ELK CITY, ID, USA
N3964V
Cessna 170
While in cruise over mountainous terrain, the pilot noticed a drop in oil pressure followed immediately by engine seizure. The pilot initiated a force landing into rugged terrain and the airplane was destroyed. Exam of the engine revealed that the no. 6 connecting rod had separated due to lack of lubrication and bearing seizure. No evidence of leaking oil was found on the underside of the airplane. The pilot stated that she checked the oil level prior to the flight and that it was adequate. The oil was last changed about one month prior to the accident. According to a maintenance log entry and the accident tachometer time, the airplane had flown about 23 hours since the oil change. The pilot stated that the airplane could not have been flown for more than about 8 hours since the inspection. The rubber seal around the no. 6 push rod tube was found detached after it was salvaged, but it could not be determined if this was a preimpact condition. The published useable oil quantity for the engine is 5.6 qts. in a 5-degree nose down attitude. Conclusive evidence for the loss of engine oil could not be determined.
On August 24, 1997, about 1700 mountain daylight time, N3964V, a Cessna 170, operated by the owner/pilot, collided with terrain and was destroyed during a forced landing near Elk City, Idaho. The forced landing was precipitated by a total loss of engine power during cruise. The commercial pilot was seriously injured and her two passengers received minor injuries. Visual meteorological conditions prevailed and no flight plan had been filed. The personal flight departed from Helena, Montana, about 1430, and was destined for Kent, Washington, with a planned stop in Grangeville, Idaho. The flight was conducted under 14 CFR 91. In an interview with the Safety Board (record of interview attached) and in a written statement (attached), the accident pilot stated that when she flew to Helena, Montana, one day prior to the accident, she did not notice any problems with the airplane or engine. She did remember, however, that she thought she may have "picked up some carb ice" a few times as she approached Helena. She noticed a slight change in the engine that was remedied by operating the carburetor heat. After visiting acquaintances in Helena, the accident pilot arrived at the Helena Airport with her passengers on the following day for the return flight to Kent. She stated that she fueled the airplane and checked the engine oil. She stated that she always checks the oil before every flight. She remembered that the oil level was "lower than full, but about a half a quart over [the specified dip stick oil level] where oil needed to be added." She did not recall exactly how many quarts were in the engine at the time. She stated that she intended on checking the oil again after she arrived in Grangeville, which is also where she also intended on fueling the airplane en route back to Kent. She stated that she did not want to overfill the engine oil level because the excess oil would spew out of the engine. The pilot stated that after departure from Helena, she flew toward the west over Butte, Montana, and Dillon, Montana. She then noticed that the oil pressure gauge began moving toward zero. The engine immediately began to run rough. She stated that she then heard a "loud bang." She stated that "it all happened very quickly," and that she was "going straight... between 9,500 and 10,000 feet" when the engine failure occurred. She stated that she was not climbing or descending, and that the weather was under visual flight rules. She initiated a forced landing onto a logging road located in rugged, mountainous terrain. She stated that during the landing, she became unconscious at impact. She was later revived by her son and her daughter. The emergency locator transmitter was activated at impact, and all three occupants were rescued the following morning. Two days after the accident, on August 26, 1997, the wreckage was examined at the accident site by a Federal Aviation Administration (FAA) aviation safety inspector from Spokane, Washington. According to the inspector (report and wreckage diagram attached), the wreckage distribution path was oriented along a magnetic bearing of 030 degrees and was about 330 feet in length. Sheared tree tops were found at the beginning of the wreckage path. The airplane was found lying on its right side (as viewed aft to forward) on top of its damaged right wing. The left wing remained completely attached to the fuselage; however, the outboard portion of the right wing was sheared off. The flaps were extended to about the 10-degree position. An examination of the cockpit instruments revealed that the tachometer time read 653.66 hours, and the altimeter read 7,546 feet. The engine, a Continental model C-145-2, was found separated from, and lying adjacent to, the airframe. No evidence of fire damage was found. The windshield was broken outward and a small amount of oil splatter was found on it. Oil splatter was also noted on the engine firewall on its left upper side. Fuel was found on board the airplane. A closer visual inspection of the engine revealed a 5-inch diameter hole in the top of the crankcase between the no. 5 and no. 6 cylinders. The connecting rod for the no. 6 cylinder had separated from its journal and was black in color. The other end of the rod remained attached to the no. 6 piston. A puddle containing about 2 quarts of black oil was found underneath the engine. An examination of the belly of the airplane did not reveal any evidence of oil splatter along its entire length. The engine was removed from the accident site for further examination. An examination of the airplane and engine maintenance records (excerpts attached), revealed that the engine had received an annual inspection and an oil change on July 24, 1997. The mechanic recorded that the tachometer time was 630.48 hours at the time of the inspection and oil change. [This figure differed by 23.18 hours as compared to the accident site reading of the tachometer time] The entry read: "Changed oil Added [blank space] qts Aeroshell 50-100. Checked & cleaned oil screen. Found OK no contamination. Checked Timing. Cleaned & gapped plugs. Compression Test #1 58/50 #2 70/80 #3 70/80 #4 65/80 #5 64/80 #6 62/80. Replaced #1 cyl aft exhaust stud." The Safety Board interviewed the mechanic (record of interview attached) who performed the most recent annual inspection and maintenance on the accident airplane and engine. The mechanic stated that he remembered performing the annual inspection. He stated that the engine did not have any major discrepancies at the time of the inspection. He stated that the "compression was pretty good" in all the cylinders, and the spark plugs were "pretty clean." He stated that there were no oil leaks on the engine, and that he "filled it" with oil during the inspection. The accident pilot stated that there should have been an adequate amount of oil in the engine because the airplane had just received an annual inspection about one month prior to the accident, and that she did not fly the airplane on more than a few occasions after the inspection. When she was told that the tachometer reading at the accident site indicated that the airplane was flown for 23 hours after the annual inspection, she refuted that time. She stated that she flew the airplane on six separate flights that totaled about 8 hours after the annual inspection. The pilot further stated that she was the only person who flew the airplane, and if someone else had flown the airplane since the annual inspection she was not aware of it. (An examination of the maintenance log book revealed that the average annual flight time of the airplane was about 25 hours per year.) She again stated that she was absolutely certain that she had an adequate amount of oil in the engine prior to departure from Helena. The engine was shipped to Mobile, Alabama, for teardown and analysis under Safety Board supervision (inspection report attached). Immediately after the engine was removed from its shipping crate, it was noted that the no. 6 cylinder push rod tube bottom rubber coupler was off the adapter attached to the crankcase. Both spring steel clamps were removed from the rubber and loose on the tube. The rubber was pushed up the tube about one inch from the attachment to the crankcase. This area did not exhibit any impact damage. The valve cover for the no. 6 cylinder was missing, and the exhaust stack was removed. Photographs taken of the engine prior to its removal from the accident site revealed that the valve cover and exhaust stacks were installed. There were no photographs available that depicted the conditions of the no. 6 cylinder push rod tube bottom rubber coupler. According to the mechanic who had performed the last inspection on the engine about a month prior to the accident, he did not touch any of the push rod tube housings, or have any need to remove the push rod tube rubber seals. Further inspection of the engine revealed that the no. 6 connecting rod bearing was destroyed and heat distressed. Fragments of the bearing were recovered from the oil sump. The no. 6 crankshaft connecting rod journal was burned and partially melted. The no. 2 and no. 4 connecting rod bearings exhibited lubrication distress signatures. All oil passages were open and unobstructed. No oil was found in the engine when the oil drain was opened. The oil sump was then filled with water and tilted forward about 15 degrees. At this position, the sump held a minimum of about 4 quarts of water when the water level was level with the oil pickup hole. According to the FAA Type Certificate Data Sheet for the Continental C145-2 engine (attached), the oil sump capacity is 8 quarts. A total of 5.6 quarts of oil are usable at a tilt angle of 5 degrees nose down. At 10 degrees nose-down, the published amount of usable oil is 3.77 quarts. No verbiage regarding unusable oil quantity is contained in the FAA-approved flight manual for the Cessna 170. The Safety Board examined the probable causes of all accidents involving the Cessna 170 since 1983. There have been 328 accidents since that time. One accident involved engine failure due to a loss of oil lubrication. In that case, the loss of oil was a result of a failure of the oil line between the engine and the oil cooler. The pilot, age 53, reported that she had a total of 1,360 hours of flight time. The pilot held a commercial pilot certificate with ratings for airplane single engine land, airplane single engine sea, and instrument airplane. She was issued the commercial pilot certificate in 1981. The pilot was also issued a flight instructor certificate with a rating for airplane single engine land in 1985. At the time of the accident, the pilot held a valid FAA Second Class Medical Certificate with the limitation that she "must wear corrective lenses."
Oil starvation of the engine due to undetermined reasons, which led to the failure and separation of a main bearing and a connecting rod. A factor contributing to the accident was the lack of suitable terrain for a successful forced landing.
Source: NTSB Aviation Accident Database
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