 UK Chinook procurement ambitions were revived in 1978 with an announced requirement for a new heavy @-@ lift helicopter to replace the Westland Wessex . Thirty Chinooks were ordered at a price of US $ 200 million . These helicopters , comparable to the CH @-@ 47C with Lycoming T55 @-@ L @-@ 11E engines , were again designated Chinook HC Mk1 , and entered service in December 1980 . Eight more Mk1s were delivered from 1984 to 1986 with the CH @-@ 47D 's Lycoming T55 @-@ L @-@ 712 turboshafts . 
