The Royal Navy built three aircraft maintenance carriers for its Fleet Air Arm before and during World War II. The Abyssinia Crisis of 1934–35 demonstrated to the Admiralty that it needed a depot ship to support the aircraft carriers in active service, just like submarine and destroyer tenders supported those types. Begun just before the start of World War II in 1939, HMS Unicorn was the first ship built in any navy that could "carry out the full range of aircraft maintenance and repair work in addition to the ability to operate aircraft from the flight deck". Unicorn proved the value of the concept and two similar support ships, Perseus and Pioneer were converted into aircraft maintenance ships by modifying light carriers while still under construction. Unlike Unicorn, neither Pioneer nor