Incident Overview
Shortly after departure, the pilot requested clearance to return for an immediate landing. A witness observed the aircraft passing overhead and heard a metallic noise. The pilot executed a steep right bank in an apparent attempt to land on Runway 29L, but the aircraft struck the ground between the parallel runways.
Engine Examination
A post-accident examination of the engine disclosed two finger-tight cylinder base nuts, as well as undertorqued base nuts and crankcase through-nuts. During the teardown, the crankshaft was found to have failed due to fatigue between the No. 3 and No. 4 connecting rod journals. Metallurgical analysis identified high localized stress-induced fatigue cracks as the origin of the failure. The engine had accumulated 266 hours of flight time since its last major overhaul on January 11, 1991.
Aircraft Modifications
The aircraft had been modified under two Supplemental Type Certificates (STCs): one for the installation of a Rayjay turbocharger and another for a Blackmac propeller. The holder of the propeller STC reported that the company would not have installed that propeller on a turbocharged engine, as certification vibratory tests had only been conducted on normally aspirated engines.
Propeller Maintenance History
The propeller was installed on January 17, 1991. It was removed on March 19, 1991, for static balancing and to "re-index prop #1 blade 180 degrees." On March 28, 1991, it was removed again for a blade angle check. A dynamic balance was performed on May 23, 1991.
Conclusion
The accident sequence involved an engine failure shortly after takeoff, attributed to a fatigued crankshaft. Contributing maintenance issues included undertorqued nuts. The modifications and propeller maintenance history were noted but not directly linked to the failure.