A quick Google search came up with this:
http://www.goodyearaviation.com/reso...raftmanual.pdf
Treadwear
Inspect treads visually and check remaining tread. Tires should be removed when tread has worn to the baseof any groove at any spot, or to a minimum depth as specified in aircraft T.O.’s.
Return To Base Limits
Goodyear tires can remain in service with visible cord in the tread area only as long as the top fabric layer is not worn through or exposed for more than 1/8 of the circumference of the tire, and not more than one inch wide. Tires within these limits can continue in service no longer than necessary to return to a maintenance base and be replaced. (This applies to the proper tires for the aircraft as specified in its Aircraft Maintenance Manual.) For all other circumstances, normal removal criteria are still recommended as per the rest of this manual. This does not apply to military tires with Maximum Wear Limits marked on the sidewall.
NOTE: Further use of tires beyond this point may render a tire unsafe or unretreadable.
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Flat Spots
Generally speaking, tires need not be removed because of flat spots due to touchdown and breaking or hydroplaning skids unless fabric is exposed. If objectionable unbalance results, however, rebalance the assembly or remove the tire from service.