What happened
On Tuesday, May 22, 2002, at 16:50, a Robin DR 221, registration F-BOZU, was performing a dual instruction flight at the Toussus-le-Noble aerodrome. During the initial climb following a takeoff from runway 25R, the Textron Lycoming O-2-35-C2A engine ceased operation at an altitude of approximately 200 feet.
The instructor took control of the aircraft and performed an emergency landing in a field located within the aerodrome perimeter, aligned with the runway. There were no injuries to the instructor or the student pilot, and the aircraft sustained no damage during the landing.
The investigation
The engine was removed and sent to the Propulsion Test Center in Saclay for technical examination. Investigators discovered that the engine had seized upon ground inspection. Detailed analysis of the crankshaft revealed a fatigue fracture on the No. 4 journal, located between the central bearing journal and the No. 3 connecting rod big end. The fatigue cracking had initiated at three adjacent sites near the rear fillet of the journal.
Further examination of the engine components revealed that the central bearing inserts had rotated, effectively erasing the positioning tabs within the half-crankcase bores. This movement caused the inserts to shift toward the rear fillet of the journal. Investigators also noted significant fretting corrosion on the mating surfaces of the central bearing and the adjacent camshaft, along with crushed seals near the assembly tie rods.
Findings
The investigation established that the engine failure was caused by the insufficient tightening or loss of clamping force of the two half-crankcase halves. This lack of structural integrity allowed micro-movements between the crankcase halves, leading to the following chain of events:
- The loss of clamping force allowed the central bearing inserts to rotate and shift axially.
- This movement resulted in the displacement of the inserts, which likely obstructed the lubrication orifice.
- The resulting lubrication disturbance led to the fatigue fracture of the crankshaft.
While the investigation identified the mechanical cause, it could not determine the specific origin of the loss of tightness in the tie rods, nor did the engine's maintenance history reveal any specific event that would explain why the clamping force was compromised.