The DHC-6 Twin Otter is a Canadian 19-passenger STOL (Short Takeoff and Landing) utility aircraft developed byde Havilland Canada and currently produced by Viking Air. The aircraft's fixed tricycle undercarriage, STOL abilities and high rate of climb have made it a successful cargo, regional passenger airliner and MEDEVACaircraft. In addition, the Twin Otter has been popular with commercial skydiving operations, and is used by theUnited States Army Parachute Team and the United States Air Force's 98th Flying Training Squadron.

Design and development
Development of the aircraft began in 1964, with the first flight on May 20, 1965. A twin-engine replacement for the single-engine Otter retaining DHC's 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 (410 kW) 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 improved reliability of turboprop power and the improved performance of a twin-engine configuration made it an immediately popular alternative to the single engine, piston-poweredOtter 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 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 680 hp (510 kW) engine that was flat-rated to 620 hp (460 kW) 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.

New production
After Series 300 production ended, the remaining tooling was purchased by Viking Air of Victoria, British Columbia, who manufacture replacement parts for all of the out of production de Havilland Canada aircraft. On February 24, 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 July 17, 2006, at the Farnborough Air Show, Viking Air announced its intention to offer a Series 400 Twin Otter. On April 2, 2007, Viking announced that with 27 orders and options in hand, it was restarting production of the Twin Otter, equipped with a more powerful Pratt & Whitney Canada PT6A-34/35 engine. As of November 2007, 40 firm orders and 10 options had been taken and a new final assembly plant established in Calgary, Alberta. Zimex Aviation of Switzerland received the first new production aircraft, serial number 845, in July 2010.

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

Operational history
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.

A number of commuter airlines in the United States got their start by operating Twin Otters in scheduled passenger operations. Houston Metro Airlines (which later changed its name to Metro Airlines) constructed their own STOLport airstrip with passenger terminal and maintenance hangar in Clear Lake City, Texas near the NASA Johnson Space Center. The Clear Lake City STOLport was specifically designed for Twin Otter operations. According to the Official Airline Guide (OAG), at one point Houston Metro operated 22 round trip flights every weekday between Clear Lake City (CLC) and Houston Intercontinental Airport (IAH, now Houston George Bush Intercontinental Airport) in a scheduled passenger airline shuttle operation. Houston Metro had agreements in place for connecting passenger feed services with Continental Airlines and Eastern Airlines at Houston Intercontinental, with this major airport having a dedicated STOL landing area at the time specifically for Twin Otter flight operations. The Clear Lake City STOLport is no longer in existence.

The Walt Disney World resort in Florida was also served with scheduled airline flights operated with Twin Otter aircraft. The Walt Disney World Airport (WDS), also known as the Lake Buena Vista STOLport, was a private airfield constructed by the Walt Disney Company with Twin Otter operations in mind. In the early 1970s, Shawnee Airlines operated scheduled Twin Otter flights between the Disney resort and nearby Orlando Jetport (MCO, now Orlando International Airport). This STOL airfield is no longer in use.

Another commuter airline in the U.S., Rocky Mountain Airways, operated Twin Otters from the Lake County Airport in Leadville, CO. At an elevation of 9,927 feet above mean sea level, this airport is the highest airfield in the U.S. ever to have received scheduled passenger airline service, thus demonstrating the wide ranging flight capabilities exhibited by the Twin Otter. Rocky Mountain Airways went on to become the worldwide launch customer for the larger, four engine de Havilland Canada DHC-7 Dash 7 STOL turboprop but continued to operate the Twin Otter as well.

Larger airlines in the U.S. and Canada also flew Twin Otters. Alaska Airlines, the original Frontier Airlines (1950-1986) and Wien Air Alaska were air carriers that flew Boeing 727 jetliners as well as earlier versions of the Boeing 737 jetliner at the time. All three airlines also operated Twin Otter aircraft. Ozark Airlines was primarily a Douglas DC-9 jetliner operator that also flew Twin Otters. In addition, Pacific Western Airlines (PWA) and Transair, Canadian air carriers that operated Boeing 737s, both flew Twin Otter aircraft as well. Two other Canadian airlines that flew Twin Otters, Time Air and Norcanair, also operated Fokker F28 Fellowship passenger jets. In many cases, the excellent operating economics of the Twin Otter allowed airlines large and small to provide scheduled passenger flights to communities that most likely would otherwise never have received air service.

Twin Otters are also 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 April 24–25, 2001, two Twin Otters performed the only winter flight to Amundsen-Scott South Pole Station to perform a medical evacuation.

The Argentine Air Force has used them in Antarctica since the 1970s with at least one of them deployed the whole year at Marambio Base The Chilean Air Force has operated the type from 1980, usually having an example based at Presidente Frei Antarctic base of the South Shetland islands.

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 (22), Trans Maldivian Airways (23), Kenn Borek Air (42) and Scenic Airlines(11). Some 115 airlines operate smaller numbers of the aircraft including Yeti Airlines in Nepal, Malaysia Airlines which uses the Twin Otter exclusively for passenger and freight transportation to the Kelabit Highlands region in Sarawak, and in the United Kingdom the FlyBe franchise operator Loganair which uses the aircraft to service the island of Barra in the Hebrides islands. This scheduled service is unique as the aircraft lands on the beach and the schedule is partly influenced by the tide tables. The Twin Otter is also used for landing at the world's shortest commercial runway on the Caribbean island of Saba, Netherlands Antilles.

Ontario's Ministry of Natural Resources is also a long-time operator of the Twin Otter.

Transport Canada still owns three DHC-6 Twin Otter aircraft, but they now see very limited flying time, as their role in Coastal Surveillance has been assumed by a fleet of DHC-8s.

The Twin Otter has been popular with commercial skydiving operations. It can carry up to 22 skydivers to over 13,500 ft (a 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.

On 26 April 2001, the first ever air rescue during polar winter from the South Pole occurred with a ski-equipped Twin Otter operated by Kenn Borek Air.

On September 25, 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 October 1, 2008, at Victoria Airport. Two days later, the aircraft departed Victoria for a ferry flight to Orlando, Florida, site of the 2008 National Business Aviation Association (NBAA) Conference and exhibition. The first new build Series 400 Twin Otter (SN 845) made its first flight on February 16, 2010, in Calgary, Alberta. Transport Canada presented Viking Air Limited with an amended DHC-6 Type Certificate including the Series 400 on July 21, 2010.

Variants
DHC-6 Series 100
Twin-engined STOL utility transport aircraft, powered by two 550 shp (432 kW) Pratt & Whitney Canada PT6A20 turboprop engines.

DHC-6 Series 110
Variant of the Series 100 built to conform to BCAR (British Civil Air Regulations)

DHC-6 Series 200
Improved version.

DHC-6 Series 300
Twin-engined STOL utility transport aircraft, powered by two 620 shp (462 kW) Pratt & Whitney Canada PT6A-27 turboprop engines.

DHC-6 Series 300M
Multi-role military transport aircraft. Two of these were produced as "proof-of-concept" demonstrators

DHC-6 Series 310
Variant of the Series 300 built to conform to BCAR (British Civil Air Regulations)

DHC-6 Series 320
Variant of the Series 300 built to conform to Australian Civil Air Regulations

DHC-6 Series 300S
Six demonstrator aircraft fitted with 11 seats, wing spoilers and an anti-skid braking system.

DHC-6 Series 400
First delivered in July 2010, powered by two Pratt & Whitney Canada PT6A-34 or optional PT6A-35 Hot & High Performance engines, and available on standard landing gear, straight floats, amphibious floats, skis, wheel skis, or intermediate flotation landing gear.

CC-138
Twin-engined STOL utility transport, search and rescue aircraft for the Canadian Forces.

UV-18A
Twin-engined STOL utility transport aircraft for the U.S. Army Alaska National Guard. Six built. It has been replaced by the C-23 Sherpa in US Army service.

UV-18B
Parachute training aircraft for the United States Air Force Academy. The United States Air Force Academy's 98th Flying Training Squadron maintains three UV-18s in its inventory as freefall parachuting training aircraft, and by the Academy Parachute Team, the Wings of Blue, for year-round parachuting operations.

UV-18C
United States Army designation for three Viking Air Series 400s ordered in 2012.