Tire Wear
Good question!!!!!
As spannersatcx stated, the tire need not be changed immediately if the cords that are showing are 'wear cords'.
sir.pratt is also correct.
With aircraft tires, there is really no decent dynamic hydroplaning tread design. (Not like car tires.) So, the hydroplaning characteristics are mainly defined by tire pressure.
It is quite normal and quite acceptable to have a bit of wear cord showing. However, the purpose of this wear cord is to alert the engineers that a tire change is necessary....perhaps at the next stop where maintenance can be performed, or on the overnight stop.
I've worked for several airlines where specific, individual policy differed. Where I work now, the tire would be considered unserviceable and must be changed immediately. But, this is company policy...not mandated by the tire engineers or FAA.
PantLoad