British CAA tyre wear limitations
http://www.caa.co.uk/docs/33/CAP455.PDF
In the absence of evidence of the safety of a lower limit for a particular aircraft/tyre/
operation combination, it is recommended that a tyre be withdrawn from service
when it is worn to such an extent that its wet runway performance would be seriously
impaired. This may be defined as when:
a) it is worn such that any groove has a depth of less than 2mm of tread for more than
one quarter of the tread circumference; or
b) at any place on the circumference the tread pattern is worn to a depth of less than
2mm across the whole width of the tread in contact with the runway.
NOTE: This is not a rigid definition and equivalence may be provided if, for example, tyre
wear is such that whilst one groove is less than 2mm all the others are 3mm or more