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The only power plant malfunctions that occurred before the accident were the separation of the throttle linkage, and a malfunction in the propeller feathering system.
The last recorded maintenance performed on the throttle linkage was at the Zantop facility in Ontario, California, during a No. 1 engine check. At that time the throttle linkage was adjusted to correct a lack of cushion on the left throttle which was discovered during the runup after inspection. This discrepancy was corrected by an unlicensed mechanic and as far as can be determined his work was not inspected. The jam nut which locks the throttle rod to the rod end at the throttle arm clevis was not properly secured. This is substantiated by the lack of burnishing of the face of the nut where it normally bears on the clevis shoulder. The control rod thread engagement inside the rod end was marginal, at best, barely reaching the witness hole. The extreme amount of wear that occurred to this rod end can be accounted for, in a large part, by the improper installation of the jam nut and by marginal mating between the threads of the rod and the rod end. This in turn allows the throttle rod to become worn to such a degree that the threads could no longer hold in the rod end and the throttle linkage separated due to normal engine Vibrations and throttle movements.
Ground tests showed that a throttle linkage disconnect, with the throttle near or forward of a climb power position, resulted in the throttle valve going to the full open position. The same tests made with the throttle aft of a climb power setting resulted in the throttle valve going to approximately 26 inches of mercury (“Hg). In either event there would be no immediate urgency to shut the engine down and feather the propeller. However, in view of the relatively short distance to McChord AFB and the single engine capabilities of the aircraft, the pilot's decision to shut down the engine and feather the propeller was in accordance with acceptable pilot procedure.
The copilot stated that, when his attention was drawn to the manifold pressure gauge by the pilot, he observed a reading of approximately 26 “Hg. No unusual noise or instrument readings were noted which would indicate an internal engine failure, nor were there any indications of engine fire. Therefore, the left engine was capable of delivering power without internal damage, prior to the initial attempt to feather and the power output of the left engine should have been controllable, in part, by proper use of the propeller control.
Examination of the feathering system indicates that it was capable of feathering the propeller. However, the high pressure setting of the propeller governor pressure cutout switch could have adversely affected the proper sequence of events during the feathering cycle. If this switch fails to open upon completion of a feathering cycle, the feathering pump motor will continue to operate and the propeller will go into an unfeathering cycle. The function of the cutout switch is to release the holding coil in the cockpit feathering switch. If this coil does not release, the feathering button will stay depressed and keep the feathering motor operating continuously throughout the feathering and unfeathering cycle. When the propeller reaches the low pitch stop the pressure cutout switch will open, the feather-unfeather cycle will terminate and the propeller will resume a constant speed operation at the r.p.m. selected by the propeller control in the cockpit. As the propeller is forced through the unfeathering cycle and reaches the low pitch stop, the propeller will overspeed until it seeks out and stabilizes at the r.p.m. selected by the propeller control. The propeller will stay at this input until some action is taken through either the feathering switch or the propeller control to change the selected pitch angle. The number of times this cycle was repeated is not known, however, the feather pump motor[1] was damaged by high
- ↑ An intermittent duty rated electric motor.
