Page:CAB Accident Report, Pan Am Flight 4.pdf/11

- 11 - difference in the thermal expansion rates of the rotating assembly and the outer turbine cases which support the inner sealing ring occurs 1-2 minutes after application of takeoff power.

The disk failure resulted in an explosive failure of the No. 4 engine and its separation from the wing due to high vibration and out of balance oscillation of the rotating parts of the engine. The right outer wing received so much damage to the lower load bearing skin and structure that the capability of the outer wing panel separated from the wing. Fuel from the engine fuel line was then being pumped directly after the engine separated and resulted in an explosive separation of a portion of the lower wing skin. The fire was sustained by the continued supply of fuel through the engine fuel line until the flight engineer or the first officer shut off the main fuel supply either by activating the fuel shutoff valve to the closed position or actuating the fire selector handle.

There was no evidence of foreign object damage to the compressor before the engine failed. The damage incurred by the compressor section of the engine was caused by impact following separation from the aircraft or was the result of turbine disintegration. There is no evidence in the combustion chambers, or on the turbine vanes or blades, that would reflect a critical overtemperature operation of the engine.

The notch on the face of the third stage turbine disk was caused by the third stage turbine inner sealing ring, which rubbed the disk during takeoff due to insufficient clearance. The minimum clearance occurs during takeoff thermal transients approximately 1-2 minutes after applying takeoff thrust.

The small depth of the notch in the turbine disk indicates that the interference was small and did not occur continuously through the 39 hours of engine operation prior to engine failure.

The third stage turbine inner sealing ring was rubbing the disk immediately prior to the disk failure. The .020 inch notch and the .125 inch annealed zone were not sufficient to cause the disk to fail at normal operating takeoff metal temperatures. The metal temperature through the web of the disk adjacent to the annealed zone had to be higher than the normal operating temperatures to allow the elevated temperature tensile shear failure that occurred. Therefore, the sealing ring was rubbing and generating heat through the web of the disk at the time of the disk failure. The disk had been manufactured to the proper specifications, dimensions, and hardness. There was no evidence of fatigue or corrosion in the fracture surfaces. Further, there was no evidence of an overspeed. The failure of the disk was associated directly with the notch, the adjacent annealed zone, and the increased temperature associated with the rubbing of the sealing ring against the disk web, which caused the disk to rupture. The entire outer web-rim section did not separate from the disk because the notch did not provide a continuous plane of maximum stress. As sections of the web-rim separated the induced bending stresses shifted the propagating crack out of the notch toward the rim.

The low pressure turbine was running too far forward based on the wear evidence observed on the spline, the second seal track, and the positioning