De Havilland Canada DHC 6 Twin Otter

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 DHC-3 Otter retaining DHC's renowned 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 550 shp (410 kW) Pratt & Whitney Canada PT6A-20 turboprop 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 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 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 subvariants (Series 310 for United Kingdom operators, Series 320 for Australian operators, etc.) sold before production in Toronto by de Havilland Canada ended in 1988.

New production

After Series 300 production ended, the remaining tooling was purchased by Viking Air of Victoria, British Columbia, which manufactures 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 Canada aircraft (DHC-1 through DHC-7). 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 more powerful Pratt & Whitney Canada PT6A-34 engines. As of November 2007, 40 firm orders and 10 options had been taken and a new final assembly plant was established in Calgary, Alberta. Zimex Aviation of Switzerland received the first new production aircraft, serial number 845, in July 2010. By mid-2014, Viking had built 55 new aircraft at its Calgary facility. The production rate as of summer 2014 was about 24 aircraft per year. In April 2015, Viking announced a reduction of the production rate to 18 aircraft per year. On June 17, 2015, Viking further announced a partnership with a Chinese firm, Reignwood Aviation Group. The group will purchase 50 aircraft and become the exclusive representatives for new Series 400 Twin Otters in China.

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 systems, and use of composites for nonload-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. Areas including Canada and the United States, (specifically Alaska) had much of the demand. Many Twin Otters still serve in the far north, but they can also be found in Africa, Australia, Asia, 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 such as Papua New Guinea. In Norway, the Twin Otter paved the way for the network of short-field airports, connecting rural areas with larger towns. The Twin Otter showed 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 (take-off, 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 a 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 February 1976 edition of the Official Airline Guide, Houston Metro operated 22 round-trip flights every weekday at this time 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), as well as to Tampa International Airport (TPA). This service by Shawnee Airlines is mentioned in the "Air Commuter Section" of the Sept, 6, 1972 Eastern Air Lines system timetable as a connecting service to and from Eastern flights. 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 of 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, as well as Canada's First Air, were air carriers that flew Boeing 727 jetliners and earlier versions of the Boeing 737 jetliner at the time. All four airlines also operated Twin Otter aircraft. Ozark Air Lines was primarily a McDonnell Douglas DC-9 jetliner operator that also flew Twin Otters. South Pacific Island Airways flew Twin Otters and Boeing 707 jets. In addition, Canadian air carriers Pacific Western Airlines and Transair, both of which operated Boeing 737-200s, flew Twin Otter aircraft. Other Canadian airlines that flew Twin Otters included Time Air and Norcanair, both of which also operated Fokker F28 Fellowship passenger jets. Twin Otter aircraft were operated by commuter or regional airlines on behalf of major air carrier partners and were painted in the liveries of such airlines as Continental Airlines, Eastern Airlines, and British Airways, with these aircraft providing connecting feeder service for these respective major airlines in the past. 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 first winter flight to Amundsen–Scott South Pole Station to perform a medical evacuation. On June 21–22, 2016, Kenn Borek Air's Twin Otters performed the third winter evacuation flight to Amundsen–Scott South Pole Station to remove two people for medical reasons.

The Argentine Air Force has used the Twin Otter in Antarctica since the 1970s, with at least one of them deployed year-round at Marambio Base. The Chilean Air Force has operated the type since 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 daily scheduled service is unique as the aircraft lands on the beach and the schedule is partly influenced by the tide tables. Trials in Barra with heavier planes than the Twin Otter, like the Short 360, failed because they sank in the sand. 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 get 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 17,000 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. Six years after, in July 2016, 100 series 400 have been delivered to 34 customers operating in 29 countries.

Variants

DHC-6 Series 100 

Twin-engine 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-engine STOL utility transport aircraft, powered by two 680 shp (715 ESHP) (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. Both have since been reverted to Series 300 conformity.

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. All have since been reverted to Series 300 conformity.

Viking Air DHC-6 Series 400

Viking Air production, first delivered in July 2010, powered by two Pratt & Whitney Canada PT6A-34 engines, and available on standard landing gear, straight floats, amphibious floats, skis, wheel skis, or intermediate flotation landing gear.

Viking Air DHC-6 Series 400S Seaplane

Viking Air seventeen-seat seaplane version of the Series 400 with twin floats and corrorsion-resistance measures for the airframe, engines and fuels system. Customer deliveries planned from early 2017. 500 lb (230 kg) lighter than the 400.

CC-138 

Twin-engine STOL utility transport, search and rescue aircraft for the Canadian Forces. Based on the Series 300 aircraft.

UV-18A 

Twin-engine 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 free-fall parachuting training aircraft, and by the Academy Parachute Team, the Wings of Blue, for year-round parachuting operations. Based on the Series 300 aircraft.

UV-18C 

United States Army designation for three Viking Air Series 400s delivered in 2013.

Operators

281 Twin Otter were in airline service in 2016, and 26 on order : 112 in North/South America, 106 in Asia Pacific & Middle East (16 orders), 38 in Europe (10 orders) and 25 in Africa. The Twin Otter has been popular not only with bush operators as a replacement for the single-engine de Havilland Canada DHC-3 Otter but also with other civil and military customers, with over 890 aircraft built. Many commuter airlines in the U.S. got their start by flying the Twin Otter in scheduled passenger operations.

Accidents and incidents

On June 29, 1972, Air Wisconsin Flight 671, a DHC-6-100 with eight people on board collided in mid-air over Lake Winnebago near Appleton, Wisconsin, with North Central Airlines Flight 290, a Convair 580 carrying five people. Both aircraft crashed into the lake, killing all 13 people on board.

On 3 May 1976, a DHC-6 Twin Otter 300, demonstration flight crashed on take off from Monze Air Force Base, Zambia, killing 11.

On January 18, 1978, a Frontier Airlines DHC-6-300 (N982FL) crashed during a training flight in Pueblo, Colorado killing all three crew members.

On September 2, 1978, an Air West Airlines Twin Otter crash in Coal Harbour (Vancouver) killed 11 people, nine of the 11 passengers and both crew members.

On July 31, 1981, Panamanian Air Force DHC-6-300 FAP-205 crashed during flight, killing all seven people on board including President Omar Torrijos (see Panamanian Air Force FAP-205 crash).

On February 21, 1982, Pilgrim Airlines Flight 458, a DHC-6-300 operating a schedule 2 commuter passenger flight, made an emergency landing on the northwest branch of the Scituate Reservoir near Providence, Rhode Island, after a fire broke out on board. One passenger was killed, eight passengers had serious injuries.

On March 11, 1982, Widerøe Flight 933 crashed into the Barents Sea near Gamvik, Norway killing all 15 people on board.

On June 18, 1986, Grand Canyon Airlines Flight 6, a DHC-6-300, collided with a Helitech Bell 206 helicopter, resulting in the death of all 20 people on board the DHC-6 and all five people on board the helicopter.

On October 28, 1989, Aloha Island Air Flight 1712 crashed into a mountain on approach to Hoolehua Airport at Molokai, Hawaii. The crash killed all 20 on board.

On April 12, 1990, Widerøe Flight 839 crashed in the ocean outside Værøy, Norway due to wind, killing all five people on board.

On April 22, 1992, a Perris Valley Aviation DHC-6-200 lost power at Perris Valley Airport in California, crashing from approximately 150 feet (46 m) coming to rest just next to the runway (near the end), killing 14 skydivers and two crew members on board; six skydivers survived.

On 10 January 1995, Merpati Nusantara Airlines Flight 6715, a Twin Otter 300, disappeared on a scheduled flight from Bima Airport to Satar Tacik Airport, Ruteng, Indonesia with the loss of 4 crew and 10 passengers. It appears to have crashed in the Molo Strait (waters between Flores and Rinca) in bad weather.

On November 30, 1996, an Aces DHC-6-300 crashed in the Cerro el Barcino mountains 8 km. from Aeropuerto Enrique Olaya Herrera in Medellin, Colombia, resulting in the death of 15 people, two crew and thirteen passengers on the ground.

On 7 January 1997 at around 11:00 local time, a Polynesian Airlines Twin Otter crashed into Mount Vaea in Samoa during bad weather conditions, a so-called controlled flight into terrain. The aircraft had been operating Flight 211 from Pago Pago to Apia, when the pilots decided to divert to Faleolo Airport. In the crash, two of three passengers and one of the two pilots lost their lives.

On March 24, 2001, an Air Caraïbes DHC-6-300 crashed in the mountains near Gustaf III Airport on Saint Barthélemy in the French West Indies, resulting in the death of 17 passengers, two crew and one person on the ground.

On August 9, 2007, Air Moorea Flight 1121 crashed shortly after taking off from Moorea Temae Airport in French Polynesia; the plane was bound for Tahiti. All 20 occupants, including 19 passengers and one crew member, were killed.

On May 6, 2007, French Air Force DHC-6-300 742/F-RACB in support of the Multinational Force and Observers in the Sinai Peninsula crashed, killing one Canadian and eight French peacekeepers.

On October 8, 2008, Yeti Airlines Flight 103, a DHC-6-300, was destroyed on landing at Lukla in Nepal; 16 passengers and two crew died in the incident, only the pilot survived.

On August 2, 2009, Merpati Nusantara Airlines Flight 9760 crashed in Indonesia about 22 kilometres (14 mi) north of Oksibil. All 16 people on board were killed.

On August 11, 2009, Airlines PNG Flight 4684 crashed on a mountain whilst en route from Port Moresby to Kokoda in Papua New Guinea, killing all 13 on board.

On December 15, 2010, a Tara Air DHC-6-310 crashed in the Bilandu Forest in Nepal. All 22 passengers and crew on board were killed.

On January 20, 2011, a Twin Otter Crashed in Ecuador. Six military passengers died.

On September 22, 2011, an Arctic Sunwest Charters DHC-6-300 float plane crashed in Yellowknife, Northwest Territories, clipping a condominium and crashing in the street killing two and injuring seven.

On 23 January 2013, C-GKBC (c/n:650), a Kenn Borek Air DHC-6 Twin Otter 300 skiplane went missing over the Queen Alexandra Range in Antarctica. On board the plane were 3 Canadians. The plane had been en route from the South Pole to Terra Nova Bay. Wreckage was found on Mount Elizabeth on the 25th, the crash was said to be unsurvivable.

On 16 May 2013, Nepal Airlines Flight 555 from Pokhara (PKR) to Jomsom (JMO) veered left off of the runway after touching down at Jomsom and went down the slope to the Kaligandaki river. The aircraft stopped at the bank of the river, with the left wing in the water. Three crew and four passengers received serious injuries, and 15 passengers received minor or no injuries. The aircraft was damaged beyond repair.

On 10 October 2013, MASwings Flight 3002, a DHC-6-310 (9M-MDM), crashed on landing at Kudat Airport; killing two of 16 on board.

On 16 February 2014, Nepal Airlines Twin Otter (9N-ABB) crashed at Dhikura of Arghakhachi on the way to Jumla from Pokhara killing 18 people including crew.

On 20 September 2014, Hevilift Twin Otter (P2-KSF) crashed on landing near Port Moresby, New Guinea. Of the nine people on board, four were killed (including both crew) and five were injured.

On 2 July 2015, Trans Maldivian Airways Twin Otter Floatplane (8Q-MAN) carrying 11 tourists crashed landed in the water near Kuredhu Island Resort in north central Maldives at 5:30pm, The crash landing occurred just a couple of miles off the Hotel, 11 passengers and three members of crew survived unharmed

On 2 October 2015, an Aviastar DHC-6 carrying 10 passengers and crews slammed onto the woods of Pajaja Mountain while en route to Makassar, killing all on board. Investigation revealed that the pilot decided to deviate the flight from its initial route without much consideration.

On 24 February 2016, a Tara Air Viking Air DHC-6 Series 400 Twin Otter carrying 18 passengers and 3 crew crashed after takeoff from Pokhara, Nepal, with none surviving. Search crews have found 19 bodies thus far and later updated total to 23 passengers as two babies were unaccounted for.

Specifications

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