The DHC-6 Twin Otter is a 20-passenger STOL (Short Takeoff and Landing) utility aircraft developed by de Havilland Canada. It has often been called the most successful aircraft program in Canada's history. The aircraft's fixed tricycle undercarriage, STOL abilities and relatively high rate of climb have made it a successful cargo, regional passenger airliner and MEDEVAC aircraft. In addition, the Twin Otter has been popular with commercial skydiving operations.
Development of the aircraft began in 1964, with the first flight on 20 May 1965. A twin-engined replacement for the single-engined Otter had been planned by de Havilland Canada. Twin engines not only provided improved safety but also allowed for an increase in payload while retaining the renowned STOL qualities. 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 550 shp Pratt and Whitney Canada PT6A-20 propeller turbine engine in the early 1960s made the concept of a twin more feasible. To bush operators, the enhanced reliability of turboprop power and the enhanced performance of a twin-engined configuration made it an immediately popular alternative to the single engine, 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 number 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 to 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 550 shaft horsepower PT6A-20 engines.
In 1969, the 300 series was introduced, beginning with serial number 231. Both aircraft performance and payload were improved by fitting more powerful PT6A-27 engines. This was a 680 hp engine that was flat-rated to 620 hp for use in the Series 300 Twin Otter. The Series 300 proved to be the most successful variant by far, with 614 Series 300 aircraft and their sub-variants (Series 310 for United Kingdom operators, Series 320 for Australian operators, etc.) sold before production ended in 1988.
Twin Otters could be delivered directly from the factory with floats, skis or tricycle landing gear fittings, making them adaptable bush planes for remote and northern areas including Canada and the United States, specifically Alaska. Many Twin Otters still serve in the far north, but they can also be found in Africa, Australia, Antarctica and other regions where bush planes are the optimum means of travel. Their versatility and maneuverability have made them popular in areas with difficult flying environments, including Papua New Guinea. In Norway, the Twin Otter paved the way for the network of short-field airports, connecting the rural areas with the larger towns with outstanding reliability, and remained in service until 2000 on certain routes. Widerøe of Norway was, at one time, the world's largest operator of Twin Otters. During one period of its tenure in Norway, the Twin Otter fleet achieved over 96,000 cycles (takeoff, flight and landing) per year.
Twin Otters, rated for flight down to −75°C (−103°F), are a staple of Antarctic transportation. Four Twin Otters are employed by the British Antarctic Survey on research and supply flights, and several are employed by the United States Antarctic Program via contract with Kenn Borek Air. On 24 April 24–25 2001, two Twin Otters performed the only winter flight to Amundsen-Scott South Pole Station to perform a medical evacuation.
As of August 2006, a total of 584 Twin Otter aircraft (all variants) remain in service worldwide. Major operators include: Libyan Arab Airlines (16), Maldivian Air Taxi (17), Trans Maldivian Airways (15), Kenn Borek Air (33) and Scenic Airlines (11). Some 115 airlines operate smaller numbers of the aircraft including Yeti Airlines in Nepal and the British Airways subsidiary airline, Loganair.
Ontario's Ministry of Natural Resources is also a long-time operator of the Otter.
The Twin Otter has been popular with commercial skydiving operations. It is generally limited to carrying 22 jumpers (a relatively large load compared to most other aircraft in the industry); presently, the Twin Otter is used in skydiving operations in many countries. The United States Air Force operates three Twin Otters for the United States Air Force Academy's skydiving team. The cadets affectionately call the aircraft the "'Twotter."
After series production ended, the remaining tooling was purchased by Viking Air of Victoria which manufactures replacement parts for all of the out of production de Havilland Canada aircraft. On 24 February 2006, Viking purchased the type certificates from Bombardier Aerospace for all the out of production de Havilland DHC-1 through DHC-7 aircraft. The ownership of the certificates gives Viking the exclusive right to manufacture new aircraft.
On 17 July 2006, at the Farnborough Air Show, Viking Air announced its intention to offer a "Series 400" Twin Otter. Restarting the production would be dependent on the ability to realize enough initial orders.
In April 2007, Viking announced that it would restart production of the Twin Otter with a more powerful PT6A-34/35 engine. At the time, Viking had 27 orders and options in hand. As of November 2007, 40 firm orders and 10 options have been taken and a new assembly plant has been established in Calgary, Alberta with with customer deliveries commencing summer 2009. The parts are made in Victoria, British Columbia then shipped to Calgary, Alberta for assembly. Zimex Aviation of Switzerland will receive the first aircraft.
On 25 September 2008, the Series 400 Technology Demonstrator achieved "power on" status in advance of an official rollout. First flight of the Series 400 technical demonstrator, C-FDHT, took place 1 October 2008 at Victoria Airport. The next day, the aircraft departed Victoria for a ferry flight to Orlando, Florida, site of the 2008 NBAA Conference and exhibition.
Major changes introduced with the Series 400 include Honeywell Primus Apex fully integrated avionics, deletion of the AC electrical system, deletion of the beta backup system, modernization of the electrical and lighting system, and use of composites for non load bearing structures such as doors.
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