What happened
On November 17, 2003, a Robin DR 400-120 aircraft, registration F-GFXO, experienced a sudden loss of power during the initial climb phase of flight. The aircraft was operating as a club aircraft near Flers, France. Following the engine failure, the pilot executed a right-hand turn and performed an emergency landing in an upward-sloping field. While the landing resulted in no injuries to the pilot, the engine sustained significant damage.
Prior to the flight, a student pilot had reported a drop in engine oil pressure after a solo flight. In response, a club mechanic tightened the engine hoses and cleaned the engine. A ground engine run was subsequently conducted in the presence of an instructor, which appeared satisfactory, with no oil leaks detected during visual inspections. To confirm the engine's operational status, an instructor took off for a test flight, during which the power loss occurred.
The investigation
Investigators examined the engine, specifically focusing on the Lycoming O 235 L 2A powerplant. The inspection revealed a significant crack on the number two cylinder. The investigation determined that the crack had initially propagated through fatigue during engine operating cycles and later progressed while the engine was static.
The origin of the crack was traced to a corroded area located near an elastomeric seal placed on the engine. This seal was positioned at the junction of the cooling cowlings and the cylinder fins, intended to channel airflow and reduce cowling vibration. However, the design of this seal caused moisture to stagnate at the base of the cylinder, accelerating the corrosion process.
Findings
- The primary cause of the engine failure was an inadequate maintenance action involving the installation of an unauthorized elastomeric seal.
- The seal, which was not recommended by the engine manufacturer, trapped moisture against the cylinder base.
- This trapped moisture led to advanced corrosion on the cylinders, eventually resulting in the structural failure of the number two cylinder.
- The installation of the seal was an unapproved modification intended to improve cooling and dampen vibrations.