The de Havilland Canada DHC-6 Twin Otter is a Canadian STOL (Short Takeoff and Landing) utility aircraft developed by de Havilland Canada, which produced the aircraft from 1965 to 1988; Viking Air purchased the type certificate, then restarted production in 2008 before re-adopting the DHC name in 2022. The aircraft's fixed tricycle undercarriage, STOL capabilities, twin turboprop engines and high rate of climb have made it a successful commuter airliner, typically seating 18–20 passengers, as well as a cargo and medical evacuation aircraft. In addition, the Twin Otter has been popular with commercial skydiving operations, and is used by the United States Army Parachute Team and the 98th Flying Training Squadron of the United States Air Force. Design and development Development of the aircraft began in 1964, with the first flight on 20 May 1965. A twin-engine replacement for the single-engine DHC-3 Otter retaining DHC's STOL qualities, its design features included double-slotted trailing-edge flaps and ailerons that work in unison with the flaps to boost STOL performance. The availability of the Pratt & Whitney Canada PT6A-20 turboprop in the early 1960s made the concept of a twin more feasible. A DHC-3 Otter with its piston engine replaced with two PT6A-4 engines had already flown in 1963. It had been extensively modified for STOL research. To bush plane operators, the improved reliability of turboprop power and the improved performance of a twin-engine configuration made it an immediately popular alternative to the piston-powered Otter which had been flying since 1951. The first six aircraft produced were designated Series 1, indicating that they were prototype aircraft. The initial production run consisted of Series 100 aircraft, serial numbers seven to 115 inclusive. In 1968, Series 200 production began with serial number 116. Changes made at the beginning of Series 200 production included improving the STOL performance, adding a longer nose that was equipped with a larger baggage compartment (except for aircraft fitted with floats), and fitting a larger door to the rear baggage compartment. All Series 1, 100, and 200 aircraft and their variants (110, 210) were fitted with the PT6A-20 engines. In 1969, the Series 300 was introduced, beginning with serial number 231. Both aircraft performance and payload were improved by fitting more powerful PT6A-27 engines. This was a engine that was flat rated to for use in the Series 300 Twin Otter. The Series 300 proved to be the most successful variant