Trans-Canada Air Lines was the first North American airline to accept delivery of the Viscount. Unlike the piston engined aircraft commonly flown by North American airlines, the Viscount was a quiet aircraft whose engines produced a minimum of vibration. Since the Viscount's Rolls-Royce Dart engines ran so much more smoothly than piston engines did, engineers at Vickers believed propeller loss would be unlikely.
Accident details
On the morning of July 9, while the aircraft was cruising at flight level 190 over the town of Flat Rock, Michigan, the No. 4 engine on the aircraft experienced a drop in RPM. The engine then sped to 14,000 RPM, significantly above the engine's normal cruise figure. As pilots attempted to feather the propeller, the engine sped up even more, the aircraft's indicated airspeed decreased, and the pilots declared an emergency and began an immediate emergency descent, depressurising the cabin as they did so. Less than a minute later and as the aircraft descended through 9,000 feet, the propeller attached to the No. 4 engine broke loose. One of the four propeller blades penetrated the passenger cabin and cut through the first row of seats, immediately killing a young woman travelling with her two small children. The blade also injured a family of three sitting across the aisle from the victim and a flight attendant who had been standing at the front of the cabin. The children of the victim were not injured. The pilots eventually landed the aircraft at Windsor, Ontario; they learned only after landing that there had been casualties in the passenger cabin. One small section of the blade remained in the cabin, while the main section of the blade and the other three blades from the propeller were found on the ground in the vicinity of Flat Rock. Canadian accident investigators found that a bevel gear in the oil pump drive had failed, shutting off lubrication to the propeller. This caused the propeller to decouple from the engine, allowing it to windmill at high speed. Moreover, during the emergency descent, the pilots had allowed the aircraft's airspeed to increase too close to the maximum allowable. This put significant strain on the windmilling propeller and in all probability caused it to fail in flight. The possibility of the bevel gear drive failing causing the propeller to windmill had not been foreseen by Vickers engineers, and there was therefore no mention of it in the training or operations manual. The accident forced aircraft designers and engineers to rethink their assumption that turbo-prop aircraft would be less likely to suffer propeller loss.
Aircraft
The aircraft was a four-engined Vickers Viscount that first flew in the United Kingdom in 1955. It was delivered new to Trans-Canada Air Lines on 21 June 1955. It was sold to a private American operator in January 1964 before being sold to Air Inter in France in June 1965. It was withdrawn from use at Orly Airport in October 1974 before being broken up and scrapped during 1975.
