Narrative:

I have made some personal observations concerning our B-737-800 fleet tires. The following observations and opinions are a concern for the safety of our B737-800 fleet in the form of revenue loss and potentially a reason for the continuing incidents of runway excursions. First; my observations; starting with tires; among the many limits that can take a tire out of service; are the tread groove limits. The company [standard] fleet number is a minimum of 2/32s' of an inch; tread groove depth at any place on the tire. The B737 only; specifically requires at least 3/32s' of an inch [tread groove remaining]. I am to assume this is due the propensity of the B737-800 fleet to hydroplane excessively. Additionally; the advent of the newer radial tires; has brought further fleet concerns due to excessive shoulder wear on these tires. Now; based on my experience as a mechanic; and quite extensive experience working with automobile front ends and alignment; I have noticed an expensive issue with the B737 tires. First; once wear on tire edges starts; it continues at an accelerated rate as the tire continues to wear on its edges or shoulders. Just as a car tire will wear further and further towards the edge of the tire; and worsen as the tire wears; I have witnessed this in our B737 tires also. On an automobile; an out of alignment of as little as 1/8' of an inch from centered on the edge of the tire; or 'toe in or toe out' error; can lead to spectacular tire wear rates. Just 1/4' of an inch out; can destroy an automobile tire in a couple of thousand miles. I have observed our tires to be as much as 1.5' inches 'out' of alignment due to the designed play built into the torque links and shimmy dampers. I have witnessed this play in the torque link bolt as it shows the lateral movement of the bottom torque link in relation to the top torque link. This gap can be witnessed just inboard of the shimmy damper assembly. The gap I have observed; has been as much as 3/4's of an inch between the damper assembly and the outboard edge of the bottom torque link. This 'play' is the play [movement]; as the torque link bolt/piston assembly slides in and out [left to right] into the damper assembly during axle castoring and lateral dampening. If one were to extrapolate the actual toe in/out error to the forward edge of the tire; it would be around 1.5 inches of 'toe error'. The above mentioned 'toe error'; is admitted to be as much as +/- 1.5 degrees of castor by design; based upon my research. This is rather large when one looks at the potential for tire wear. Another concern of mine; for another discussion; to only be mentioned here; is the ease of introduction of hydroplaning on a 'crabbed; or misaligned toe in/out' tire during touchdown. Please note that once hydroplaning begins; it is very hard to stop; especially in the case of a 'crooked' tire as described earlier; since this phenomenon will continue until the aircraft is stopped; as compared to allowing the wheels to spin up again as in; for example; during reverted rubber hydroplaning. The reason I bring up this issue in this discussion; is the fact that we are experiencing shallower shoulder grooves on these tires and in my opinion; very possibly introducing hydroplaning earlier in the landing roll due to the following:1) the tire may be toed 'in' or 'out'. This may; or seems; could lead to the introduction of premature hydroplaning as the edge of the tire 'skids' and slightly buckles under; skidding even more; during touchdown. 2. The lack of tread groove on the edge; combined with this 'skidding'; introduces hydroplaning over about eight inches laterally of tread; whereby the other 'good' grooves are unable to sufficiently shed or displace water due to the skimming effect of the skidding tire edge. In the last two weeks; I have written-up four separate tires for what appeared to be 'out of limits' wear. All four tires were worn excessively on the 'shoulders'. The shoulder; bymy definition at least; is the area between the most outboard tread groove and the tire side wall. What I have been noticing; is the shoulders have been wearing and the rest of the tire not wearing at all in comparison. I have witnessed; on the four previously mentioned tires; virtually pristine center tread and completely worn out shoulders. Two of the tires I wrote-up to be inspected; had about 3/8's of an inch of tread groove on the outer grooves; and had very pronounced wear on the shoulders. The third [flight] cycle later; these two tires were replaced. A previous aircraft two weeks earlier; had a tire that was literally brand new; except for the inboard shoulder. This particular tire; had been worn to the point of reinforcing plies showing around the entire circumference (out of limits by this fact alone); and absolutely no outer tread groove showing whatsoever. This particular tire obviously had many cycles on it; operating in an 'out of service' condition before my first officer (first officer) found it. The main gear axle will self-caster upon landing. Once castered one way or the other upon landing; the wheel will stay in that condition until aircraft tracking allows otherwise. Note: the pressure inside the shimmy damper assembly is only 75 psi max...hardly enough pressure to override a cocked; or castoring condition by itself. My aircraft; a B737-800; august 2011; ZZZ1 to ZZZ2; had about 3/16's of an inch of outer tread groove on the # 2 tire when we left ZZZ2. On return to ZZZ1; this 3/16' had worn down to a minus 1/16' of an inch; on the outer edge of the outer tread groove; requiring its replacement in ZZZ1; a 92-minute delay was incurred. Synopsis of my observations and recommendations: 1) we are experiencing costly wear on our tires; causing schedule disruptions. 2) I believe there are factors including tire design; gear design and shimmy damper design; leading to these issues. 3. I believe an education of both pilots and mechanics is warranted in the 'out of service' limits of the shoulder grooves on our tires. Maintenance controller X at base agreed that the tread depth limits requires the depth to be measured from the center of the groove and up both sides of the groove. In the issue of shoulder wear; one can see full tread groove depth on the inboard side of the groove; and have over half of the groove missing on the outboard side due to shoulder wear. This phenomenon is very misleading to crews and mechanics as to the serviceability of our tires. 4) I believe that excessive hydroplaning is occurring due to issues discussed above.

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Original NASA ASRS Text

Title: A Captain discusses how excessive shimmy damper movements of the Main Landing Gear (MLG) upper and lower torque links contribute to unusual inboard or outboard main tire groove shoulder wear on their B737-800 aircraft. The excessive shimmy damper play allows the tires to set in a crab-like toe 'in' or toe 'out' condition contributing to uncontrolled dynamic viscous hydroplaning upon landing.

Narrative: I have made some personal observations concerning our B-737-800 fleet tires. The following observations and opinions are a concern for the safety of our B737-800 fleet in the form of revenue loss and potentially a reason for the continuing incidents of runway excursions. First; my observations; starting with tires; among the many limits that can take a tire out of service; are the tread groove limits. The company [standard] fleet number is a minimum of 2/32s' of an inch; tread groove depth at any place on the tire. The B737 only; specifically requires at least 3/32s' of an inch [tread groove remaining]. I am to assume this is due the propensity of the B737-800 fleet to hydroplane excessively. Additionally; the advent of the newer radial tires; has brought further fleet concerns due to excessive shoulder wear on these tires. Now; based on my experience as a Mechanic; and quite extensive experience working with automobile front ends and alignment; I have noticed an expensive issue with the B737 tires. First; once wear on tire edges starts; it continues at an accelerated rate as the tire continues to wear on its edges or shoulders. Just as a car tire will wear further and further towards the edge of the tire; and worsen as the tire wears; I have witnessed this in our B737 tires also. On an automobile; an out of alignment of as little as 1/8' of an inch from centered on the edge of the tire; or 'toe in or toe out' error; can lead to spectacular tire wear rates. Just 1/4' of an inch out; can destroy an automobile tire in a couple of thousand miles. I have observed our tires to be as much as 1.5' inches 'out' of alignment due to the designed play built into the Torque Links and Shimmy Dampers. I have witnessed this play in the torque link bolt as it shows the lateral movement of the bottom torque link in relation to the top torque link. This gap can be witnessed just inboard of the shimmy damper assembly. The gap I have observed; has been as much as 3/4's of an inch between the damper assembly and the outboard edge of the bottom torque link. This 'play' is the play [movement]; as the torque link bolt/piston assembly slides in and out [left to right] into the damper assembly during axle castoring and lateral dampening. If one were to extrapolate the actual toe in/out error to the forward edge of the tire; it would be around 1.5 inches of 'toe error'. The above mentioned 'toe error'; is admitted to be as much as +/- 1.5 degrees of castor by design; based upon my research. This is rather large when one looks at the potential for tire wear. Another concern of mine; for another discussion; to only be mentioned here; is the ease of introduction of hydroplaning on a 'crabbed; or misaligned toe in/out' tire during touchdown. Please note that once hydroplaning begins; it is very hard to stop; especially in the case of a 'crooked' tire as described earlier; since this phenomenon will continue until the aircraft is stopped; as compared to allowing the wheels to spin up again as in; for example; during reverted rubber hydroplaning. The reason I bring up this issue in this discussion; is the fact that we are experiencing shallower shoulder grooves on these tires and in my opinion; very possibly introducing hydroplaning earlier in the landing roll due to the following:1) The tire may be toed 'in' or 'out'. This may; or seems; could lead to the introduction of premature hydroplaning as the edge of the tire 'skids' and slightly buckles under; skidding even more; during touchdown. 2. The lack of tread groove on the edge; combined with this 'skidding'; introduces hydroplaning over about eight inches laterally of tread; whereby the other 'good' grooves are unable to sufficiently shed or displace water due to the skimming effect of the skidding tire edge. In the last two weeks; I have written-up four separate tires for what appeared to be 'out of limits' wear. All four tires were worn excessively on the 'shoulders'. The shoulder; bymy definition at least; is the area between the most outboard tread groove and the tire side wall. What I have been noticing; is the shoulders have been wearing and the rest of the tire not wearing at all in comparison. I have witnessed; on the four previously mentioned tires; virtually pristine center tread and completely worn out shoulders. Two of the tires I wrote-up to be inspected; had about 3/8's of an inch of tread groove on the outer grooves; and had very pronounced wear on the shoulders. The third [flight] cycle later; these two tires were replaced. A previous aircraft two weeks earlier; had a tire that was literally brand new; except for the inboard shoulder. This particular tire; had been worn to the point of reinforcing plies showing around the entire circumference (out of limits by this fact alone); and absolutely no outer tread groove showing whatsoever. This particular tire obviously had many cycles on it; operating in an 'out of service' condition before my First Officer (FO) found it. The main gear axle will self-caster upon landing. Once castered one way or the other upon landing; the wheel will stay in that condition until aircraft tracking allows otherwise. Note: the pressure inside the Shimmy Damper assembly is only 75 psi max...Hardly enough pressure to override a cocked; or castoring condition by itself. My aircraft; a B737-800; August 2011; ZZZ1 to ZZZ2; had about 3/16's of an inch of outer tread groove on the # 2 tire when we left ZZZ2. On return to ZZZ1; this 3/16' had worn down to a minus 1/16' of an inch; on the outer edge of the outer tread groove; requiring its replacement in ZZZ1; a 92-minute delay was incurred. Synopsis of my observations and recommendations: 1) We are experiencing costly wear on our tires; causing schedule disruptions. 2) I believe there are factors including tire design; gear design and shimmy damper design; leading to these issues. 3. I believe an education of both pilots and mechanics is warranted in the 'out of service' limits of the shoulder grooves on our tires. Maintenance Controller X at Base agreed that the tread depth limits requires the depth to be measured from the center of the groove and up BOTH sides of the groove. In the issue of shoulder wear; one can see full tread groove depth on the inboard side of the groove; and have over half of the groove missing on the outboard side due to shoulder wear. This phenomenon is very misleading to crews and mechanics as to the serviceability of our tires. 4) I believe that excessive hydroplaning is occurring due to issues discussed above.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.