Dade City, FL, USA
N5944J
CESSNA 172N
During the accident flight, the exhaust gas temperature on the No. 3 cylinder began as the coldest and then transitioned to the hottest. Subsequently, the cylinder head temperature on the No. 3 cylinder also transitioned to the hottest. According to the pilot, the engine then began to run roughly, and he adjusted the throttle and mixture positions to gain control of the malfunctioning engine. During emergency communications with air traffic control, the engine lost all power, and the pilot conducted an emergency landing to a field. Examination of the engine revealed that the No. 3 cylinder exhaust valve rocker arm stud had backed out of the cylinder, which prevented the exhaust valve from opening properly. Specifically, the loose rocker arm stud prevented the pushrod from depressing the exhaust valve far enough in to release exhaust from the cylinder, which damaged the intake valve pushrod and caused a complete loss of engine power. The No. 3 cylinder had been replaced about 14 months before the accident and had accumulated 480.9 hours since installation. It is likely that the rocker arm and nut assembly became loose over time; the initial torque value could not be determined.
On April 27, 2013, about 1800 eastern daylight time, a Cessna 172N, N5944J, was substantially damaged during a forced landing to a field following a total loss of engine power about seven miles north of Zephyrhills, Florida. The flight instructor, student pilot, and private pilot passenger received minor injuries. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the instructional flight that was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. The flight departed Hernando County Airport (BKV), Brooksville, Florida at 1746.According to the pilot, about 19 minutes after departure from BKV, the airplane was climbing through 4,500 feet mean seal level (msl) for 5,000 feet msl. Suddenly, the engine started to "rumble" and run rough. The pilot stated that he reduced engine power, applied carburetor heat, and "played" with the mixture because the number three engine cylinder was not indicating any exhaust gas temperature or cylinder head temperature. The pilot added that he informed Tampa Air Traffic Control (ATC) that he had a rough running engine and that he needed to land as soon as possible. ATC offered the pilot an alternate airfield for landing but the pilot stated that the more engine adjustments that he made, the worse the engine responded. The pilot stated that engine power decreased to near zero, he tuned code 7700 on the airplane transponder, notified ATC of his location, selected a landing site, and ran the emergency checklist from memory. The pilot stated that after the engine quit and during the forced landing sequence, he left the fuel selector on the "BOTH" tank position and extended the flaps to 40 degrees. As he made the approach to the field, he realized that a tailwind was disrupting his intended landing spot and that a ditch came into view that he had not previously recognized. The pilot flared the airplane for touchdown in the field, pulled the control yoke all the way aft and braced for impact. The airplane impacted the ditch and flipped end over end before coming to rest. The high-wing, tricycle gear equipped airplane, serial number 17273846 was manufactured in 1980 and was powered by a Lycoming O-320-H2AD 150-hp engine, serial number L-7539-76. The engine had accrued 2,815 total flight hours at the time of the accident. A review of the airplane and engine maintenance records revealed that the last annual inspection was completed on March 14, 2013 with an airframe total time of 2,747.9 hours and an engine total time of 2,621 hours. The No. 3 engine cylinder was replaced on December 7, 2011 at a recorded time of 2,207.5 hours. Examination of the airplane revealed that the fuselage was buckled, the main landing gear boxes were crushed, and the overhead section of the cabin was detached from the door frames. The propeller spinner was crushed. The engine was equipped with a 2-blade McCauley propeller. One blade was bent aft approximately 10 degrees beginning about 10 inches outboard of the blade root. The other blade exhibited a gouge four inches outboard of the blade root. The right fuel tank contained twenty gallons and the left tank contained fifteen gallons of a blue fluid that was similar in color and smell to 100LL aviation fuel. Crankshaft continuity was observed by removing the top spark plugs and turning the attached propeller by hand. The rocker covers were removed and valve action was observed through several engine rotations confirming drivetrain continuity. Thumb compression was confirmed on Cylinder Nos 1, 3, and 4. Cylinder No. 2 was filled with compressed air and was noted as escaping past the piston rings. The spark plug leads were removed and spark was observed on each lead when the engine was rotated. The carburetor was removed, found full of a blue fluid similar to 100LL aviation fuel, and free of contaminates. The oil filter was removed and no contaminants were noted. Further examination of the engine revealed that the No. 3 cylinder intake push rod was bent and the exhaust push rod was unremarkable. The No. 3 cylinder exhaust rocker arm was observed to have about one-half of an inch of free play as compared to the other cylinder exhaust rocker arms. The No. 3 cylinder was removed and the piston was observed to have normal wear. No foreign objects were noted in the cylinder intake and exhaust ports. The push rod tappets operated normally as the propeller was turned. The crankshaft intake and exhaust lobes were unremarkable and operated normally when the propeller was turned. Examination of the intake and exhaust rocker arms noted that the fulcrum of the exhaust rocker arm displayed wear on the top and bottom surfaces and that the assembly was dislodged from the fulcrum mounting rail. The mounting rail also displayed wear on the top and bottom surfaces. The rocker arm and fulcrum are attached to the cylinder via a metal stud that runs into the cylinder head. The intake rocker arm measured 1 1/4 of an inch from the mounting rail to the side of the rocker arm. The exhaust rocker arm was measured to 1 3/8 of an inch from the mounting rail to the side of the rocker arm. The exhaust rocker arm adjusting nut was rotated and the mounting stud rotated with the nut. The stud and nut were removed and were unremarkable. According to the engine manufacturer, if the exhaust rocker arm stud were to back out, the push rod will still receive commands from the crankshaft lobes. However, due to the increased free play with the exhaust side fulcrum, the push rod will not be able to sufficiently push the rocker arm far enough down for the exhaust valve to release cylinder pressure. Therefore, the intake push rod will not be able to depress the intake valve with the cylinder head pressure and the intake push rod will bend as a result. The airplane was equipped with a JP Instrument EDM 800 engine monitor and was downloaded and examined by the NTSB Vehicle Recorder Laboratory in Washington, D. C. Figure 4 of the report shows a plot of the flight immediately prior to the accident flight, which occurred earlier on the same day. Exhaust Gas Temperature (EGT) 3 was generally operating colder than the EGTs associated with the other cylinders. Cylinder Head Temperature (CHT) 3 generally operated hotter than other the CHT associated with the other cylinders. Figure 5 of the report shows a plot for the accident flight EGT-3 began as the coldest EGT, and transitioned to the hottest EGT between about 1745:50 and 1757:16 EDT, when CHT-3 also became the hottest CHT. At about 1757:16 EDT, EGT-3 increased momentarily, and then began to decrease rapidly. Coincident with the EGT-3 change at about 1757:16 EDT, all the CHTs began to decrease. A detailed report of the data is contained in the public docket associated with this accident. Lycoming Service Bulletin (SB) 412B, with an effective date of March 10, 1978, mandated replacement of the intake and exhaust valve rocker arm retaining studs. The SB applied to all O-320-H engines with serial numbers L-101-76 through L-2182-76 inclusive. Lycoming SB 412B did not apply to the accident engine. According to the engine maintenance logs, the No. 3 cylinder was replaced with a serviceable cylinder 14 months prior to the accident flight. According to paragraph 5 of SB 412B, the No. 3 cylinder was also exempt from the SB due to the letter "L" stamped onto the milled rail between the 2 retaining studs. At the time of the accident, the No. 3 cylinder accumulated 480.9 total hours since installation on the accident engine.
The in-flight loosening of the No. 3 cylinder exhaust valve rocker arm stud, which resulted in a complete loss of engine power.
Source: NTSB Aviation Accident Database
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