Pensacola, FL, USA
N79509
CESSNA 172K
During the training flight, the single-engine airplane was in cruise flight about 1,000 feet above ground level when the engine stopped operating. The flight instructor assumed control of the airplane and unsuccessfully attempted to restart the engine while descending for an emergency landing. He ditched the airplane into a nearby bay. The airplane sustained substantial damage to the windshield during the accident sequence and was submerged in salt water. Postaccident examination of the engine revealed that the bolt or lockplate, which attaches the crankshaft gear to the aft end of the crankshaft, was missing. Further, the alignment dowel was fractured across about one-third of the cross-sectional area. The bolt and lockplate were likely either not installed or were improperly installed, which would allow the bolt to loosen and become disengaged. Measurements of the dowel, pilot flange, and counterbore recess diameters were all within the manufacturer's specifications; the dowel also met the manufacturer's hardness requirement. Because the components were within manufacturer's specifications, it is likely that the components did not have any preexisting wear or manufacturing issues. Additionally, wear marks on the crankshaft, dowel, and gear were likely caused by the newly installed gear and were consistent with either no installation or improper installation of the bolt or lockplate. Therefore, it is likely that when the wear on the dowel had proceeded through about one-third of the cross-section, the stresses in the dowel became sufficiently high to initiate fatigue cracks at the base of the worn part of the dowel and eventually led to a fatigue fracture. The evidence is consistent with the failure of maintenance personnel to install, or properly install, the bolt and lockplate during the engine’s last overhaul, which resulted in the crankshaft gear becoming separated from the crankshaft and a total loss of engine power. If the engine had failed at a point that would have allowed the pilot to land the airplane on land, the airplane damage is less likely to have been substantial; therefore, contributing to the accident was the lack of suitable terrain.
On March 16, 2012, about 1015 central daylight time, a Cessna 172K, N79509, experienced a total loss of engine power while maneuvering near Pensacola, Florida. The pilot subsequently ditched the airplane into Pensacola Bay. The certificated flight instructor (CFI) and student pilot were not injured. The airplane sustained substantial damage to the windshield and was submerged in salt water. The airplane was registered to, and operated by, a local flight school under the provisions of Title 14 Code of Federal Regulations Part 91 as a local instructional flight. Visual meteorological conditions prevailed and no flight plan had been filed. The flight departed from the Pensacola Gulf Coast Regional Airport (PNS), Pensacola, FL about 0915. According to both the CFI and the student, while cruising about 1,000 feet above ground level, the engine "hesitated then quit." The CFI assumed control of the airplane and during the subsequent descent, the CFI attempted to restart the engine; however, the restart was unsuccessful. The airplane was ditched in the bay. The airplane struck the water and came to rest, both occupants egressed, and the airplane subsequently sank. The CFI held a commercial pilot certificate with ratings for airplane single-engine land, multiengine land, and instrument airplane. As well as a flight instructor certificate with ratings for airplane single-engine land, multiengine land, and instrument airplane. He reported over 4000 total hours of flight experience, of which, over 2000 total hours were in the accident airplane make and model. The student pilot held a student pilot certificate which was issued on February 5, 2012. He reported 3 total hours of flight experience, of which, all were in the accident airplane make and model. Post accident examination of the engine by the NTSB investigator in charge revealed that the engine remained attached to the airframe via the engine mounts. The engine was rotated by hand utilizing the propeller which remained attached to the propeller flange. Thumb compression was confirmed on the No. 4 cylinder; however, no compression was noted on the other cylinders. Following the removal of the engine accessories, the engine was rotated by hand utilizing the attached propeller; however, continuity was not obtained to the rear accessory pad. Removal of the accessory pad revealed that the crankshaft gear lockplate and bolt were absent from the crankshaft gear, attached to the aft end of the crankshaft. The lockplate and bolt were not able to be located throughout the engine. Rotation of the crankshaft gear revealed that the stepped dowel had been fractured. The crankshaft, step dowel, and crankshaft gear were sent to the NTSB's Materials Laboratory for further examination. No other defects were noted with the engine. A detailed report of the engine examination is included in the docket that accompanies this accident. Examination of the airframe and engine maintenance records revealed that a 50 hour inspection had been accomplished on February 23, 2012. At the time of the inspection the recorded engine time since overhaul was 534.39 hours total time in service. An entry dated August 8, 2010 recorded that the engine was rebuilt in accordance with Lycoming factory recommended procedures. The entry also stated that "all recommended parts listed in the Lycoming overhaul manual and service bulletins were installed." The NTSB's Materials Laboratory visually examined the crankshaft gear; however, no witness marks or wear was noted in the area where the lockplate and bolt make contact with the gear. No deformation, fracture, or wear was found in the threaded bolt hole, located in the end of the crankshaft. Examination of the dowel and pilot hole on the gear revealed matching wear marks. Further examination of the dowel revealed that the wear mark covered about one-third of the dowel's cross sectional area and the remaining area was fractured with ratchet and crank arrest marks as well as individual fatigue striations. The crankshaft, dowel, and gear were all measured and found to be within limits set forth by the manufacturer. A detailed examination report is included in the docket that accompanies this accident. According to Lycoming Service Bulletin 475C, which states in part "assemble the gear to the crankshaft using both a new lockplate and bolt…tighten the bolt to 125 in-lbs torque, then with a hammer and brass drift, tap lightly around the pilot flange of the gear and listen for sharp solid sounds from the hammer blows that would indicate that the gear is seated against the crankshaft…bend the lockplate against the bolt head. A logbook entry, specifying the final bolt torque, verify that the lockplate was properly bent in place against the bolt head and that the inspections and reword required by Lycoming Service Bulletin No. 475C were accomplished…" No entry was located in the engine logbook that indicated the final bolt torque.
A total loss of engine power due to the improper installation of the crankshaft gear bolt or lockplate by maintenance personnel. Contributing to the accident was the lack of suitable terrain for a forced landing.
Source: NTSB Aviation Accident Database
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