nuts & bolts
technical counselor
Tips to Improve Tire Life Inflation is key GEORGE R . WIL H E L M SE N, E A A 4 4 2 6 6 4
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ires are an essential part of an airplane. Without tires it would be difficult, if not impossible, to accelerate to takeoff speed or land without damaging the aircraft, unless it has floats or skis as its primary landing gear. In spite of this, many people are unaware of what is needed to properly maintain tires and thus cause damage to these components. To resolve this, here are some important tips on tire care and maintenance. These tips are not exclusive to airplanes; they also extend to tires used on cars, trucks, and other vehicles. Inflation We aren’t talking about rising prices but rather the need to keep aircraft tires properly inflated. The inflation pressure of tires serves several important purposes, the primary purpose being to distribute the load on the tire in the manner it was designed. This keeps the tire rolling freely, which means it is less likely to generate excessive heat as it rolls. By load, we mean the weight of the equipment that is being supported by the tire. In a fully loaded aircraft, this may range from as little as several hundred pounds for some light planes to more than a thousand pounds for twin-engine aircraft. According to tire manufacturers, the leading cause of reduced tire life is improper inflation. Underinflated tires develop more heat during acceleration, rolling, and deceleration. Heat causes rubber compounds to break down more rapidly than they would otherwise, causing damage that reduces the life of the tire. Proper inflation also reduces rolling friction between the tire and the pavement, allowing a vehicle to accelerate as it was designed to. In the world of airplanes, a little extra friction can slow down your takeoff roll and 106
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increase the amount of runway needed to get airborne, potentially threatening your ability to clear obstructions at the end of the runway. While this is a subtle point, it needs to be kept in mind, since it can have a direct impact on whether a flight can be completed safely. Once we’ve inflated our tires, why do we need to check them again? A little known fact about tires is that they are designed to leak. The rubber in tires and tire tubes leaks slight amounts of air through diffusion. Tires with tubes actually have vent ports in the side of the tires, near the rim, that are designed to vent this air as it escapes. While the amount of air that escapes is small, it needs to be replenished to keep the tire at the proper pressure. In addition, the ideal gas law is in play here. This law is pure physics and relates that as the temperature
According to tire manufacturers, the leading cause of reduced tire life is improper inflation.
changes, so will the air pressure in tires. In general, for every 5 degrees Fahrenheit of temperature change, tire pressure will change by approximately 1 percent. As an example, if you fill your tires when the temperature is 90 degrees F, and the temperature drops by 30 degrees, your tire pressure will drop by 6 percent, which is substantial. Conversely, if the temperature increased by 30 degrees, your tire pressure would rise by 6 percent. It is also important to inflate tires while they are mounted on the plane and loaded. For example, if you were to take the average tire for an airplane and fill it on
LEFT: Cracked tires are often the result of underinflation and/or excessive sunlight. CENTER: This valve is easy to reach, and will not need to be bent to add air. RIGHT: It is easy to see the under inflation of this tire, with the sidewalls in contact with the asphalt.
a bench and then install it on an airplane, the tire pressure would rise about 4 percent when the weight of the plane is loaded on the tire. Check your tires when they are installed and loaded with the expected weight. Underinflated tires also put more stress on the sidewall of the tire, resulting in cracking of the sidewalls and excessive wear on the outer edges of the tire. Overinflated tires will cause more wear on the crown, or middle tread, of the tires. Check Inflation Regularly Improper inflation is the leading cause of reduced tire life; for that reason, it is important to check tire pressure regularly. Major tire manufacturers are fairly consistent in their recommendations to check tire pressure before every flight or daily for planes that fly several times a day. This will assure that the tires are properly inflated and maximize their life. Tire pressure should be checked when tires are cool. After a flight, wait at least three hours to allow the tires to release any heat developed during taxiing, takeoff, and landing. Using a good tire pressure gauge is important. If your existing “stick” type gauge allows the stick to fall out, it isn’t going to provide an accurate measurement since the friction of the stick is part of the pressure measurement. Make sure your gauge is operating properly by checking your car tires following service. Ask the shop what air pressure they filled your tires to and then check them with your gauge. If there is more than a 5 percent difference, consider getting a new gauge. Finding the Right Pressure To find the required pressures for tires, look in the pilot’s operating handbook. This number should take into consideration the average load placed on a tire and what air pressure is needed to carry the design load. For planes built from scratch, the best approach is to place the plane on a scale, determine the average loaded weight of each wheel, and then determine the correct tire pressure from an aircraft with a similar load. As an example, if your plane had a load of 1,000 pounds per main wheel and 500 pounds on the tail wheel, you could determine the correct tire pressures by finding another taildragger with similar weights, and then looking at the type of tires and the pressures used. The type of tire is important because tires have different ply ratings. A higher number of plies results in a stiffer sidewall, which may have been part of the
design consideration. Unless you have information that indicates otherwise, to safely transfer the pressure, stick with a tire with the same number of plies. Tube Tips Unlike most cars, which migrated away from tires with tubes years ago, most aircraft tires still use them. Most pilots don’t know that the tubes need to be replaced at the same time as the tires. The reason is that tubes actually take a “set” when they are installed in tires and conform to the interior of the tire. If the tube is not replaced, it can form a fold inside the new tire, which can result in the failure of the tube. When new tubes are installed in tires, be sure to select the right tube and valve stem combination. For example, some rims are designed for the standard- or centermounted stem for filling the tire, while others require a right-angle stem. While you can get a standard stem to bend into a rim, every time you exert force on that stem it will cause stress at its base. Over time the stresses will cause cracking, until the stem breaks off or begins to leak excessively. Before tubes are installed in tires, they need to be lightly coated with tire talc and then inflated slightly. This talc, which is similar to baby powder, allows the tube to slip inside the tire as it is filled and helps to avoid any folds. As the tire and tube are installed, it is important to line up the tube’s “heavy point” with the rim of the tire. Most tires have a stripe or dot, usually yellow or red, to indicate the light spot. For tubes without a mark, the valve on the tube is considered the heavy spot and is aligned with the mark on the tire that represents the light spot. By doing this, you will end up with the best balance in the tire. Finally, once a tire is initially filled and loaded, it is important to check the tire pressure frequently over the following days, as the tire will usually have several air pockets between the tube and the tire. These air pockets migrate out the tire vents and reduce tire pressure. Care and Upkeep of Tires The number one cause of damage and reduced life in tires after underinflation is the environment. Exposure to sunlight damages rubber and can result in weather checking of the tires and sidewalls. Excessive weather checking requires replacement of the tire. Several manufacturers are now making tire covers to protect them from sunlight. If a plane is stored outside and its tires exposed to sunlight, these covers, when properly annotated with a “Remove Before Flight” streamer, can improve the life of the tires. EAA Sport Aviation
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technical counselor While it’s common to clean the tires on a car, the manufacturers of aircraft tires do not recommend cleaning or treating them with tire shine products. The detergents used in cleaning products can strip away the protective coatings that protect tires from the elements. Without these coatings, a tire can be damaged more quickly than a tire that has not been cleaned. The manufacturers stress that if cleaning is required, use denatured alcohol followed by a washing with a mild soap and water solution. Landings and Brakes It may come as no surprise that the better your landing is, the longer your tires will last. When you land, your tires spin up from idle to your landing speed. Touching down lightly will reduce this stress on your tires and allow them to last longer. Similarly, applying moderate rather than hard brake pressure will extend tire life. Locking up the brakes can cause a tire to skid, which can quickly chew through the tread and into the cord, requiring the tire to be scrapped. In extreme cases, the cord can be breached, causing the tube to fail, which may cause the tire to go flat and result in damaging ground loops in some aircraft. Working Out a “Cold Set” Tires that are not operated regularly can take on a “cold set”; that is, the tire shape will remain in its resting position. Tire manufacturers say this may be worked out with some taxi time, as the tire comes up to speed. Of course, you may feel a few bumps as the tire rolls out of its cold set condition. In the end, the life of your tires can be optimized through a combination of proper inflation, reduced exposure to elements, proper installation, a light touch on landing, and reasonable braking.
George Wilhelmsen holds a commercial certificate, airplane single engine land, with an instrument rating, and he has more than 1,000 hours of flight experience. He has a bachelor’s degree in engineering technology with a background in DC, analog, and digital controls. He flies a Beech Debonair. 108
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