What happened
On August 14, 2010, a DHC-1 Chipmunk, registration LN-DHC, was conducting a private flight near Kjeller airport. Shortly after departing runway 30 and climbing to approximately 2,000 feet, the pilot noticed the engine performance degrading, characterized by rough running and decreasing RPM, similar to carburetor icing. Despite attempts to use carburetor heat, the condition persisted. The passenger observed smoke emanating from beneath the rear instrument panel, prompting the pilot to deploy a fire extinguisher toward the rear of the cockpit.
As the pilot attempted to return to Kjeller, the engine performance worsened. While flying over dense forest, the pilot increased throttle to reach a suitable landing site in the Gansdalen area. At roughly 1,600 feet, a loud bang and mechanical rattling occurred, followed by a total loss of engine power. The pilot declared an emergency on the Kjeller frequency, notifying controllers of the engine seizure and the intention to perform an emergency landing near Kukollen and Øyeren.
The pilot selected a large grain field with no visible obstacles. After a wide approach to ensure safety, the pilot deployed full flaps and performed a tail-low landing into rising terrain. The aircraft came to a stop after rolling approximately 80 meters. There were no fatalities and no injuries to the two occupants.
The investigation
An investigation by the Norwegian Safety Investigation Authority (NSIA) revealed extensive mechanical destruction within the engine. The rear cylinder had detached from the crankcase, causing the intake manifold and exhaust system to bend. The piston and part of the connecting rod remained inside the cylinder, while the remaining portion of the rod had fractured the crankcase and snapped the camshaft into two pieces. The damage was so severe that both rear engine mounts failed, causing the engine to sag.
Upon disassembly and reconstruction of a new engine, investigators discovered that the oil pump's bypass valve was prone to sticking. The engine was an Experimental-category aircraft, and the cylinders had been overhauled approximately 83 flight hours prior to the incident.
Findings
- The engine failure was caused by overheating of the piston in the rear cylinder, which led to excessive friction and the eventual fracture of the connecting rod.
- A primary contributing factor may have been a malfunctioning oil pump bypass valve that stuck in the open position, potentially leading to insufficient oil pressure and inadequate cooling of the pistons.
- The rear cylinder is inherently more vulnerable to heat due to poorer airflow and the presence of a drainage hole in the intake manifold that allows "false air" into the system, resulting in a leaner fuel-air mixture.
- Other potential contributing factors included the possibility of a lean fuel mixture during flight or the failure of piston rings, which would increase combustion gas leakage and heat transfer.