In the late 1980s, Tridair announced a program to install two Rolls-Royce 250-C20Rs and a new transmission on the proven Bell 206L3 airframe. The proposed modification would add twin-engine capability at a reasonable cost and with very little increase in weight or operating cost. An intriguing part of the design called for the ability to shut down one engine in cruise to conserve fuel. A few years later, Tridair and Bell announced an agreement whereby Tridair and Bell would cooperate on this program. Tridair would take the lead on development and certification and would have all rights to the retrofit market. Bell would then use the same technology on new production aircraft. The Tridair retrofit aircraft would be referred to as Gemini ST and would be based on Bell 206L1 and L3 models. Bell would base their new manufactured model on the Bell 206L4 and call it the Bell 206LT. Both are in the light twin-engine helicopter class.
Other than the engines and transmission, all details with respect to dimensions, cabin configuration, drivetrain and systems are the same as for the Bell 206L1 and L3 (Gemini ST) and the 206L4 (206LT).
While the concept was potentially sound, the airframe did not permit an increase in maximum takeoff gross weight. At the same time, empty weight increased by about 600 pounds. This put a serious penalty on the payload carrying capability of this helicopter and effectively reduced its utility to a few specialty applications. As a result, sales of both the Gemini ST and the 206LT were very limited.
The design of the Bell 206LT and Gemini ST uses the same proven two-bladed teetering rotor used on the Bell 206L3 and L4. Two Rolls-Royce 250-C20R engines are used in addition to a redesigned transmission that accommodates two engines. The transmission design allows single-engine operations in cruise flight as a routine operation. A two-bladed tail rotor provides directional control and the main and tail rotor blades use an extruded aluminum spar with a honeycomb core and bonded skin. The fuselage is made of conventional aluminum alloy. The cabin has two seats in front and an aft cabin with two seats facing aft and a three-seat bench facing forward. The cabin is long enough to allow carrying of two stretchers, one on top of the other, on one side of the cabin. This leaves room for two medical attendants.
Design of the Gemini ST was launched in the late 1980s. Certification was received in 1993 and deliveries started in 1993. Certification for use of a single-engine in cruise was obtained in 1994. Bell delivered 12 206LTs in 1993 and 1994, after which production ceased. To date, 18 206L3 have been converted to Gemini ST.