Major changes were made to the Twin Commander with the introduction of the model 840 in 1979. The wings were increased 30 inches and canted winglets were added to increase total wing area. The 254K variant of the AiResearch gas turbine TPE331 engines were used that generated 717.5 shaft horsepower each with new Dowty full feathering and reversible propellers. Wet wings were added outboard of the nacelles increasing fuel capacity from 384 to 482 gallons. The increased fuel capacity of the wet wings and fuel efficiency of the new engines allowed the 840 to have a maximum range of 2,040 nautical miles. It was the first Twin Commander to break the 2,000 nautical mile range barrier. Aircraft dry weight decreased by 507 lbs. over the 690B.