De Havilland Canada DHC-6 Twin Otter

In 1964 de Havilland Canada announced that it was developing a twin-turboprop high-wing monoplane with STOL capability to provide accommodation for 13 to 18 passengers. Identified as the de Havilland Canada DHC-6 Twin Otter, the first of an initial batch of five made its maiden flight on 20 May 1965. Powerplant of the first three aircraft consisted of two 432kW Pratt & Whitney Aircraft of Canada PT6A engines, but the fourth and subsequent examples of this first Twin Otter Series 100 production version had PT6A-20 engines of similar output. Design of the aircraft's wing includes double-slotted trailing-edge flaps and ailerons which can be drooped simultaneously with the flaps to enhance STOL performance. Fixed tricycle landing gear can have optional float or ski installations, as well as the standard wheels.

Intended primarily for service with commuter or third-level airlines, the Twin Otter has nevertheless seen wide use with air forces and government agencies. Military operators include Argentina, Canada, Chile, Ecuador, Ethiopia, France, Haiti, Jamaica, Nepal, Norway, Panama, Paraguay, Peru, the USA and Venezuela.

The first Twin Otter Series 100 entered service in 1966, and, following manufacture of 115 of that version, production switched to the Twin Otter Series 200. It differed by having increased baggage capacity in a lengthened fuselage nose and was certificated for operation at a higher gross weight. After 115 had been built the production Twin Otter Series 300 was introduced, this having more powerful PT6A-27 engines which make possible an increase of almost 454kg in maximum take-off weight. Later production aircraft had a 20-seat commuter interior as standard and all floatplane versions, irrespective of series, retain the shorter fuselage nose of the original Series 100. Specialised equipment that has been developed to enhance the capabilities of these popular aircraft includes a ventral pod to carry 272kg of freight and an expendable fabric membrane tank holding 1818 litres of water for water-bombing fire-fighting operations.

In 1982 DH Canada offered two specialised military versions designated Twin Otter Series 300M basic military transport or COIN version armed with cabin-mounted machine-guns and underwing hardpoints, and the 300MR maritime reconnaissance version equipped with search radar, comprehensive avionics and a wing-mounted searchlight. Although a prototype 300MR was flown, the only buyer was Senegal with a single aircraft. When the last Twin Otter was delivered in December 1988 production had reached 844. Studies for a successor are designated DHC-9.

VARIANTS

Twin Otter 300S: designation of six aircraft fitted with 11 seats, an improved high-capacity anti-skid braking system and wing spoilers, built for the 1973 experimental Air Transit service linking downtown STOL airports in Montreal and Ottawa.

Twin Otter Series 400: proposed development to meet US FAR 36 noise regulations. Not built.

UV-18A: two standard Series 300s delivered October 1976 onwards to the US Army Alaska National Guard, followed by four more in 1979 and 1982. Operating on wheels, floats or skis they are used for command, personnel or logistic flights within Alaska.

UV-18B: two standard Series 300s delivered to the US Air Force Academy in 1977 and used for sporting parachuting activities.