Because the taller passenger cabin adds both weight and drag to the airplane, other elements of the 1900D were also changed. More powerful engines and modified propellers were installed, winglets were added to reduce drag and increase the wings' efficiency, and the tail was made larger in response to the more powerful engines. The cockpit was updated with an Electronic Flight Instrument System (EFIS). The 1900D was certified under the then-new FAR Part 23 "Commuter Category" standards, which had replaced the earlier SFAR 41C. Since the UD serial numbers were already in use by the military 1900s, the 1900D airplanes have serial numbers beginning with UE. The 1900D is the most popular version of the airliner, with 439 of the 1900D built.