Accident Details
On the morning of the accident, a Gogal Air Services Limited Noorduyn Norseman Mark V (registration C-FECG, serial number N29-43) departed Burntwood Lake Lodge, Manitoba, for a 20-minute flight to Snow Lake. The aircraft was carrying the pilot and seven passengers. Approximately 10 minutes into the flight, the engine began to sputter and lose power. The pilot attempted to restore power by pushing the mixture control to full rich, applying carburetor heat, and switching fuel tanks, but engine performance did not improve. At 0900 central daylight time, the pilot executed a forced landing 15 nautical miles north of Snow Lake into a sparsely-wooded marshy area. The aircraft touched down on marsh grass, travelled approximately 200 yards through reeds, and entered a stand of trees at about 30 mph. The wings and floats absorbed most of the impact, and the aircraft came to a gradual stop with the right float beneath the fuselage. All eight occupants exited without injury and were later flown to Snow Lake by a company helicopter. The aircraft sustained substantial damage, but impact forces were low and the emergency locator transmitter did not activate.
Aircraft and Engine Examination
The aircraft was examined on site, and a clean, bright flow of fuel was present at the main airframe fuel strainer. The engine cowling was removed, and an initial examination of the Pratt & Whitney R1340 AN-1 engine (serial number 18620) revealed that the exhaust ear on the number 2 cylinder had cracked and split open. The engine was transported to the TSB Central Region wreckage facility for further examination. Rotating the engine showed that the number 2 cylinder exhaust valve did not move from the closed position. The separation of the ear had increased clearance between the exhaust valve push rod and rocker arm, preventing the valve from opening. During operation, a closed exhaust valve can force hot pressurized exhaust gases into the common induction system through the intake valve, disrupting the fuel/air mixture to the remaining cylinders and causing engine roughness and power loss. The number 2 cylinder was sent to the TSB Engineering Laboratory for analysis.
Probable Cause
Laboratory analysis determined that the number 2 cylinder exhaust ear failed under fatigue in an area of casting porosity and geometric stress concentration. The casting porosity was likely introduced during manufacture of the cylinder, estimated to have occurred 50 to 80 years prior. The cylinder's time in service could not be established due to lack of tracking, but numerous repair shop markings suggested high-time usage. The fatigue crack had been present for a considerable time before failure, as indicated by corrosion and staining on the fracture surface. The crack was not detected during a cylinder overhaul 595 hours before the accident, likely because it had not reached a detectable stage. The failure of the exhaust ear prevented the exhaust valve from opening, leading to a disruption in the fuel/air mixture and loss of engine power. Records showed that three of the engine's nine cylinders had failed within 600 hours of overhaul, with previous failures also occurring in top cylinder positions, resulting in two documented forced landings without damage. The engine power loss occurred in an area where a damage-free forced landing was improbable.