Firstly don't be confused between tread reinforcing cords and the actual carcass plies, this is mainly for the B747, Airbus is slightly different. The first ones you see are the tread reinforcing cords which are nylon the main carcass cords are steel plies. As an Engineer I am governed by the Maintenance Manual and not the manufacturers books.
The following is from the Boeing MM.
Wear removal limits
Main Base
Centre Rib tyres
Tyres should be removed from service when either of the following conditions is noted:
(a) Either of the grooves adjacent to the center rib is worn away for a length of 3 inches (7.5 cm) at any one location, or,
(b) The first layer of reinforcing fabric is exposed at the center of the tread for a length of 3 inches
(7.5 cm) at any one location.
Center Groove Tyres
As per (a) or (b) above except use center groove for wear determination.
LINE STATIONS
Center Rib Tyres
Tyres can remain in service as long as there is some evidence of the grooves at the center of the tread remaining at various points around the tyre provided there are no obvious flat spots.
Center Groove Tyres
As per above except use the center groove for wear determination.
Note 1: If passengers compliant of vibration immediately after rotation in the door 3 and 4 area are registered, check the main landing gear tyres for even tyre tread depth around the circumference of each tyre. If the tread depth over approximately 1/4 of a tyre circumference differs from the remaining 3/4 of the tyre circumference by 0. 1 80 (3/16) inch or more, replace the tyre.
Note 2: Tyres with flat spots through skids, aquaplaning etc may continue in service until return to Base as long as the tread grooves are still visible at the part of greatest wear. Note however, that excessively uneven wear on NLG tyres can cause NLG shimmy or vibration.
Note 3: TRAINING FLIGHTS. When an aircraft is scheduled for a training detail, the Ground Engineer should establish with Operations/Movements the estimated number of landings planned for the detail.
Tyres should have sufficient tread, above operators limits, to cover A) the scheduled training detail and B) the next scheduled commercial flight if a short transit time is anticipated.
Hope that helps
