Trans-Canada Air Lines Flight 304

Trans-Canada Airlines CF-TGR

Occurrence
Date	July 9, 1956
Summary	Mechanical failure
Site	over Flat Rock, Michigan, USA
Aircraft type	Vickers Viscount 700
Operator	Trans-Canada Airlines
Registration	CF-TGRdisaster
Passengers	31
Crew	4
Fatalities	1
Injuries	5
Survivors	34

CF-TGR was a Vickers Viscount 700 aircraft owned by Trans-Canada Airlines. On July 9, 1956 the No. 4 propeller of the aircraft tore loose from its engine over Flat Rock, Michigan during a flight from Chicago, Illinois to Toronto, Ontario and Montreal, Quebec; one blade of the propeller sliced through the passenger section of the cabin, killing one passenger and injuring four passengers and one flight attendant. The aircraft diverted to Windsor, Ontario and the pilots carried out an emergency landing. The accident was the first to involve a Vickers Viscount aircraft and was the first incidence of a propeller loss on a turbo-prop aircraft.

Trans-Canada Airlines was the first airline to accept delivery of the Viscount in North America. Unlike the piston engined aircraft (such as the Douglas DC-6 and the Lockheed Constellation) commonly flown by North American airlines, the Viscount was a (for the time) 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 that propeller loss would be unlikely.

On the morning of July 9, while CF-TGR 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. 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 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, in their haste to land, the pilots had allowed the aircraft's airspeed to increase to close to the maximum allowable (often called the V^NE, or the Velocity Not to Exceed) during the emergency descent. 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.

Trans-Canada Airlines did not use flight numbers as of the date of this accident.

Information from the Aero Transport Data Bank[1] shows that CF-TGR was subsequently registered in the United States as N911H and in France as F-BNAX before being scrapped at Orly Airport by Air Inter Europe, then a subsidiary of Air France, at some point in or after 1965.

External links and references

• Plane Crash: The Mysteries of Major Air Disasters and How They Were Solved, by Clayton Knight and K. S. Knight; Greenberg Press, 1958. No ISBN available.
• Air Disaster, Vol. 4: The Propeller Era, by MacArthur Job, Aerospace Publications Pty. Ltd. (Australia), 2001 ISBN 1-875-67148-X
• Details of accident from airdisaster.com database