What happened
On August 23, 2009, an experimental, amateur-built Vans RV-9A, registration N362TS, was involved in a forced landing near Marianna Municipal Airport (MAI) in Florida. The flight, which began from Tri-County Airport (1J0) in Bonifay, Florida, was being conducted under visual meteorological conditions.
While cruising at approximately 3,500 feet, the pilot heard a loud bang. Although the engine continued to operate normally and the pilot could control engine RPM, the propeller stopped producing thrust, and the propeller RPM could no longer be controlled. The pilot initially attempted to reach MAI, reporting engine problems on the common traffic advisory frequency. However, approximately five miles west of the airport, the pilot determined the airport was unreachable and prepared for an off-airport landing.
The aircraft struck trees approximately 60 feet above the ground and came to rest in a swamp about three miles west of the airport. The pilot sustained one serious injury and was evacuated from the wreckage via a medical helicopter.
The investigation
Investigators examined the aircraft's engine and propeller speed reduction unit (PSRU) gearbox. The engine was a modified Subaru automobile engine equipped with a non-certificated PSRU gearbox to drive the propeller at a lower speed.
Post-accident disassembly of the gearbox revealed that a gear on the common shaft was completely devoid of teeth. The gearbox housing contained a large amount of metal flakes ranging from 1/16 to 1 inch in size. The investigation confirmed the unit was a "Gen-I" version of the Eggenfell to gearbox.
GPS data from a handheld unit showed that after the loss of thrust, the aircraft remained airborne for approximately seven minutes, traversing 10.2 miles with a steady descent rate of about 620 feet per minute. Analysis of the terrain revealed that several suitable landing sites, including large fields and straight segments of highway, were overflown during the descent.
Findings
- The propeller speed reduction gearbox failed catastrophically, preventing the engine from transmitting power to the propeller.
- The pilot was aware of a manufacturer mandate to replace the Gen-I gearbox with the Gen-III version but chose not to do so due to the cost of the upgrade and the low number of hours currently in service (less than 200 hours).
- The manufacturer had previously notified owners that the Gen-III upgrade was mandatory and had expressed safety concerns regarding older models, but did not provide owners with specific failure rate data or technical substantiation to justify the mandate.
- The pilot overflew multiple suitable off-field landing sites while attempting to reach the distant airport.