What happened
On January 16, 1971, a student pilot initiated a solo VFR training flight from the Hausen a.A. airfield in a Siat 223 Flamingo, registration HB-EVM. Shortly after takeoff, at an altitude of approximately 50 meters, the engine began to run unevenly with a noticeable loss of power. Although the pilot managed to temporarily stabilize the engine by applying full power, the malfunction recurred during the departure turn at about 100 meters, accompanied by severe vibrations.
Recognizing the danger, the pilot attempted to maintain altitude to reach the airfield, but the engine failure persisted. This forced an emergency landing in a snow-covered field northwest of Kappel a.A. The aircraft impacted the soft ground at a speed of approximately 50 km/h, coming to a stop without flipping or catching fire. While the pilot escaped uninjured, the aircraft sustained heavy damage.
The investigation
The investigation focused on the mechanical failure of the Lycoming IO-360-CIB engine. Investigators examined the engine on a test bench and determined that the primary cause of the power loss was the displacement of a rocker arm axis cap. This allowed the rocker arm to shift, causing the intake valve to become stuck in the open position. This mechanical blockage created compression back-pressure, which interfered with the fuel metering valve and throttled the fuel supply.
Investigators also reviewed recent maintenance history, noting that all rocker arms had been removed and reinstalled approximately 15 flight hours prior to the accident following a manufacturer service bulletin. While no significant oil leaks were noted prior to the flight, the inspection of the engine components revealed critical errors in the recent maintenance work.
Findings
- The engine failure was triggered by the loss of a rocker arm axis cap.
- Improperly performed maintenance work contributed to the accident; specifically, the securing washers for the mounting nuts were old and reused, and the nuts were not tightened to the required torque of 125 in. lbs, measuring only between 25 and 100 in. lbs.
- The pilot's decision to execute an emergency landing was appropriate given the circumstances.
- The snow-covered terrain, which was approximately 10 cm deep, was not visually distinguishable from the air, complicating the landing approach.