A Cessna 208B had taken off from Tulsa International Airport (Oklahoma, USA) for a post-maintenance test flight at 15:oo lt (local time). On board the aircraft, a pilot and a mechanic.
The aircraft wreckage (Source: news9.com © Unknown)
The flight preparation and take-off had been uneventful. Shortly after take-off, during his instrument scan the pilot observed the engine EGT (Exhaust Gas Temperature) at 640 ºC with the torque being low at 65%, but steady. As the aircraft continued to climb, both the EGT and the engine torque were observed to be decreasing. The pilot realised that the performance was decreasing at such a rate that he would not be able to make it back to Tulsa International Airport and elected to make an emergency landing at a nearby private airstrip. The engine performance deteriorated rapidly and even the private airstrip became unreachable. A forced landing in a nearby field became the only option. Shortly after touchdown, the aircraft collided with trees, resulting in the left wing being torn from the fuselage and the right wing folded backwards. There was no post-crash fire. The pilot sustained minor injuries in the forced landing, the mechanic was not injured. An investigation was launched by the National Transportation Safety Board with the initial on-scene inspection of the airplane being conducted under the supervision of a Federal Aviation Administration Inspector. A short overview of the main on-scene inspection findings from the NTSB report;
The high-pressure fuel lines on the engine were nearly empty
Four gallons of fuel were drained from the fuel reservoir tank
40 gallons of fuel remained in the left-wing tank
20 gallons of fuel remained in the right-wing tank
Fuel valves were found in the "ON" position
There were no pre-crash anomalies found with the fuel system
There were no signs of an uncontained engine failure or engine fire
The engine had run for 9.6 hours since being installed as a new engine
The aircraft wreckage (Source: baaa-acro.com © Rogers County Sheriff's Office)
The Honeywell TPE331-12JR engine was shipped to the manufacturer for detailed analysis. During this part of the investigation, the fuel pump was disassembled. There was wear and material missing from the gear teeth of the high-pressure drive gear. The driven gear exhibited little to no visible wear. A metallurgical examination of the gears revealed the damaged drive gear was made of a material similar to 300 series stainless steel instead of the specified M50 steel which is a harder material. The driven gear was made of a material similar to the specified M50 steel. The manufacturer subsequently inspected its stock of gears and issued notices to customers that had engines with fuel pumps installed with the same part number gear set as the one installed on the accident airplane. The manufacturer also issued a service information letter and service bulletins regarding the fuel pump gear set for engines used in civilian and military applications. As of the date of this report, the two remaining gears have not been located.
The National Transportation Safety Board concluded that the probable cause(s) of this accident to be:
The fuel pump gear manufacturer’s allowance of set-up gears made from a nonconforming material to be put in the production inventory system, the installation of a nonconforming gear in the accident airplane’s production fuel pump, and the gear’s failure, which resulted in a loss of fuel flow to the engine and the subsequent loss of engine power.
The NTSB Aviation Accident Final Report which served as a source for this blog, contains more details about the investigation and can be accessed by clicking on the .pdf file below;