It also recommended that the USMC form an experimental helicopter squadron. HMX-1 was commissioned in 1947 with Sikorsky HO3S-1s.[11] In 1948 the Marine Corps Schools came out with Amphibious Operations—Employment of Helicopters (Tentative), or Phib-31, which was the first manual for helicopter airmobile operations.[12] The Marines used the term vertical envelopment instead of air mobility or air assault. HMX-1 performed its first vertical envelopment from the deck of an aircraft carrier in an exercise in 1949.