What happened
On August 11, 2000, at approximately 12:30 PM, a Glaser-Dirks DG-400 motor glider departed from the Jesenwang airfield (EDMJ) for a private recreational flight. The initial climb proceeded normally until the pilot began a left turn for the departure pattern. At that moment, the pilot experienced intense vibrations, which were suspected to originate from the engine.
In response to the instability, the pilot reduced the engine to idle and initiated a right-hand turn. Realizing that returning to the airfield was not safely possible, the pilot opted for an emergency landing in a nearby cornfield approximately 2.4 km northwest of the aerodrome. During the landing, the right wing dipped into the 80 cm high crop, causing the aircraft to spin on its axis. There were no injuries resulting from the incident, though the aircraft sustained light damage and some damage occurred to the field.
The investigation
The BFU examined the aircraft and the engine components following the accident. The investigation focused on the engine's auxiliary drive system. Physical inspection revealed that the rear mounting plate of the propeller boom had broken away from the engine block. Additionally, the trailing edges of the two-bladed wooden propeller were splintered.
Microscopic analysis of the broken mounting plate fragments, conducted via scanning electron microscopy (SEM), identified the failure as fatigue cracking. The cracks originated at the surface, specifically near the circular impressions left by the washers. The investigation also reviewed the pilot's experience, the aircraft's maintenance history, and the engine's operational hours since its last overhaul.
Findings
- The primary cause of the engine failure was fatigue cracking within the propeller boom mounting plate, which progressed until the component fractured.
- The fatigue was driven by bending moments caused by propeller thrust.
- High local stress concentrations were caused by a stiffness change at the edge of the washer and the preload of the connection bolt, likely exacerbated by misalignment during assembly.
- The design of the boom assembly as a statically indeterminate, rigid system allowed vibrations and assembly-induced forces to impact all components.
- The pre-flight inspection procedures provided in the flight manual were insufficient to detect the impending failure, as the cracks were not easily visible near the washer contours.
Safety action
- The manufacturer of the DG-400 should implement improved connection elements that provide a more secure and tolerant interface between the propeller boom and the engine to better withstand vibrations and assembly errors.
- It is recommended that all DG-400 motor gliders equipped with auxiliary drives be retrofitted with these improved components.