After acquiring a licence in 1956 to manufacture the Sikorsky S-58 helicopter, Westland imported one of these aircraft in HSS-1 configuration. Given the British serial number XL.722, this aircraft was test-flown for a time with its original 1525hp Wright R-1820-84 engine before being modified to accept a 1100shp Napier Gazelle NGa.11 gas turbine. In its new form it was flown for the first time on 17 May 1957, and was later joined by two pre-production Wessex HAS Mk.1's for Naval trials; the first of these flew on 20 June 1958. The HAS Mk.1 went into production in 1959 for the Royal Navy as a submarine search and strike helicopter equipped with dipping Asdic and provision for one or two homing torpedoes. Powered by a 1450shp Gazelle Mk.161 engine, it began service trials with No.700H Flight in April 1960 and has since been delivered to Nos. 706, 737, 771, 815, 819 and 848 Squadrons. The first of these to commission, in July 1961, was No.815; the Wessexes of No.848 Squadron were for commando assault duties aboard H.M.S. Albion, having the ASW gear removed to make room for 16 troops or 8 stretchers and a medical attendant in the main cabin. Alternatively, a slung load of 1814kg can be suspended from an under-fuselage hook. From January 1967 the Wessex Mk.1's have been joined in service by the HAS Mk.3, which is powered by a 1600shp Gazelle Mk.122, and has an extended rotor head fairing and large dorsal radome. Twenty-seven HAS Mk.31's supplied to Royal Australian Navy since August 1962 are similar to the HAS Mk.1 apart from their 1540shp Gazelle Mk.162 engines.
All other Wessex variants so far announced have two coupled Gnome engines in place of the single Gazelle. These include the RAF's HC Mk.2, flown for the first time in production form on 5 October 1962 and entering service with No.38 Group in February 1964; the Navy's HU Mk.5, for which two orders have been placed and which entered service in summer 1964 as a commando-carrier assault transport; twelve Mk.52's for the Iraqi Air Force, three Mk.53's for the Ghana Air Force, and one Mk.54 for the Brunei government. Seven Wessex Mk.60's have been built for Bristow Helicopters Ltd. These are 10-passenger commercial equivalents of theMk.2 and operate in support of the oil and gas drilling rigs in the North Sea.
Wessex HAS.Mk 1: Royal Navy ASW version with Napier Gazelle Mk 161 powerplant
Wessex HC.Mk 2: high-performance development of Wessex HAS.Mk 1 for the RAF with two coupled Bristol Siddeley Gnome Mk 110/Mk 111 turboshafts, each rated at 1007kW; used primarily as transports (16 troops) or air ambulance (eight stretchers)
Wessex HAS.Mk 3: advanced Royal Navy ASW version with 1193kW Napier Gazelle Mk 165 and a comprehensive automatic flight-control system
Wessex HC.Mk 4: two aircraft as Wessex HC.Mk 2 but with VIP interiors for service with The Queen's Flight
Wessex HU.Mk 5: troop-carrying assault helicopter for the Royal Marine Commandos; similar to Wessex HC.Mk 2
Wessex HAS.Mk 31: 27 built for Royal Australian Navy, similar to Wessex HAS.Mk 1 but with 1174kW Napier Gazelle Mk 162 flat-rated to 1148kW; delivery began in August 1962 and when later given updated ASW systems became redesignated Wessex HAS.Mk 31B
Wessex Mk 52: 12 similar to Wessex HC.Mk 2 for Iraqi air force
Wessex Mk 53: three similar to Wessex HC.Mk 2 for Ghana air force
Wessex Mk 54: one similar to Wessex HC.Mk 2 for service in Brunei
Wessex Mk 60: civil version seating 10 to 16 passengers according to role, 15 survivors in rescue operations, or as an air ambulance can carry eight stretchers, two sitting casualties and a medical attendant.