The engine and propeller were dismantled under the supervision of the Board's powerplant specialist. At the conclusion of the tear-down it was determined that the master rod had failed at the crankshaft end. This master rod was turned over to the Metallurgical Division of the National Bureau of Standards for a detailed analysis.
The metallurgical examination of the failed master rod indicated that the failure was caused by a fatigue fracture which originated at the inner surface of the bore of the master rod behind the steel—backed bronze bearing sleeve. Fitted and scored areas on the surface of the bearing sleeve contained chromium flakes deposited from the rod. The nucleus of the fatigue fracture was centered in a pit in the bore or bearing surface of the master rod, the fracture progressing under the action of fluctuating stresses until all but a small portion of the rod section had been penetrated. This part then failed suddenly, completing the fracture. The sequence of events leading to the pitted condition of the chromium—plated surface on the master rod and the condition of the mating surface of the steel back of the bearing sleeve is uncertain, but the fact that the fatigue fracture originated at point of stress concentration in a pit in the chromium plating, indicated that the pit was present at the time the fatigue fracture started.
It was evident that the damage to the engine was the result of the fatigue failure of the master rod. The failure apparently stopped the crankshaft so abruptly that the inertia of the propeller twisted off the nose section. The fuel characteristics were within proper limits and the condition of the sump and oil tank samples was not abnormal.
Disassembly and examination of the propeller by the Bureau of Standards revealed that the propeller was in normal operating condition up to the time of the powerplant failure, all damage to it being incurred by impact with the ground. This propeller did not contribute in any way to the powerplant failure.
A review of the air carrier maintenance and service records indicates that the failure of the master rod on this engine was an isolated case. Precautionary measures have been taken by TWA by altering their overhaul procedure on the type engine involved, to include complete disassembly of the master rod at each overhaul.
PROBABLE CAUSE: Failure of the master rod bearing.
BY THE BOARD:
/s/ Darwin Charles Brown
Secretary
