Page:CAB Accident Report, Pan Am Flight 4.pdf/9

- 9 - 0.020/-0.025 inch deep. Radial fractures occurred at each end of both the pieces examined. There were numerous cracks in the groove that appeared to be typical of brittle, intergranular cracks that occur while metal is at a high temperature. The surfaces of the groove and a portion of the circumferential fractures, about 0.1 inch, adjacent to the groove were coated with a black, high temperature oxide scale. Distinct temper colors on circumferential fracture started at the edge of the black scale and extended to the backface of the disk. Temper colors on the radial fractures extended 0.6-0.9 inch from the circumferential fracture. These temper colors indicate that an abnormal temperature gradient existed at the time the fractures occurred. The maximum temperature was at the groove and the gradient extended through the disk to the backface and to about one inch radially from the groove.

The circumferential fracture surface, near the groove, showed evidence of a brittle intergranular type separation, while the fracture adjacent to the backface of the disk showed evidence of a tensile shear failure. There was no evidence of fatigue or corrosion.

Examination of the microstructure of the metal on a section through the groove, and the circumferential fracture, revealed evidence of a heat affected zone in the metal adjacent to the groove. The temperature in this zone had been high enough to change the etching characteristics and reduce the hardness of the metal. The evidence indicated that the metal had been heated to 1,700°F or higher. Hardness testing of the metal 0.1 inch from the groove was found to be around 149-184 on the Brinnell scale. Hardness testing of disk material remote from the groove area gave Brinnell readings of 336-362 compared to the specification of 302-388 Brinnell.

The engine, less exhaust case, was returned to the manufacturer where it was disassembled and examined under Board supervision. Special attention was given to the operating position of the low pressure turbine assembly. The wear pattern was measured on the low pressure compressor turbine drive shaft and found to be 3.233-3.257 inches long. Measurements of similar drive shafts in the Pan American jet overhaul base ranged from 3.120-3.212 inches with an average of 3.159 inches. Comparative measurements of the wear pattern on 3 drive shafts in Pratt and Whitney production engines ranged from 3.151-3.170 and averaged 3.162 inches.

The three knife edges on the second stage turbine seal left only two tracks on the shoulder of the low pressure compressor turbine drive shaft beginning 0.350 inch aft of the front shoulder. A survey of similar drive shafts revealed three tracts on the shoulder. Tracks on the No. 4-1/2 seal liner of the accident engine were compared to a production engine and estimated to be 0.250-0.040 inch forward of similar marks on the production engine. The third and fourth stage inner sealing ring track marks were comparable to similar marks on other engines.

A review of the carrier's jet engine buildup procedures for this engine was conducted, and depositions taken from the personnel who assembled the engine. The low pressure turbine shaft was installed twice in the course of determining the correct size shaft positioning spacer. This spacer determines the actual