Aircraft Tyres Richard Skiba Skiba, R. (1999). Aircraft Tyres: Differences Between Aircraft and Automotive Tyres, Pacific Flyer, September. Skiba, R. (2000). Aircraft Tyres', Gyro News, Vol.14, No. 3, Spring (Sep/Oct/Nov). Aircraft tyres are different to car tyres and on this basis, maintenance requirements are different. Differences Between Aircraft and Automotive Tyres Tyres used for automotive and aviation purposes vary quite significantly in their structure. To point out some of the differences, the following is a comparison based on information available from Goodyear North American Tyres (http://www.goodyear.com). The first difference is in the ply rating. This is the term used as an index to the load rating of the tyre or tyre strength/load carrying capacity. It does not actually refer to the number of fabric plies in the tyre as is did in the past. The ply rating for a standard aircraft tyre of with a diameter of 25 and a section width of 6½ is 18 compared to that of a standard passenger car tyre, with the same dimensions, of 4. It should be noted that these ratings will vary with size and application - We don't usually fit 25 X 6½ tyres to ultralights! Tyson with a Goodyear 15X6 aircraft tyre at Essendon Airport. Page 1 of 5
This means that an aircraft tyre with the same dimensions as an automotive tyre has 4½ times the load carrying capacity. The next difference, load rating, is the maximum allowable load that the tyre can carry at a specified inflation pressure. The load rating for the aircraft tyre is 13,000 pounds compared to the automotive tyre with 835 pounds. Difference also occurs in inflation pressure. Our 25 X 6½ aircraft tyre is normally inflated 300 psi where the car tyre is inflated to 32. The two tyres can also be compared based on deflection. This refers to the amount of deflection or alteration the tyre sees under load. This can be shown diagramatically as in figure 1. Rotation Traction Wave Runway Figure 1 Tyre deflection during landing Deflection Distance The figure shows that a landing tyre deflects towards the centre of its mounting point as it comes into contact with the ground. The deflection allowed in the aircraft tyre is 32% - 35% as compared to 17% for an automotive tyre of the same size. (As an aside, to calculate the deflection of a tyre, the free height minus the loaded free height must be divided by the free height. To find the free height, the outside diameter of the tyre is subtracted from the flange diameter then divided by 2, where the flange diameter is the measurement taken at the top of the wheel rim flange. The loaded free height is calculated by subtracting the flange diameter from the static Loaded Radius and dividing by 2. The static loaded radius is the measurement from the centre of the axle to the runway when the tyre is under load). The two tyres being compared also differ in their maximum speed rating. For the same size tyre, the aircraft tyre is generally rated to 275 mph where the car tyre is rated to 100 mph. The aircraft tyre then, is able to carry approximately 16 times the weight of a car tyre at almost 3 times the speed. As such, aircraft tyres operate under severe conditions compared to automotive tyres. Aircraft tyres can not be treated as car tyres with regard to their maintenance, simply based on the extreme conditions they must withstand. Page 2 of 5
Visual Inspections Given the role that tyres play in the survival of the aircraft and pilot, there are a number of key indicators to tell us when a tyre should be removed and replaced and these will now be considered. Foremost, the tread should inspected for wear. A tyre which has worn to the base of any groove should be replaced, or minimum technically specified limit for that aircraft. Tyres with tread worn on one side may be turned around as long as they are not excessively worn. The tread should also be inspected for cuts and the tyre removed where fabric can be seen without spreading the cut. The tyre should also be removed where a tread cut extends more than half the width of a rib and deeper than 50% of the remaining tread. Sidewall damage may also necessitate the removal of a tyre. Where there is weathering, cracking or cuts which go down to the ply, the tyre should be replaced. Likewise, any tyres with bulges in the tread, sidewall or bead area (along where the tyre comes in contact with the flange) should be removed as the bulge indicates separation. The bead areas need also to be checked for damage due to excessive heat which may be due to heavy braking. The tyre may need to be removed depending on the extent of the damage. Inspection must also be made to ensure that the tire does not come into contact with any part of the aircraft, such as gear and wheel wells, which may rub on the wheel or tyre. The wheels must also be checked for damage and removed where necessary. Groove cracking is another condition which may warrant tyre replacement. This is a crack which may occur at the base of the groove and is usually the result of over or under inflation or overloading. If fabric is visible along the crack, the tyre should be considered unusable and removed. Flat spots may also arise on tyres due to skidding or tread rubber reversion (rubber is burned due to hydroplaning during landing on wet or icy runways). Tyres with flat spots should be removed if they effect balancing or fabric is exposed. Weather cracks may occur with time as a result of ozone. The deterioration shows as a random pattern of superficial cracks along the sidewall. If fabric is visible through these cracks then the tyre should be removed. Use the Correct Tyre Page 3 of 5
It is essential that the correct tyre, not a near best fit is used for the application. Given the pressures that an aircraft tyre is subjected to, an incorrectly sized tyre, although it looks as if it fits correctly, may not provide adequate deflection or support and end in fatal consequence as pointed out by Mike Garrett's article. As such, the aircraft specifications need to be consulted before making any changes to the tyres. The tyres are marked with all the relevant information so you can check that the right one is fitted and that you buy the right one for replacement. On each tyre you will find the manufacturer, size, load rating, speed rating, molded skin depth, Tyre Size Manufacturer printed on sidewall Load Rating, Ply Rating, Speed Rating part number and serial number. Information such as plant identification (where the tyre was manufactured) and other markings may also be included. Ply rating and AEA codes likewise could be included. AEA codes define new tyre casing and tread construction. Goodyear serial numbers also contain a lot of information within them. They are an eight character number with the first character signifying the year of manufacture. So in the serial number: 6 019 1234, using the example provided by Goodyear (http://www.goodyear.com), the year of manufacture would be 1996. The second, third and fourth characters outline the day of the year using a Julian date format, so in our above example, the 19 th day of the year or 19 th January. The fifth position identifies the plant of manufacture. If the letter is 'A' this is Akron, 'B' for Brazil, 'G' for Luxembourg and 'T' for Thailand (although commercial aircraft tyres are no longer produced in Brazil and Luxembourg). Where there is a number rather than a letter in the fifth position than the tyre was produced in Danville as is the case for our example. If the tyre was produced in Thailand, the serial number would have been 6 019 T 234. The ply type of the tyre is also an important consideration and needs to be identified from the sidewall. Check whether the airframe Page 4 of 5
requires a bias ply or radial ply. Radial ply tyres should not be mounted on wheels which require bias ply and vice-a-versa. In conclusion, tyres for aircraft must be considered as an essential element of any aircraft preflight. Just like damaged or incorrect shoes make for unhappy people, damaged or incorrect tyres make for unhappy aircraft. Safe Flying. # Page 5 of 5