Stukas and HMS Illustrious.

[Graham Boak]

Illustrious's flight deck had 3in armour, intended to protect against a 500lb SAP bomb. There does not seem to have been such a hit. There are more details of the armour coverage in Friedman. The lift was not protected. Only one of the bombs dropped on 10th penetrated the armoured deck, hit 4 - possibly hit 2 as well - with one other such hit on the 16th.

I don't know how they judged the matter, but they are likely to have been experienced in such matters. Discussions about damage to USS Franklin, on one of the warship sites (modelwarship.com, I think) quote a report which compared the size of the entry holes, rather than the damage. From memory, this was part of a discussion on whether one or two bombs had been dropped. One hole was determined to have been made by a rocket that "cooked off" in the fire.