These units were specially adapted to be mounted on the side of the ship and used as heavy duty lifts from the ship bottom up to the hanger deck.
The main purpose being if the ship had an engine failure they could respond quickly with an engine change. They are also used for general duties for moving goods round the ship.
In June 2010 the HMS Illustrious under went a major refit, during this Matterson got the contract to overhaul its 4 Units. These 4 units were made in 1977 and are still operating as good today 34 years on………