Accident
On June 26, 2011, at about 1015 Pacific daylight time, a Christian Industries Inc. Pitts S-1T, registration N77MN, experienced a partial loss of engine power during cruise flight at 6,500 feet mean sea level, resulting in a forced landing approximately 26 miles northeast of Alkali Lake, Oregon. The airplane, operated by the pilot under 14 CFR Part 91, sustained substantial damage. The commercial pilot, the sole occupant, reported minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The cross-country flight originated from Burns, Oregon, around 0950, with an intended destination of Lakeview, Oregon.
Pilot Report
The pilot stated that during cruise flight, a large amount of oil suddenly coated the windscreen. She turned away from the edge of a plateau and observed that the engine oil pressure gauge indicated zero. During the off-airport landing, she attempted to raise the nose to drag the tail on the ground to slow the airplane, but reported no response from the elevator or ailerons. Subsequently, the airplane impacted the ground in a nose-low attitude and cartwheeled before coming to rest upright.
Examination Findings
Examination by the pilot revealed that the upper and lower right wings were folded upwards and the fuselage exhibited structural damage. The wreckage was recovered for further examination.
The recovered Lycoming AEIO-360-A1E engine (serial number L-24247-51A) showed heavy oil and oil-covered dirt on the forward portion of the engine and cowling. All accessories remained attached, cylinders were secure to the crankcase, and the propeller remained attached to the crankshaft flange. All oil line fittings were tight and secure.
The propeller and top spark plugs were removed. The crankcase front bearing oil seal was partially extruded from its bore. Rotational continuity was established throughout the engine and valve train, and thumb compression and suction were obtained on all four cylinders.
Front Bearing Oil Seal
The front bearing oil seal was removed and examined. The spring was intact and undamaged, and a slight amount of sealant material was found around the forward circumference. Compressed air applied to the crankcase breather fitting resulted in air expelled from both the breather fitting and the nose seal bore area. The reason for the seal extrusion was undetermined.
Maintenance History
Review of the airplane's maintenance logbooks indicated that the front bearing oil seal was replaced on September 14, 2010, at tachometer time 1,845.4 hours, and again on June 8, 2011, at tachometer time 1,910.2 hours. At the time of the accident, the airplane had accumulated 10.1 hours since the last replacement.