Event Description
Approximately 422 flight hours before the incident flight, a Smith Aerostar 601P had been modified in accordance with Machen STC SA980NW. During the flight, while leveling off at 16,500 feet above mean sea level, the pilot observed blistering and discoloration on the number one engine nacelle. In response, the pilot shut down and feathered the number one engine, then returned to the departure airport and landed without further incident.
Post-Landing Examination
After landing, an examination of the aircraft revealed that an in-flight fire had occurred, resulting in minor damage to the engine accessory section. Investigators found two exhaust clamps loose at the exhaust stack waste gate connection. Hot gases escaping from the loose connection had impinged upon the turbocharger oil supply hose (Machen part number CAA04S160). This hose was wire reinforced and fitted with a fire sleeve, but it was located in close proximity to the exhaust stack. There was evidence that the hose had ruptured due to prolonged exposure to heat.
Fire Origin
The escaping oil from the ruptured hose came into contact with the hot exhaust stack and turbocharger, which resulted in a fire. The fire was contained to the engine accessory section and did not spread further. The exact duration of the heat exposure leading to the hose rupture was not specified, but the configuration placed the hose near a heat source, contributing to the failure.