Page:CAB Accident Report, Pan Am Flight 4.pdf/10

- 10 - position of the low turbine shaft assembly. Measurements and calculations were performed to determine the correct size of this spacer. The calculations for determining the turbine shaft positioning spacer were not recorded, nor was the size of the installed spacer. The mechanic who did this work testified that the spacer was within tolerance but on the low side. Measurement after the accident showed this spacer was 0.258. The blueprint specifications for the spacer was 0.256 to 0.366 inch.

The following shift removed the turbine rotor to adjust the position of the second stage turbine outer air seal, and then reinstalled the same rotor. The second stage turbine outer air seal was installed using a pusher tool with ratchet adapters rather then knurled nuts. The clearance from the rear of the second stage turbine disk to the rear flange of the turbine nozzle case was not measured and the third stage nozzle vanes were assembled. The shop work card used for this operation stated in item 21C "Repeat the position measurement, if the rotor was removed and reinstalled." This was not done by the second shift.

A measurement was taken from the turbine nozzle case rear flange to the inner sealing ring to establish that adequate clearance existed between the third stage nozzle vanes/inner sealing ring assembly and the third stage disk. It was performed after the inner sealing ring was pulled rearward by use of "hand force" in accordance with the approved overhaul manual.

The inspector on duty during the reinstallation of the turbine rotor testified that he signed off work he had not inspected. He also lined-out a mechanic's initials placed on the work card to indicate that the work had been accomplished on the previous shift. He later voided this line-out when the work was reaccomplished by a different mechanic.

The engine was run on a test stand without any reported discrepancies and was installed on N761PA two days later, operating 39.21 hours without a reported discrepancy until the accident occurred.

Laboratory tests of fuel samples taken from the six remaining fuel tanks on the aircraft revealed no significant deviation from the specification established for a Jet A turbine engine fuel. It was estimated that the fuel temperature in the tanks of N761PA at the time of the accident was between 70° and 80°F. The flammability limit of Jet A fuel was reported by the FAA to be from 90-170°F. This figure does not take into account variations to the flammability limits of fuel in a tank due to volume, size, shape, agitation, and other factors that affect a fuel tank vapor space when the aircraft is in flight.

2.1

The number three turbine disk in the No. 4 engine of N761PA failed due to a localized reduction in its cross-sectional area and overheated conditions due to rubbing between the turbine disk and the third stage turbine inner sealing ring immediately forward of the disk. This rubbing was the result of a transient loss of clearance between these parts on takeoff. The maximum