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 as the de Havilland Canada DHC-6 Twin Otter. 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.

Jointly funded by the Canadian Department of Defence and de Havilland Canada, the first of an initial batch of five made its maiden flight on 19 May 1965. The first three aircraft were powered by 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.

Its design philosophy is directed towards operation from short semi-prepared runways for services in localities where air connections have not previously been practical, and its emphasis is on engineering simplicity and operational versatility. This is reflected by such features as urethane blocks as shock absorbers in the main undercarriage shoulders. Another selling point is the quickly convertible 384 cu.ft cabin from passengers to a maximum freight load of 4250 lb carried over 100 nm stage lengths with reserves.

The principal impact of the Twin Otter, and a feature which has already resulted in sales and options for about 30 aircraft, is probably its STOL performance to and from 50 ft of about 1000 ft. The CAR 3 distances are still only 1700 ft for take-off and 2160 ft for landing. At the start of its European tour at the Hanover Air Show, the Twin Otter demonstrator was able to join the Do27s and Porters ferrying passengers across the town to the small airstrip of the main industrial fair, and it was also able to get in and out of Heligoland's 190m strip off the German coast.

The Twin Otter has full-span double-slotted flaps, with the outer sections also operating as ailerons, but has no exotic high lift devices. Its tricycle undercarriage has an electrically-driven hydraulic pump providing power for steering, toe brakes and flaps, with a nosewheel - tiller on the port control wheel. The Twin Otter cockpit has entry doors each side so that the crew can get in and out when the cabin is packed to capacity with freight.

The Twin Otter has roof-mounted engine controls, which increase available cockpit space and the PT6s can be left in the flight idle gate for taxying except for occasional aft movement into the Beta range, to prevent excess speed building up on the ground. Further aft movement of the power levers through the idle gate selects reverse pitch, which enables the Twin Otter to back out of restricted parking spaces.

There are two main power references-torque, in lb/sq.in, and turbine gas temperature, with respective limits of 42.5 and 750 deg C.

During take-off, it would be easy to over boost the engines by opening the power levers to their full extent, so if the outside air temperature is on the low side, the torque-meters have to be watched to avoid exceeding the limit. In hot weather the limiting factor becomes exhaust temperature.

After raising flap, the Twin Otter climbs at 110 kt on 41 lb/sq.in torque, with the airscrew rpm pulled back to 90 per cent and climbed at nearly 2 000 ft/min. Levelling out with power reduced to 33 lb/sq.in torque and 78 percent rpm (330 shp per engine) gives an indicated 140-145 kt., which trued out to 155-160 kt. for a fuel flow of 225 lb per hour per engine.

One of the most useful aspects of Twin Otter performance is the'large usable speed range between Vno Of 160 kt. and a flapdown stall with power of well below 40 kt. IAS, including a manoeuvre speed for bush flying of around 55 kt. with about 25 per cent power and full flap.

To eliminate large changes of trim with flap movement, the Twin Otter retains the same trim tab interconnection that was introduced in its single-engine predecessor. This ensures a comfortable limit to, the nose-up pitch accompanying flap retraction during an overshoot.  Clean and power off, slight aileron snatch precedes pre-stall buffet, with some lateral hunting at 65 kt. which continues after the mild break-away at about 60 kt. IAS. With the full 40 degrees of flap extended, a gentle and straight stall break occurs at about 42 kt. IAS, power oft. Aerodynamic buffet gives about 8 kt. stall warning in all configurations

With 30 degrees (take~off) of flap extended, Vmc is 64 kt., and single-engine flight is completely uncomplicated. Feathering the port (critical) propeller from a 92 kt. climb resulted in light rudder loads, even before wind milling stopped, although the Twin Otter runs out of rudder trim at just below 85 kt, a long way before Vmc. There was no difficulty in showing a single engine en route climb of more than 300 ft/min.

Before landing, the nosewheel tiller has to be checked for centring, and then a spectacularly steep approach is achieved with full flap and just a trickle of power at only 65 kt. With restricted reverse pitch a 250 ft ground run is achieved.

The price, in basic form, in 1966 was (Canadian) $275000 approx. The PT6A-20 starts operational life with an overhaul period of 1200 hours.

Some 30 Twin Otters were due for completion by the end of 1966, and production was to continue at the rate of about six per month.

Delivered in July 1966, 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 652 shp 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.
The 300S model features several improvements: high-capacity brakes, an antiskid system, wing spoilers, refined electrical and hydraulic systems, propeller automatic feather, and improved fire protection. In addition, six 300S enhanced STOL performance DHC-6-300s were built in the mid 1970s. Earlier models, the series 100 and 200, were fitted with 570 shp engines. Access to the two man cockpit is through a car-like door on each side or through the cabin. The standard “Commuter” version will accommodate 20 passengers, but optional arrangements are available.
In 1982 DH Canada offered two specialised military versions designated Twin Otter Series 300M basic military transport (15 troops, 20 passengers or 2,270kg of cargo) or COIN version armed with cabin-mounted machine-guns and four underwing hardpoints, and the 300MR maritime reconnaissance version equipped with undernose Litton AN/APS-504 360 degree scan search radar, comprehensive avionics, an infrared linescanner, and a wing-mounted searchlight. Although a prototype 300MR was flown, the only buyer as Senegal with a single aircraft.
The Twin Otter has 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.
When the last Twin Otter was delivered in December 1988 production had reached 844.
Field Aviation Co of Mississauga, Ontario, developed an update package for the aircraft which has prompted a first contract by Wideroe Flyveselskap to modernise that airline's fleet of nine aircraft. Principal differences are the four-bladed propellers on the two Pratt & Whitney Canada PT6A-27 turboprops and a brand new interior.

By 2016 the Twin Otter series 400 was in series production by Viking Air and a number were in service, including two operated by Loganair, Glasgow, Scotland.

Gallery

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.

Specifications:

De Havilland Canada DHC 6 Twin Otter
Engine: 2 x Pratt & Whitney Canada PT6A 20, 643 shp
Wingspan: 65 ft (19.81 m)
Length: 49.5 ft (15.1 m)
Height: 18.6 ft (5.67 m)
Wing area: 420 sq.ft (39.02 sq.m)
Max take off weight: 11,000 lb (4990 kg)
Weight empty: 5850 lb (2653 kg)
Max. fuel: 2488 1b (1128 kg)
Max. payload: 4250 lb (1928 kg)
Max. speed: 239 kt / 442 km/h
Max. cruise at 10,000 ft (3050 m): 158 kt (293 kph)
Service ceiling: 25,500 ft (8354 m)
Single-engine ceiling: 8500 ft (2590 m)
Initial climb: 1550 ft/min (7.87 m/sec)
Single-engine climb: 315 ft/min (1.59 m/sec)
Cruising altitude: 10007 ft / 3050 m
Wing loading: 25.01 lb/sq.ft / 122.0 kg/sq.m
Range full fuel / 2420 lb (1105 kg) payload: 710 nm (1320 km)
Range with max. payload: 100 nm (185 km)
STO to 50 ft (15 m): 1090 ft (332 m)
Short landing from 50 ft (15 m): 980 ft (298 m)
CAR 3 distance take-off: 1700 ft (518 m)
CAR 3 distance landing, 2 160 ft (658 m)
Crew: 2+20

DHC-6 Twin Otter
Engine: 2 x PT6A-27, 620 hp.
Seats: 22.
Wing loading: 29.8 lb/sq.ft.
Pwr loading: 10.08 lb/hp.
Gross wt: 12,500 lb.
Empty wt: 7387 lb.
Equipped useful load: 4944 lb.
Payload max fuel: 1754 lb.
Range max fuel/cruise: 763nm/4.1hr.
Range max fuel / range: 895nm/6.1hr.
Ceiling: 26,700 ft.
Max cruise: 182 kt.
Max range cruise: 148 kt.
Vmc: 68 kt.
Stall: 58-74 kt.
1.3 Vso: 75 kt.
ROC: 1600 fpm.
SE ROC: 340 fpm @ 80 kt.
SE ceiling: 11,600 ft.
Min field length: 1500 ft.
Fuel cap: 2583/3190 lb.

DHC-6 Twin Otter
Engines: 2 x Pratt & Whitney (UACL) PT6A-27 turboprop, 652 shp.
Prop: 8 ft 6 in (2.59 m) dia 3-blade.
Wing span: 65 ft 0 in (19.81m).
Length: 51 ft 9 in (15.77 m).
Wing area: 420 sq ft (39.02 sq.m).
Gross weight: 12,500 lb (5,670 kg).
Max cruising speed: 210 mph (338 kph) at 10,000 ft (3,050 m).
Range: 745 miles (1,198 km) with 3250 lb (1474 kg) payload.
Crew: 1 or 2
Accommodation: 20 passengers or equivalent freight load.

DHC 6-300
Engines: 2 x P&W PT6A-27, 620 shp / 486kW.
Props: Hartzell 3-blade, 102-in.
Seats: 20.
Wingspan: 19.81 m / 65 ft 0 in
Length: 15.77 m / 52 ft 9 in
Height: 5.94 m / 20 ft 6 in
Wing area: 39.02 sq.m / 420.01 sq ft
Wing aspect ratio: 10.1.
Maximum ramp weight: 12,508 lbs.
Maximum takeoff weight: 5670 kg / 12,500 lbs.
Standard empty weight: 6873 lbs.
Maximum useful load: 5635 lbs.
Zero-fuel weight: 12,300 lbs.
Maximum landing weight: 12,300 lbs.
Wing loading: 29.8 lbs/sq.ft.
Power loading: 10.1 lbs/hp.
Maximum usable fuel: 3190 lbs.
Best rate of climb: 1600.
Service ceiling: 8140 m / 26,700 ft.
Maximum single-engine rate of climb: 340 fpm @ 79 kts.
Single-engine climb gradient: 258 ft/nm.
Single-engine ceiling: 11,600 ft.
Maximum speed: 182 kts.
Normal cruise @ 10,000ft: 182 kts.
Fuel flow @ normal cruise: 664 pph.
Endurance at normal cruise: 4.3 hrs:
Stalling speed clean: 74 kts.
Stalling speed gear/flaps down: 58 kts.
Turbulent-air penetration speed: 136 kts.