I suggest two things:

1) rough runways - cut the grass shorter guys, and get rid of pebbles and sticks
2) problems with the fibreglass work - too much resin, not enough glass. Remember that two layers of 0.75 oz cloth is better than 1 layer of 1.5 oz when it comes to crack resistance. Also, that the resin contributes little to strength but will in fact be somewhat brittle on its own.

Of course, I could be completely out to lunch on this