What happened
On August 6, 2010, a PZL M-18 aircraft, registration N70461, was performing an agricultural application flight near Meridian, California. Shortly after departing runway 16 at Sanborn Airport (38CN), the pilot heard a loud crack and experienced a sudden deceleration as the engine stopped developing power.
To avoid a levee, the pilot executed a forced landing in a water-filled rice paddy located south of the airstrip. Upon touchdown, the aircraft nosed over onto its back. The pilot sustained one minor injury, and the aircraft was substantially damaged. The flight was being operated by Martin's Dusters under VMC weather conditions.
The investigation
Investigators examined the engine, a Lycoming T53-L-7A turboshaft, which had been installed in the aircraft via an FAA field approval approximately 18 years prior. The investigation revealed that the engine was a military-design variant, and maintenance guidance for its "hot end" was primarily available through U.S. military documentation.
Detailed examination of the engine at a Honeywell facility found that all power turbine blades were separated from the root platform or severely damaged. Material analysis indicated that the first-stage turbine blades failed due to short-term exposure to temperatures exceeding 2,000 degrees Fahrenheit. The investigation also found that all fuel vaporizers in the combustor exceeded allowable damage limits and showed signs of long-term thermal damage. Additionally, maintenance records showed that the specified 600-hour inspection interval for the engine's hot section had been exceeded at least twice in the preceding 14 years, and the engine had exceeded its overhaul interval by approximately 300 hours at the time of the accident.
Findings
- The engine's fuel vaporizers had undergone long-term deterioration.
- The damaged vaporizers altered the combustor exit temperature profiles.
- These altered temperature profiles caused the failure of the first-stage turbine blades.
- The failure of the turbine blades resulted in a complete loss of engine power.
- The engine's hot section inspection intervals had been exceeded.
- The engine's overhaul interval had been exceeded.