Initially assembled with its Wright Cyclone, it was demonstrated to the British armed services leading to a preliminary order for the Royal Navy.

The decision made by Westland to install a modern gas-turbine powerplant gave the Wessex a greater load capacity, was quieter and generated less vibration, the latter quality being highly beneficial when treating casualties during flight.

These same qualities that made the Wessex well-suited to the anti-submarine role also lent themselves to the search and rescue (SAR) mission, which the type would become heavily used for.

As one of the RAF's standing duties, multiple Wessex helicopters were permanently kept on standby to respond to an emergency located anywhere within 40 miles of the British coastline within 15 minutes during daytime.

[17] The Wessex's service career featured long-term deployments to both Hong Kong and Northern Ireland to support internal security operations, performing transport and surveillance missions.

[18] In Northern Ireland, the use of helicopters for supply missions proved a viable alternative to vulnerable road convoys; operations in this theatre led to the employment of various defensive equipment and countermeasures against the threat posed by small arms and shoulder-launched anti-air missiles.

By February 1964, a large number of RAF and RN helicopters, including Westland Wessex, were operating from bases in Sarawak and Sabah to assist Army and Marine detachments fighting guerilla forces infiltrated by Indonesia over its one thousand mile frontier with Malaysia.

Having removed much of the anti-submarine equipment to lighten the aircraft, during the campaign in Borneo the Wessex was typically operated as a transport helicopter, capable of ferrying up to 16 troops or a 4,000-pound payload of supplies directly to the front lines.

[31] Bristow flew them from various UK airfields and helicopter pads to support the growing North Sea Oil industry until they were withdrawn in 1982.

In April 1961, the Royal Australian Navy (RAN) announced that they had selected the Westland Wessex to become the standard service helicopter from their ships and its intention to purchase roughly 30 for anti-submarine patrols, casualty evacuations, and fleet communications duties.

[34][35] The Wessex was a major operational shift for the Fleet Air Arm, enabling the RAN to proceed with the conversion of the aircraft carrier HMAS Melbourne as an anti-submarine platform.

[35] Performing search and rescue sorties became another valued role of the type; in 1974, multiple Wessex helicopters participated in the relief effort in Darwin in the aftermath of Cyclone Tracy.

[35] While the Wessex proved to be too large to reasonably operate from most of the RAN's destroyers, it was found to be well suited as a troop-transport helicopter from heavy landing ships and larger vessels.

[30] Data from Westland Aircraft since 1915[102]General characteristics Performance Wessexes portrayed the visually similar CH-34 Choctaws in Stanley Kubrick's 1987 film Full Metal Jacket.

These are powered by the coupled-twin de Havilland Gnome[105] with a distinctive long nose and single large turbine exhaust on each side, distinguishing them from the CH-34.