What happened
On a flight from Burntwood Lake Lodge to Snow Lake, Manitoba, a float-equipped Noorduyn Norseman Mark V, registration C-FECG, experienced a sudden engine malfunction. Approximately ten minutes after departing, the engine began to sputter and lose power. Despite the pilot's attempts to restore power by adjusting the mixture, applying carburetor heat, and switching fuel tanks, the engine performance failed to improve, leading to a loss of altitude.
Unable to reach a nearby lake, the pilot executed a forced landing in a marshy, sparsely wooded area roughly 15 nautical miles north of Snow Lake. The aircraft struck the marsh grass and traveled approximately 200 yards through reeds before hitting a stand of trees at roughly 30 mph. While the aircraft sustained substantial damage, the zero fatalities and zero injuries were due to the low impact forces. The pilot and seven passengers were able to exit the aircraft safely and were later transported to Snow Lake via helicopter.
The investigation
Investigators examined the Pratt & Whitney R1340 AN-1 engine and discovered that the exhaust ear on the number 2 cylinder had cracked and split open. Further analysis at the TSB Engineering Laboratory revealed that the failure was caused by fatigue in an area containing casting porosity. This porosity was likely a manufacturing defect present for 50 to 80 years.
Testing showed that the separation of the exhaust ear increased the clearance between the push rod and the rocker arm, preventing the exhaust valve from opening. This failure allowed hot, pressurized exhaust gases to flow back into the induction system through the intake valve. This backflow disrupted the air/fuel mixture for the remaining cylinders, causing the engine roughness and power loss observed during the flight.
Findings
- The exhaust ear failure on the number 2 cylinder was the primary cause of the engine power loss.
- The failure was driven by fatigue originating from casting porosity and geometric stress concentration within the cylinder.
- The engine power loss occurred in a flight area where a damage-free landing was unlikely.
- The age and high service time of the original P&W R1340-series cylinders increase the risk of similar fatigue-related failures.