During the most critical period of the Battle of the Atlantic a number of British merchant ships were equipped with flight decks for launching and landing of aircraft which were carried as a defensive armament . The ships known as Merchant Aircraft Carriers ( or MAC ) ships combined the functions of laden merchant vessel with those of miniature escort carriers . Their original purpose was to provide air cover for both themselves and other vessels in the 500 mile gap in the Atlantic which was at that time outside the range of shore - based aircraft . The Fleet Air Arm pilots operating from ships have to face many difficulties . Even the largest MAC ship is considerably smaller than the smallest aircraft carrier and in one case pilots have a flat space of only 400 feet from which to take off and on which to land . Here HMS Alexia a tanker MAC ship .