire part numbers. These manufacturers did tons of testing to get the airplane certified for use on various runway surfaces and conditions, and whatever tires they used during those tests are what’s approved for use on the airplane.
If the manufacturer only used Michelins to meet their certification standards, Michelin tires are your choice. If they threw a set of Goodyear tires on the airplane and repeated all the tests, you’re welcome to use either tire as long as they’re installed as a matched set.
It’d be great if we could mix and match Michelin and Goodyear tires, but if Textron Aviation (or Pilatus or Embraer) wanted us to use any tire installed in any position, they would have certified every conceivable combination of Michelins and Goodyears. That’d be a very expensive endeavor for a very small return.
Now that we know what tires are made of and which ones you can use let’s take care of them. The best way to get the most life out of your tires is to perform a good preflight inspection. If you own your airplane or are part of a flying club, you’ll see the same tires each time you fly, so you’ll know the history and pedigree of the tires. That is, you’ll know when something has changed, like a cut that wasn’t there last time or tread starting to separate. These are just a few clues that you’ll use to know when it’s time to do something or not.
Speaking of inspection criteria, if you need help determining if a cut is superficial or if that tire is toast, there are ways to find out for sure. Some aircraft manuals have somewhat useful tire criteria. FAA AC20-97B is a good source of information available online for free. I recommend using your Google machine to download the fantastic guides that Goodyear and Michelin have each created and made available as free pdf downloads. Other manufacturers may produce something similar, but these are the two with which I’m familiar. They contain tons of useful information along with pictures of anomalies so you can make an informed decision. These guides can also educate you enough so that when your airplane is down for maintenance, you don’t have to take your IA’s word as to the condition of your tires; you’ll know their status before you even show up at the shop.
If you rent airplanes from a flying school, you won’t have as good a grasp on the history of the tires, so you’ll have to look a little closer during your preflight. It’s almost impossible to tell just by looking at a tire if it’s over or under-inflated. The rule of thumb is that if you can visibly tell that the tire is under-inflated or takes full power to taxi out of the chocks, the tire pressure is dangerously low and, in some cases, may be unairworthy. Most tire manufacturers publish a pressure that if the tire falls below while installed on the airplane, the tire must be replaced.
Anyone who pulls a wagon or pushes a wheelbarrow with flat tires will instantly understand why this is the case. When tires are under-inflated, they’re put under much more strain and are subjected to loads for which they’re not designed, especially in the sidewalls. Under-inflated tires also generate more heat than properly inflated tires. In other words, no good will come from not keeping your tires properly inflated. You can find the proper tire pressures in the aircraft POH, maintenance manual or even on some checklists, but I’d recommend verifying those before you dig out the air chuck.
There was a crash of a Lear 60 in 2008 where one of the main tires “separated from the wheel and likely struck the underside of the airplane.” The NTSB’s probable cause of the crash was “due to severe underinflation, and the captain’s execution of a rejected takeoff after V1…” There were a few other links in the accident chain, but the underinflated tires set everything in motion.
The tires on the accident airplane had been replaced recently, and the pressures hadn’t been checked in the weeks leading up to the accident. Once new tires are mounted onto serviceable wheels and initially inflated, the new tire/wheel combo stretches and settles while the newly pressurized gases inside the tire cools and contracts. All of this leads to what can be a significant drop in pressure within the first 24 hours of a tire change.
The last thing I’ll say about taking care of your tires is this – every tire manual I’ve ever read has at least one note, usually in several places, telling people to not stick sharp objects into cuts of INFLATED tires in order to see if the cord is visible. At the risk of being repetitive and getting myself reported to The Department of Redundancy Department, I’d like to stress the fact that you shouldn’t do that. If you think a cut is severe enough to warrant probing, just assume that you need a new tire or at least a second opinion from a technician.
Speaking of new tires, I’ve known of many people over the past few years who have grounded their airplanes due to worn tires where no replacements were available. Attempted tire orders have been met with lead times of 6 to 12 months. These weren’t people who operate some obscure airplane either. They were Citation and Falcon operators flying airplanes currently in production.
All that is to say, if you own an airplane, you might consider keeping at least one full set of tires, O-rings, tubes, and whatever else is associated with a tire change on hand. I understand that not everyone has the luxury of keeping their airplane in their own hangar with a built-in tire rack along the wall, but I think it’s worth the effort to find a spot to keep a full set of rubber. The worst-case scenario is that you’ll have to use your “extra” set of tires while your order for new tires is delayed. Best case scenario is you’ll always have a new set of tires in your way.
Tubes, tires and O-rings are easy to store because they usually don’t have a shelf life as long as they’re stored properly. Keeping your tires and tubes out of direct sunlight and fluorescent light is the best thing you can do for them. Light deteriorates rubber faster than almost anything else. They don’t even have to be kept in a climate-controlled environment, although that wouldn’t hurt. When you stack your tires, don’t stack them horizontally, meaning, don’t lay them sidewall-down on the concrete. Stand your tires vertically, just like they’d be installed on your airplane. Laying a tire sideways on the floor will deform the sidewall over time, and that’s not a part of the tire you want to weaken.
With a bit of care and forethought, you can get the maximum life and safest operation possible out of your tires. If you keep a close eye on them, keep them properly inflated, and don’t go sticking pointy things into them, they’ll serve you well for many flights.
