Armoured flight deck

An armoured flight deck is an aircraft carrier flight deck that incorporates substantial armour in its design.

Comparison is often made between the carrier designs of the Royal Navy (RN) and the United States Navy (USN). The two navies followed differing philosophies in the use of armour on carrier flight decks, starting with the design of the RN's Illustrious class and ending with the design of the Midway class, when the USN also adopted armoured flight decks. The two classes most easily compared are the RN's Illustrious class and Implacable class and their nearest USN contemporaries, the Yorktown and Essex classes. The Illustrious class followed the Yorktown but preceded the Essex, while the Implacable class design predated the Essex but these ships were completed after the lead ships of the Essex class. The development of armoured flight deck carriers proceeded during World War II and before the end of World War II, both the USN, with USS Midway, and the Imperial Japanese Navy (IJN), with Taiho and Shinano, would also commission armoured flight deck carriers, while all USN fleet aircraft carriers built since 1945 feature armoured flight decks. The remainder of the IJN carrier force during World War II had unarmoured flight decks just like the Yorktown and Essex classes of the USN.

In choosing the best design for their carriers, the British had to consider the advantages and disadvantages of hangar design. There was a choice between open or closed hangar and the position of the armour. The placing of the strongest deck affected the strength of the hull. The further apart the deck and the keel, the stronger the design. If the flight deck was placed above the main deck then it had to be built to allow for movement with expansion sections. A closed hangar design was the strongest structurally and made for a lighter hull and the RN carried this concept one step further and designed the armoured flight deck to also act as the strength deck without any underlying plating, thus achieving an armoured flight deck on the lowest possible displacement.

...
Wikipedia