St. Agatha, ME, USA
N7213W
PIPER PA28
During an instructional flight while maneuvering, the student pilot moved to increase the throttle, which was followed by a pop that was felt and heard from the engine. Subsequently, the power became stuck at idle and would not produce additional power, even with several troubleshooting actions performed by the flight instructor. The flight instructor completed a forced landing in a field, where the landing gear collapsed, and the fuselage was substantially damaged. Examination of the engine revealed that the crankshaft gear would not engage and, as a result, the engine exhibited no compression and all components to the engine’s accessory gear box would not rotate. The crankshaft gear bolt was found loose about 2 to 3 threads, the lock plate holding the crankshaft gear bolt was loose, and the alignment dowel had sheared. Further forensic review of the dowel revealed that it exhibited signatures consistent with fatigue crack propagation and overstress fractures. It is likely that this crankshaft alignment dowel failure resulted in the loss of engine crankshaft continuity and the inability of the engine to produce power when commanded. The engine manufacturer had published a service bulletin that likely would have required replacement of the alignment dowel; however, the bulletin was only required to be completed during overhaul, after a propeller strike, or whenever the crankshaft gear was removed. There was no record in the maintenance records that the service bulletin was complied with and the most recent overhaul inspection was completed prior to the publication of the service bulletin.
On August 5, 2020, about 1245 eastern daylight time, a Piper PA28-180 airplane, N7213W, was substantially damaged when it was involved in an accident near St. Agatha, Maine. The flight instructor and student pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 instructional flight. The flight instructor reported that he and the student pilot planned to complete a "refresher flight" to continue the student's advancement to his checkride. After completing a variety of maneuvers and basic navigation procedures, the flight instructor had the student pilot demonstrate flight at less than cruise airspeed. The flight instructor believed the airplane had slowed down to about 90 to 100 mph, the engine power was reduced to about 2300 rpm, and the carburetor heat was on. The flight instructor subsequently told the student pilot to increase the power to maintain about 3,000 ft mean sea level (msl); however, as the student did so, a “pop” from the engine was heard and felt and engine power went to idle. The flight instructor characterized the engine power as not sufficient to maintain altitude; however, it was still "running perfectly" at idle and all engine instruments were in the normal range. The flight instructor took over the flight controls and "checked everything." He reported that switching fuel tanks, adjusting mixture and power settings, checking the primer, and adjusting carburetor heat did not increase engine power. He subsequently identified an open field and completed a forced landing. During the landing roll through rough terrain, the airplane's landing gear collapsed, which resulted in substantial damage to the fuselage. An examination of the engine supervised by a Federal Aviation Administration inspector revealed that the crankshaft would rotate when the propeller was rotated by hand; however, the crankshaft gear would not engage and, as a result, the engine exhibited no compression and all components to the engine’s accessory gear box would not rotate. The crankshaft gear bolt was found loose about 2 to 3 threads, the lock plate holding the crankshaft gear bolt was loose, and the alignment dowel had sheared. All other engine components were examined, and no anomalies were observed. The National Transportation Safety Board (NTSB) Materials Laboratory further examined parts from the aft section of the crankshaft, which included a crankshaft drive gear, attachment bolt, lock plate, and a fractured dowel remnant. No significant anomalies were observed with the crankshaft drive gear, its teeth, bolt, or lock plate. The fractured crankshaft dowel remnant exhibited banded crack arrest marks, consistent with crack propagation, and ratchet marks, consistent with multiple crack initiation sites. These features were determined to be consistent with fatigue crack propagation. The fracture surface exhibited two opposite-facing fatigue cracks, whose features were consistent with having initiated at the dowel surface and propagating inward. A thin middle region exhibited overstress fractures. Figure 1 shows a drawing of the rear end area of the crankshaft in addition to NTSB Materials Laboratory photographs and annotations of the fractured dowel. Figure 1: View of a drawing of the crankshaft drive gear and NTSB photographs of the alignment dowel In January 2003, Lycoming published Mandatory Service Bulletin No. 475C, Crankshaft Gear Modification and Assembly Procedures, which applied to the accident engine. The service bulletin was to be complied with, “During overhaul, after a propeller strike, or whenever crankshaft gear removal is required.” The service bulletin further stated Damage to the crankshaft gear and the counterbored recess in the rear of the crankshaft, as well as badly worn or broken gear alignment dowels are the result of improper assembly techniques or the reuse of worn or damaged parts during reassembly. Since a failure of the gear or the gear attaching parts would result in complete engine stoppage, the proper inspection and reassembly of these parts is very important. The procedures described in the following steps are mandatory. Review of the airplane’s maintenance records revealed that an engine overhaul was last performed in October 2001 and the time since that overhaul was about 660 hours. The maintenance records were reviewed from the overhaul to the most recent annual inspection, which occurred on February 26, 2020. No maintenance entries referenced work performed on the crankshaft, nor was there any record that would indicate compliance with the service bulletin or that compliance was required.
The failure of the crankshaft gear alignment dowel due to fatigue crack propagation and overstress, which resulted in a partial loss of engine power.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
