Aircraft Tyres by hjkuiw354

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									                                     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.




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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


                                                              Deflection Distance
              Figure 1 – Tyre deflection during landing

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.


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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




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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
                                                                       Load Rating,
                                                                       Ply Rating,
                                         Manufacturer printed          Speed Rating
                                         on sidewall


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 19th day of the year or 19th 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




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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.


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