USAF announced plans to replace aging HH-60G helicopters, and deferred secondary combat search and rescue requirements calling for a larger helicopter. Sea Chinook For years the U.S. Navy has been operating different versions of the single-rotor CH-53 helicopter as its heavy-lift helicopter. CH-47s regularly conduct ship-based operations for U.S. Special Forces and other international operators. Due to budget issues, technical problems and delays with CH-53K, the director of the Pentagon's cost assessment office directed US Navy to consider maritime versions of CH-47. Naval versions must be protected against the corrosive seaborne environment and be able to operate from aircraft carriers and amphibious ships. Other export models The Royal Air Force version of the CH-47C is designated Chinook HC1; its versions of the CH-47D are designated Chinook HC2 and HC2A. The export version of the CH-47C Chinook for the Italian Army was designated "CH-47C Plus". The HH-47D is a search and rescue version for the Republic of Korea Air Force. The CH-47DG is an upgraded version of the CH-47C for Greece. While the CH-47SD (also known as the "Super D") is a modified variant for Singapore of the CH-47D, with extended range fuel tanks and higher payload carrying capacity; the CH-47SD is currently in use by the Republic of Singapore Air Force, Hellenic Army and the Republic of China Army. Eight CH-47Cs were delivered to the Canadian Forces in 1974. These helicopters were in Canadian service until 1991, with the designation CH-147. These aircraft were subsequently sold to the Netherlands and operated by the Royal Netherlands Air Force as CH-47Ds. All aircraft were phased out in 2021 and replaced by CH-47Fs. Civilian models Model 234LR (long range): Commercial transport helicopter. The Model 234LR can be fitted out as an all-passenger, all-cargo, or cargo/passenger transport helicopter. Model 234ER (extended range): Commercial transport version. Model MLR (multi-purpose long range): Commercial transport version. Model 234UT (utility transport): Utility transport helicopter. Model 414: The Model 414 is the international export version of the CH-47D. It is also known as the CH-47D International Chinook. CU-47: Built by Unical from parts from ex-Canadian Forces CH-47D/CH-147D for Coulson Aviation to be used for aerial firefighting. One delivered in 2020 with 11 others to be delivered. Derivatives In 1969, work on the experimental Model 347 was begun. It was a CH-47A with a lengthened fuselage, four-blade rotors, detachable wings mounted on top of the fuselage and other changes. It