The tyres used on DC-8s have a tread portion that contains three fabric re-enforcing layers. These layers protect the tyre carcass to some extent against foreign object damage. A tyre tread, however, does not contribute to the carcass integrity; even when a tyre is worn to the third fabric layer, the normal change point, it does not lose any structural integrity. The mandatory change point for a tyre is when it is worn to the carcass layer; however, at this point, the tyre could not undergo a retread.