What happened
During the initial climb phase following departure, the pilot notified air traffic control of an engine overboost issue and requested permission to return to the airport. Due to prevailing instrument meteorological conditions, the aircraft was cleared to perform an ILS approach for runway 26. An observer on the ground noted the aircraft descending through low cloud cover in a level attitude before it suddenly pitched into a near-vertical descent. The aircraft subsequently struck a sign at a shopping center.
Findings
Investigation of the wreckage revealed significant mechanical failures within the engine's turbocharging system. The left turbocharger wastegate was positioned in the open state, while the right turbocharger wastegate was found in the closed position, creating an overboost condition. Specifically, the right turbocharger's butterfly valve was found to be free to rotate on its shaft because the pin was missing and the valve itself had suffered severe erosion. This malfunction resulted in a hole being burned through the exhaust valve of the number 1 cylinder on the right engine.
Both engines and their associated turbochargers were identified as original equipment that had not undergone an overhaul in 21 years. Additionally, both propellers were operating in a low pitch, high RPM range at the time of the event. Post-accident toxicology results indicated the presence of 0.564 mcg/ml of sertraline, an antidepressant, in the pilot's system, a medication that is not authorized for use during flight operations.