Page:CAB Accident Report, Pan Am Flight 4.pdf/15

- 15 - (b) Probable Cause

The Board determines that the probable cause of this accident was the failure of the third stage turbine disk. This failure was caused by a transient loss of operating clearance between the third stage turbine disk and the third stage inner sealing ring. This loss of clearance resulted from a combination of improper turbine rotor positioning during engine assembly, the use of serviceable worn parts, and an operating clearance which was less than predicted in design analysis.

Corrective Action

Following this accident PAA personnel testified that the inspection procedures had been changed to require duplicate measuring and calculations of spacer sizes and clearances by the mechanic and the inspector, working separately. Additionally the PAA maintenance manual and work card was changed to reflect this requirement.

A Pratt and Whitney witness testified that a revision in the procedures for the assembly of their engines was made to preclude the possibility of future disk failures.

BY THE CIVIL AERONAUTICS BOARD: