Page:CAB Aircaft Accident Report, Trans-Canada Air Lines Flight 304.pdf/6

 shear pattern on the threads of the hub bore and on the blade retention nuts. All blades were intact except the previously reported tip portion of No. 2 which was found inside the fuselage. No irregularities of the propeller were evident except for impact damage.

The propeller control unit was functionally tested, disassembled, and examined. All was normal except that the pressure switch setting was 345 p. s. i. The specified setting is 460 plus or minus 20 p. s. i.

Weather conditions in the area of the accident were reported as ceiling estimated 3,500 feet, broken, 8,000 feet, overcast; visibility in excess of 15 miles; very light rain showers. The crew reported that although cruise flight was conducted above a cloud layer, breaks in the clouds permitted the entire descent to be made with visual reference to the ground.

It was not possible to determine whether the momentary drop of 200 to 300 r. p. m. in No. 1. engine had any connection with events that followed. The initial overspeed of No. 4 engine to 13,900 or 14,000 r. p. m. undoubtedly occurred when the normally fixed bushing turned and failure of the driven bevel gear followed to the extent that rotation of the bevel box drive was completely stopped. At this stage of the engine difficulty the propeller could have been feathered.

The fuel pump, oil pump, and propeller control unit are driven by the bevel box drive. Following failure of the