De Havilland Canada Dash 7

Design and development

In the 1960s, de Havilland Canada was already well known worldwide for their series of high-performance STOL aircraft, notably the very popular DHC-2 Beaver and DHC-6 Twin Otter. However, these aircraft were generally fairly small and served outlying routes, as opposed to the busier regional airliner routes which were already well served by larger, higher-performance turboprop aircraft such as the Fokker F27, Fairchild F-27, Convair 580, Convair 600 and Hawker Siddeley 748.

The de Havilland Canada company personnel felt they could compete with these designs in a roundabout way. With their excellent STOL performance, their designs could fly into smaller airports located in city centres and smaller, outlying, more austere airports having runways that the other aircraft could not easily use (unpaved, unimproved). The original specification called for an aircraft that could carry 40 passengers and operating from runways only 2,000 ft long (610 m), and designed for, with a full load of passengers, the ability to fly 700 nautical miles (1296km), or a range of 805 miles (1,296 km).

With new noise restrictions coming into effect throughout the 1970s, an aircraft tailored for this role would also have to be very quiet. Propeller noise is a factor of blade length and chord and the speed at which it rotates. To meet these new regulations, the new design used much larger (oversized) propellers geared to rotate at a lower speed than is normally designed. Much of the problem sound from a typical propeller is generated at the tips of the blades which are rotating at or just beneath the speed of sound. By using oversize propeller blades, there is no need to have the blade tip reaching near the speed of sound, and the rotating speed can therefore be reduced without sacrificing thrust. In reducing the speed, this noise is reduced substantially. The Dash 7 often landed at only 900 rpm, and took off at only 1,210 rpm.

In other respects, the new DHC-7 was essentially a larger, four-engine version of the Twin Otter: the general layout remained similar, with a high aspect ratio, high-mounted wing, and similar details of the cockpit and nose profile. Changes included the addition of cabin pressurization (requiring a switch to a fuselage with a circular cross-section), landing gear that folded forward into the inner engine nacelles, and a large T-tail intended to keep the elevator clear of the propwash during take-off (the Twin Otter's empennage was a cruciform arrangement).

The Twin Otter incorporated "flaperons" that drooped the ailerons as part of the flaps, but these were not included in the Dash 7 due to weight and complexity. Instead, the ailerons were reduced in size to allow more flap area, and were augmented with two sets of roll spoilers, or spoilerons. The inboard roll spoilers operate at all speeds. while the outboard roll spoilers only operate at speeds less than 130 knots (150 mph; 240 km/h) indicated air speed to allow for more roll control at slower speeds. Upon touchdown, both the inboard and outboard roll spoilers extend in unison to aid in destroying lift created by the wing. Each wing also includes two ground spoilers which only extend on touchdown. Most of the trailing edge is spanned by a complex, double Fowler flap arrangement for high lift at low speed. During a typical STOL landing, flaps are selected to the 45° position, generating more lift and drag, thus allowing for steeper descents and lower approach speeds. Depending on weight, the VREF speed with flaps at 45° is between 70 and 85 knots (81 and 98 mph; 130 and 157 km/h). On touchdown, through "squat switches" in the main gear, the flaps automatically retract to the 25° position, thus reducing lift once on the runway and producing better braking performance. The flaps also retract to 25° when engine power is increased during a go-around procedure. The four-engine layout aids lift at low speeds due to the wide span of the propellers blowing air over the wing ("propwash"). When reverse thrust is selected on landing, the props reverse pitch, push air forward, and slow the aircraft very effectively along with the antiskid main wheel brakes. More importantly, if an engine fails, the asymmetric thrust is much less than on a twin-engine layout, thereby increasing safety and allowing for a lower minimum control speed with an engine inoperative.

Operational history

Development started in 1972 and the prototype first flew on March 27, 1975. Testing went smoothly, and the first delivery took place to Rocky Mountain Airways on February 3, 1978. The Dash 7 enabled Rocky Mountain Airways to operate scheduled passenger air service from Denver into the Avon STOLport in Colorado which was controlled by the airline. The Avon STOLport was located in a mountain valley in close proximity to the Vail, CO ski resort. Another close-in ski resort airfield served by Rocky Mountain Airways with Dash 7 flights from Denver was Steamboat Springs Airport in Colorado. With a relatively short runway length of 4,452 feet and an airfield elevation of 6,882 feet, the Dash 7 was well suited for operations from this airport located in the Rocky Mountains.