The problem with very light twin-engine helicopters is payload and range. The extra weight of the second engine, combining gearbox, extra fuel tanks and related systems severely limits the airframe’s payload. Also, that smaller payload capacity will need to include the weight of more fuel. As such, carrying enough fuel for an acceptable range normally leaves little weight capacity for passengers. Operators of small helicopters who wanted the redundancy of a twin have to make some big trade-offs.
In the late 1980s, Doug Daigle, a 14,000-hour helicopter pilot and owner of Tridair Helicopters, came up with an idea that tried to solve this problem. He took a popular single-engine helicopter, the Bell LongRanger (206L3), and removed the 650-shaft-horsepower Allison 250-C30 engine (the 500-shp C28 for the older L1s) and added a pair of 450-shp Allison 250- C20R engines to create a twin with good single-engine performance. However, what was different about his design was it was the first twin to be certified for normal operation on one engine in all areas of flight. Certified by the FAA in 1994, he named it the Gemini ST conversion.
When operating both engines, the Gemini consumes fuel at 45 gallons per hour, compared to the LongRanger at 38 gph. Because both helicopters have the same fuel capacity (he did not add extra fuel tanks), endurance drops from 2.9 hours for the LongRanger to 2.5 hours for the Gemini. However, the Gemini’s C20R engine burns only 28 gph in single-engine mode, and this increases the endurance to 3.9 hours. Daigle believed long trips will normally have some extended cruise flight, where the pilot could choose to fly on one engine. The other engine can be started for critical maneuvers that require the redundancy of a second engine.
The extra weight of the engine and related systems does increase the empty weight, so there is some trade-off with the Gemini. It has a smaller useful load of 1,610 pounds, compared to 2,175 pounds for the LongRanger. The Gemini can carry 740 pounds of fuel (the same as the LongRanger), which leaves a payload capacity of 870 pounds.
Bell Helicopter then entered into an agreement with the company that supported Tridair’s conversion of existing LongRangers while Bell would build a new production model, the 206LT TwinRanger, under Tridair’s STC. Bell only delivered 13 airframes and the concept of shutting one engine down in flight never caught on. This model was replaced by the Bell 427 which was under development for the EMS market. The 427’s cabin proved too small and Bell canceled the program and introduced the larger, single-pilot IFR certified 429 in 2009.