Early Type 23 flight decks are described as having been arranged for Sea King landing loads. Merlin placed its wheels, and therefore its concentrated loads, in a different arrangement. The remedy was a substantial steel reinforcement beneath the aft part of the deck. Sailors moving between ships reportedly struck their heads on that added structure when they expected the clearance of an unmodified vessel. The example explains why being able to land vertically does not make an aircraft compatible with every ship's flight deck.
Notable Point · 3:53:52-3:54:37
Merlin landing loads and Type 23 deck reinforcement
A change in helicopter wheel arrangement can require structural reinforcement even when a frigate already has a flight deck.