NTSB Factual Report

On October 14, 2005, about 0930 Alaska daylight time, a tundra tire-equipped amateur built experimental Crisenbery Northman 2+2 airplane, N239PT, sustained substantial damage following a loss of engine power during cruise flight, about 23 miles west of Nenana, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country personal flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by the pilot. The private certificated pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. The flight originated about 0915 from the pilot's home on Iksgiza Lake, about 10 miles west of the accident site. No flight plan was filed.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on October 17, the pilot reported that in preparation for the flight, he preheated the engine for about 1.75 hours, conducted a preflight, and started the engine to warm it up until the cylinder head temperature was about 250 degrees. He stated that the outside ambient temperature was about 12 degrees F. After warming the engine, he departed and leveled the airplane in cruise flight about 1,000 feet above ground level, (agl), en route to Fairbanks, Alaska. About 15 minutes after takeoff, the engine lost power, and the propeller stopped. The pilot stated he attempted to restart the engine, but it would not rotate, and he selected an area of small trees for a forced landing. Following the accident, the pilot said he noticed oil covering the belly of the airplane, primarily along the right side.

The airplane was equipped with a Lycoming O-320 engine, and had a remote oil filter housing installed in 1995 by the builder of the airplane. In a telephone conversation with the builder on November 9, he indicated that the oil filter was an Oberg 600 designed for automotive use that was mounted on the firewall for ease of servicing. The filter is comprised of a rectangular cast aluminum housing. The center of each half of the housing has a raised horizontal rib along the longest axis of the case that incorporates an oil passage. Each end of the case has oil-line fittings threaded into the oil passage. Cast cooling fins are molded into each case half, oriented perpendicular to, and from the center rib to the outer edges of the housing. The two halves of the housing bolt together at each corner to internally retain a round metal screen. The 60 micron screen has a center hole that is positioned over a port incorporating a spring-loaded ball valve. The inner radius of the center hole, and the outer circumference of the screen, are edged with rubber gasket material. The inner and outer gaskets rest in adjacent molded channels within the filter housing to provide a seal. The outer mating surfaces of the housing do not have any gasket material. All the sealing surfaces are around the edge of the filter screen as it is pressed together by the housing halves.

The builder of the airplane reported that he experienced one previous oil leak along the mating seam of the filter housing. He reinstalled the filter screen, and had no further difficulties.

The pilot of the accident airplane reported that he purchased the airplane in October, 2004. He experienced a leak along the mating seam of the filter housing in about November or December of 2004. He reinstalled the filter, and had a small leak about one week later. In both cases, reinstallation resulted in no further difficulties. The pilot said that he installed a new filter element during an annual inspection in April, 2005. He had no indication of a leak.

Following recovery of the airplane, a Federal Aviation Administration (FAA) air safety inspector from the Fairbanks Flight Standards District Office (FSDO), examined the engine and reported that it exhibited damage consistent with a lack of oil lubrication.

The pilot reported that he discovered a visible crack on the exterior of the upper half of the filter housing. An examination of the housing by the NTSB IIC revealed a vertical, 1 1/4 inch crack in the center rib of the upper housing half. The crack was also visible on the inside of the housing in the center of the horizontal oil passage. The filter screen had small metal flakes adhering to the screen, and an odor of burned oil. The inner and outer screen rubber gaskets were undamaged. There was a visible gap between the mating surfaces of the two halves of the housing, along the longest sides of the housing. The mechanism for the initiation of the crack was not determined.

During telephone conversations with the NTSB IIC, the manufacturer of the filter indicated that the filter was produced for racing, automotive, and industrial applications about 15 years ago. At that time, the housing was cast aluminum. A new version of the filter is available for aviation use under a supplemental type certificate (STC), which incorporates a slightly different internal filter retaining design, and a filter bypass annunciator light, but utilizes the same metal filter screen. The new housing is made of milled aluminum.

NTSB Probable Cause

A loss of engine power during cruise flight due to a cracked oil filter housing and subsequent loss of engine oil, which resulted in a forced landing in trees.