History of the Flight
The aircraft, with an instructor pilot and private pilot on board, departed Ottawa, Ontario, for a planned night cross-country training flight to Cornwall and Mirabel, Quebec, and return to Ottawa. During climb to cruise, a smell of hot oil was detected; oil temperature was high but pressure normal. The smell was attributed to possible spillage during oil addition before departure. The flight continued. At Cornwall, a touch-and-go landing was completed on runway 28. At about 300 feet AGL, oil temperature was again high and pressure now lower than normal. At about 500 feet AGL, the decision was made to return to land on runway 10. The private pilot turned back but was high on approach and entered a forward slip. The instructor determined a landing could not be completed without a runway excursion, took control, and initiated an overshoot. Shortly after, the engine lost all power, and a forced landing was made in a field. During landing, the nose gear was torn off, causing the aircraft to flip over. The crew sustained minor injuries; the aircraft was substantially damaged.
Crew and Aircraft Information
The instructor pilot and private pilot were qualified and certified in accordance with regulations. The private pilot was receiving dual instruction toward a night rating.
Engine Examination
The aircraft engine was examined. A clamp on the No. 2 exhaust pipe had been chafing the outboard end of the pressure oil filter. This chafing, from engine vibration, had worn a hole in the filter case, allowing oil to escape and causing diminished oil pressure. The No. 2 exhaust pipe is fastened to the cylinder head by M8 self-locking nuts (Rotax P/N 942-035). These nuts had lost torque and become loose, allowing the exhaust pipe to contact the oil filter. Oil sprayed over the windshield. Exhaust nuts on cylinders 1 and 4 were also loose; those on cylinder 3 were tight. Maintenance records revealed work on cylinders 1, 2, and 4 involving removal and installation of exhaust nuts.
Maintenance Issues
The locking mechanism of the M8 lock nut is an oval-shaped thread hole that pinches the stud thread upon installation. Removal reduces the pinch and diminishes locking capability. The aircraft maintenance manual states that self-locking nuts must be replaced after removal if friction torque has diminished. Testing determined that each removal reduces friction torque. The occurrence engine had re-installed exhaust nuts, and the maintenance facility had no M8 self-locking exhaust nuts in stock.
Oil Filter Considerations
A spring in the original oil pressure filter (Rotax P/N 825-700) caused cracks in the filter housing. Diamond Aircraft recommended replacement with oil filter Rotax P/N 825-701 via Alert Service Bulletin No. DA20-79-04A. The replacement filter is longer, reducing end clearance with the No. 2 exhaust pipe. Each filter replacement requires removal and reinstallation of the No. 2 exhaust pipe.
Decision-Making
The instructor pilot's decision to overshoot was examined. When low oil pressure was noted, he instructed the student to turn back for landing, indicating emergency awareness. The instructor did not immediately take control because the student was maintaining control and completing the turn. Due to darkness and mist on the canopy, it is unknown when the instructor could clearly determine height and position. When the instructor believed the aircraft was too high, he assumed control. Faced with landing straight ahead (likely runway excursion and damage) or overshooting, he chose the latter. With the engine still producing power, he believed a safe landing was possible. This decision is consistent with framing bias: choosing an uncertain loss over a sure loss. The engine lost power shortly after.
Causes and Contributing Factors
The self-locking exhaust nuts, re-installed against the aircraft manufacturer's recommendation, came loose and allowed the exhaust pipe to move into contact with the pressure oil filter housing. The vibration of the engine during operation caused a clamp installed on this pipe to chafe a hole into the housing, allowing the engine lubricating oil to escape. This resulted in the loss of engine oil pressure. Contributing to this occurrence was the increased length of the replacement oil filter and the lack of replacement nuts.