What happened
A night cross-country training flight departed Ottawa, Ontario, bound for Cornwall and Mirabel, Quebec. The crew, consisting of an instructor pilot and a private pilot, initially noted a smell of hot oil and high engine oil temperature during the climb, which they attributed to potential spillage during pre-flight oil addition. The flight continued to Cornwall, where a touch-and-go landing was performed.
During the subsequent takeoff from Cornwall, the crew observed that the engine oil temperature remained high, but the oil pressure had now dropped below normal levels. At approximately 500 feet above ground level, the pilot attempted to return to the airport to land on the reciprocal runway. However, the aircraft remained too high on the approach. The instructor pilot took control to perform an overshoot maneuver to avoid a runway excursion. During this maneuver, the Bombardier-Rotax GMBH Type 912A3 engine lost all power, forcing the crew to perform a landing in a field adjacent to the airport. The impact caused the nose gear to detach and the aircraft to flip. The two crew members sustained minor injuries, though the aircraft suffered substantial damage.
The investigation
The investigation focused on the engine's mechanical failure and the maintenance history of the components. Investigators examined the engine and found that a clamp on the No. 2 exhaust pipe had been chafing the end of the pressure oil filter. This chafing, driven by engine vibration, created a hole in the filter case, leading to the loss of oil pressure.
Technical analysis revealed that the M8 self-locking nuts securing the exhaust pipe to the cylinder head had become loose. While the nuts on the No. 3 cylinder were secure, the nuts on cylinders 1, 2, and 4 were loose. Maintenance records showed that recent work on these cylinders had involved removing and reinstalling these nuts. Testing confirmed that the friction torque of these specific lock nuts diminishes upon removal, and the manufacturer's manual requires replacement with new hardware after removal. The investigation also noted that the maintenance facility lacked the necessary replacement M8 nuts in their stock.
Additionally, the investigation looked at the role of the oil filter. A replacement filter, which was longer than the original part, had reduced the clearance between the filter and the exhaust pipe, contributing to the contact that caused the chafing.
Findings
- The primary cause of the oil pressure loss was the re-installation of self-locking exhaust nuts that had not been replaced, which allowed the exhaust pipe to move and contact the oil filter housing.
- The vibration of the engine caused a clamp on the exhaust pipe to chafe a hole through the filter housing.
- The increased length of the updated oil filter assembly reduced the clearance between the filter and the No. 2 exhaust pipe.
- The maintenance facility did not have the required new M8 self-locking nuts available in their inventory.