Page:CAB Accident Report, Pan Am Flight 4.pdf/12

- 12 - spacer dimension of 0.258 inch measured after the accident. This spacer size is on the low side when compared with the computed spacer requirements taken after the accident and calculations performed by the manufacturer.

The cause for this apparent mispositioning lies in the assembly of the engine during overhaul. Certain of the overhaul shop procedures could have resulted in the mispositioning of the turbine rotor although the individual procedure causing it cannot be accurately pinpointed. The shop practices in question involved reinstalling the turbine rotor without position measurements and installing the turbine inner sealing ring without depth measurements. Furthermore, the use of a rachet wrench to position the outer turbine air seal can crimp the damping spring and cause improper seating of the seal and nozzle guide vanes. The inspector signoff of the work sheets for the low pressure turbine was improper and precluded any assurance that the engine was properly assembled in that section.

In addition to the loss of clearance due to the apparent mispositioning of the turbine rotor, some loss of clearance may also have been introduced by adherence to the assembly procedure specified in the approved engine manufacturer's overhaul manual. To insure the correct positioning of the inner sealing ring a depth measurement was taken from the turbine nozzle case rear flange to the inner sealing ring. In taking this measurement, the third stage vanes and inner sealing ring were pulled rearward by "hand force" is not always adequate to insure seating of the most adverse combination of new and worn guide vanes. Lack of seating could result in a measurement which was not indicative of the true position of the inner ring.

Also, laboratory deflection tests performed after the accident showed that the deflection of the inner sealing ring caused by the use of vanes worn 0.007 inch or more was greater than previous analytical studies predicted by the engine manufacturer.

Another factor determined by tests conducted after the accident was that the actual operational clearance between these parts was less than that predicted analytically for a properly assembled engine.

The exact cause of the loss of operating clearance could not be defined, but apparently the cumulative effects of the normal actual clearance which was less than that predicted in the design analysis, worn vanes and the resultant added deflection of the inner sealing ring, and a forward positioned turbine rotor associated with assembly procedures resulted in the loss of sufficient transient operating clearance.

The disintegration of the third stage turbine disk cut the engine in two pieces and threw turbine chunks into the wing inboard of the engine pylon. The two engine sections, each supported by only one mount on the strut, began to oscillate and separated from the wing in approximately four seconds. The strut failures were caused by the oscillation, possibly coupled with mechanical damage from flying engine parts. The engine fuel line pulled from the strut closure rib when the engine separated from the wing. Fuel was pumped through this line for an estimated 99 seconds at a rate of approximately 30,000 pounds per hour, until the fuel valve was shut off by the action of either the first officer or