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that impact forces could have sheared the bolt and that the pieces could have fallen free and could have been lost. In this contention it was surmised that the bolt could have sheared without causing any damage to the attachment components. The contention concluded that the recovered bolt was not the one installed in the assembly but rather it was one left in the tall compartment during the course of normal maintenance activity. It was stated that in the opinion of Associated Airmotive the radio transmission made by Captain Bagus, hereinbefore quoted, indicated without doubt that Captain Bagus had elevator control while using autopilot and that such control was not possible assuming the disconnected link assembly-clevis attachment. They therefore concluded that the failure must have existed forward of the autopilot servos.
In an effort to satisfy the aforestated contention, the longitudinal control system was reexamined in its entirety. In addition, the stress analysis data on the pertinent parts of the link assembly-clevis attachment were carefully reviewed and tests to failure were made on the attachment assembly. The tests were performed by the National Bureau of Standards at the request of the Civil Aeronautics Board.
The stress analysis data showed that the strength of the AN5-12 bolt was greater than the strength of several other components which comprise the link assembly-clevis attachment; this therefore showed other components in the assembly would fail first. As stated, all components of the attachment recovered from N 5140B were undamaged.
The tests to failure confirmed the stress analysis data. With the attachment assembled, compression loading caused the aluminum alloy push-pull tube to buckle. (See attachment 2-C.)[1] Under tension loading applied to the assembly, the rivets attaching the forked-end component to the forward end of the push-pull tube sheared and the unit pulled off. (See attachment 3-A.) At the end of these tests the AN5-12 bolt used was marked but otherwise undamaged. Another test was then made in which loading was applied to the Clevis unit and bearing end fitting with the bolt in place. The units were positioned in the test as they are installed in the aircraft. This test resulted in a failure of the shank of the clevis unit and a complete functional destruction of the bearing. (See attachment 3-B.) At the completion of the test the bolt was only slightly bowed.
In the course of the examination and reexamination of the other portions of the longitudinal control system, a fracture was found in the forward link assembly which is located below the cockpit area and should not be confused with the link assembly-clevis attachment hereinbefore described. In addition to the fracture, a short section of the aluminum alloy tube was missing. Because the failed surfaces under visual inspection exhibited certain characteristics which resembled a fatigue fracture, it was also examined by
- ↑ In the testing when failure was clearly shown by a drop in the measured loading, the loading was relieved to preserve the parts for further tests.
