As originally designed an Essex Class carrier had three elevators to move aircraft. Two were centerline, fore and aft, and a third was deck edge on the port side after being proved successful on CV-7 USS Wasp. The deck edge created more lift capacity and storage space for aircraft without compromising the flight operations of the carrier. Machinery was a little less complicated and required 20% less man hours of maintenance.
The obvious compromise of the centerline elevators was in having one in the wrong position or inoperable due to battle damage. One can see some famous pictures of the USS Enterprise being hit by a kamikaze carrying a 550 lb. bomb which exploded under the forward elevator. It was blown 700 feet high and the Big E was now done for the remainder of the war.
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| A U.S. Navy Vought SB2U-2 Vindicator (BuNo 1376) from scouting squadron VS-72 pictured on the deck edge elevator of the aircraft carrier USS Wasp (CV-7) at Quincy, Massachusetts, in June 1940. The elevator consisted of a platform for the front wheels and an outrigger for the tail wheel. The two arms on the sides moved the platform in a half-circle up and down between the flight deck and the hangar deck. (US Navy photo) |
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| Another view of the same U.S. Navy Vought SB2U-2 Vindicator (BuNo 1376) from scouting squadron VS-72 pictured on the deck edge elevator of the aircraft carrier USS Wasp (CV-7) at Quincy, Massachusetts, in June 1940. (US Navy photo) |
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| Another view of the same U.S. Navy Vought SB2U-2 Vindicator (BuNo 1376) from scouting squadron VS-72 pictured on the deck edge elevator of the aircraft carrier USS Wasp (CV-7) at Quincy, Massachusetts, in June 1940. (US Navy photo) |
Through most of World War II, carriers operated with their flight deck in either a receiving or launching state.
In a launching state, planes are stored and readied for launch at the rear end of the deck as the front and middle sections of the deck are used for take off.
The rear elevator is used to move planes from the hangar to the rear of the deck so they can be readied for launch.
In a receiving state, planes are stored at the front of the deck as the rear and middle sections of the deck are used for landings.
The forward elevator is used to move these landed planes down to the hangar.
The middle elevator, if it exists, isn't used during launching or receiving operations. It's to increase the number of planes that can be moved to and from the deck before launch operations start or after landing operations finish.
There are a few advantages to a side elevator:
There isn't a hole in the middle of the flight deck that represents a structural weakness.
A malfunction when in a down state doesn't leave a hole in the flight deck that renders the whole thing inoperable.
The elevator only needs to be as large as the plane's landing gear rather than as large as the whole plane. The tail of the plane can hang off the side.
Nowadays, carriers can launch and receive aircraft simultaneously. Catapults mean that aircraft can launch from just the forward part of the deck whilst an angled flight deck and arrestor wires mean aircraft can land on just the rear and middle of the deck without disrupting launch operations.
The elevators can be used throughout all of this since they're not obstructing the landing deck or the catapults.