BONNERS FERRY, ID, USA
N19427
Cessna 150L
During the initial climb following a touch-and-go landing, the engine sputtered twice and the pilot-in-command initiated an emergency landing. The field of choice was obstructed with construction equipment and she chose an alternate field. She retarded the throttle to idle, lowered full flaps, and slipped the aircraft to land as expeditiously as possible; and then steered the aircraft into small trees and brush to avoid going over an embankment. Post-crash examination revealed all 3 bosses of the #3 cylinder broken, thus rendering the intake/exhaust valves permanently closed. Metallurgical examination revealed that the separation of the rocker arm shaft bosses was initiated by fatigue cracking in the boss located adjacent to the exhaust valve. Further examination revealed the presence of heavy tool marks on the bore surface of all three bosses. The heavy manual cutting of the bore surface is not authorized by the engine manufacturer and the surface created by this procedure did not meet the engine manufacturer requirements for surface finish in the bore of the bosses. A review of the aircraft's engine log revealed an overhaul 645.4 hours previous to the accident. The overhaul facility reported that all 4 cylinders were outsourced during the overhaul procedure.
On September 29, 1998, approximately 1454 Pacific daylight time, a Cessna 150L, N19427, registered to and being flown by a private pilot, incurred substantial damage when it rolled into brush and small trees during a forced landing. The forced landing followed a loss of power immediately after a touch-and-go landing at the Boundary County airport, Bonners Ferry, Idaho. The pilot was uninjured. Visual meteorological conditions existed, and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and originated from the Boundary County airport approximately 1345. The pilot reported that during the initial climb after a touch-and-go landing, the engine faltered briefly. She made a minor throttle adjustment, after which the engine faltered again. The pilot then decided to execute an emergency landing in an open field immediately ahead, as the field across the highway was obstructed with construction equipment. The pilot retarded the throttle to idle, lowered full flaps, and slipped the aircraft to get on the ground as expeditiously as possible; and then steered the aircraft into small trees and brush to avoid going over an embankment (refer to photograph 1). Post-crash examination, followed by a test run of the engine, revealed a cold number three cylinder, as well as rough-running performance when the engine was operated at or above 1,000 RPM. The rocker box cover for the number three cylinder was removed, and all three bosses retaining the single shaft for both rocker arms (intake and exhaust valves) were observed to be broken off, allowing the rocker shaft to release from the bosses (refer to SCHEMATIC I). This action would have rendered both the intake and exhaust valves permanently closed due to spring pressure and no rocker arm action. The cylinder was sent to the Safety Board's Office of Research and Engineering, Metallurgical laboratory, for further examination. Inspection of the boss fracture surfaces revealed heavy helical tool marks on the bore surface of all three bosses. Examination of the bore surfaces in all three bosses with the aid of a low power binocular microscope disclosed that these marks were generated by a tool that was rotating in a counter clockwise direction. The marks had similar widths and profiles, suggesting that they were created by the same cutting tool. Also, it was noted that the heavy tool marks were spaced unevenly, indicating that the tool was fed by hand. Average depth of the marks measured about 0.008 inches. Review of the TCM requirements for the repair of rocker arm bosses showed that the surface finish in the bore of the bosses "must be 60 rms" (root mean square). The bore surface of all bosses in the areas located between the heavy tool marks had a surface finish of 64 rms or better. Examination of these tool marks at higher magnifications revealed ladder cracking at the bottom of each mark. The presence of ladder cracks is indicative that the tool that created these marks was dull and/or that the cutting was performed using excessive pressure on the tool. The surface of the cylinder in the area of the exhaust valve contained oil deposits. However, the fracture faces on the bosses were relatively clean with no evidence of long term exposure to engine oil. Binocular microscope examination of the boss located closest to the exhaust valve revealed that the fatigue cracking initiated from the exterior surface of the boss. In each of these fractures, the fatigue propagated throughout the entire cross section of the boss. Examination of the bore surface revealed that the "original" machining marks were completely abraded from rubbing against the mating bushing (refer to attached Report No. 99-116). The aircraft's engine log was examined and the engine was reported as being "completely dis-assembled for overhaul by Flying Colors Aviation" (refer to ATTACHMENT EL-I). The overhaul was dated March 17, 1993, and the engine, which had a total time of 1033 hours, was zero timed on that date. A review of the engine logbook from the overhaul date forward to March 1, 1999, (at which time the engine was removed from the aircraft) showed a total time of 645.4 hours. There were no logbook entries between these dates indicative of any maintenance work on the number three cylinder. An inspector from the Federal Aviation Administration's Spokane Flight Standards District office visited the overhaul facility where the aircraft engine was zero timed. The owner reported that all four cylinders had been outsourced at the time of the overhaul (1993) and that he did not recall who provided the overhaul service for them.
Improper maintenance by an outsourcing facility which resulted in a lack of identification of scoring of the interior of the #3 cylinder rocker shaft bosses. The scoring resulted in a fatigue fracture and subsequent permanent closure of both intake and exhaust valves. Contributing factors were the lack of a suitable landing site and trees.
Source: NTSB Aviation Accident Database
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