Oloh, MS, USA
N739KW
CESSNA 172
The pilot reported that, while the airplane was about 3,500 ft above mean sea level, the engine produced a "very rapid metallic banging/clanging noise," along with an instant loss of engine rpm. The engine was creating a "violent shaking" of the entire airframe. After about 20 seconds of applying carburetor heat, the pilot applied full mixture and started adjusting the throttle to obtain more power. He reported that, with more power applied, the shaking became more violent, along with a loss of engine rpm. The pilot realized that the engine power at this point was not sufficient to hold altitude. During a forced landing to a field, both wings and the fuselage sustained substantial damage. Examination of the airframe and propeller found no preimpact mechanical malfunctions or failures that would have precluded normal operation. Examination of the engine revealed the No. 3 piston area displayed internal impact marks from the piston impacting the exhaust valve. The No. 3 exhaust valve was open and displayed impact marks from contact with the piston. The No. 3 valve spring seat (upper exhaust) was impact damaged with fractured pieces of the valve key (exhaust) and the valve stem cap (exhaust) laying on the bottom of the rocker area of the cylinder head. The No. 3 exhaust valve assembly moved freely inside the valve seats and was contacting the top of the piston. Based on the available evidence, the partial loss of engine power was likely due to the No. 3 valve key (exhaust) failing, which resulted in a substantial reduction in engine power output. The reason for the failure was undetermined, as the service history of the valve key was unknown.
HISTORY OF FLIGHTOn November 25, 2017, about 1455 central standard time, a Cessna 172N airplane, N739KW, sustained substantial damage following a partial loss of engine power and forced landing about two miles south of Oloh, Mississippi. The private pilot in the left front seat and the passenger in the front right seat sustained no injury, and the passenger in the left rear seat sustained minor injuries. The airplane was registered to a private individual and was operated by the pilot as a Title 14 Code of Federal Regulations Part 91 visual flight rules personal flight. Day visual meteorological conditions were present at the time of the accident and no flight plan was filed. The airplane departed from the McComb-Pike County Airport (MCB), McComb, Mississippi, about 1405. The pilot reported the purpose of the flight was for sightseeing with the two passengers. The airplane departed with 40 gallons of 100 low lead fuel. After departing MCB, the pilot was flying enroute to the Hattiesburg Bobby L. Chain Airport (HBG), Hattiesburg, Mississippi. At 3,500 ft above mean sea level with the power at about 85 percent, the mixture leaned out slightly, and with the engine gas temperatures observed at normal, the engine produced a "very rapid metallic banging/clanging noise" along with an instant loss of engine RPM. The pilot immediately applied the carburetor heat and checked the engine oil pressure, the engine oil temperature, and the engine gas temperature and noticed all gauges were in the normal operating range. According to the pilot, the engine was creating a "violent shaking" of the entire airframe. After about 20 seconds of the carburetor heat applied, the pilot applied full mixture and started adjusting the throttle to see if he could obtain more power. He reported that with more power applied, the shaking became more violent along with a loss of engine RPM. The pilot realized that the power the engine was producing at this point was not sufficient to hold altitude. He then reduced the power and the shaking subsequently reduced, but the shaking did not stop. The pilot assessed his current location, and the distance from the destination airport and determined that the airplane would be unable to glide the full distance. The pilot also assessed turning back to another airport and determined that the airplane would be unable to make this distance. The pilot decided to conduct a landing to a nearby open field. He reported at this point, the engine was just above idle, but still not producing sufficient power to hold existing altitude or assist in gliding to an airport. As the pilot was deciding which field he was going to land to, he made wide, slow turns holding as much altitude as possible. The pilot selected a flat field and steeped his turn for an emergency landing profile. Once the airplane descended and the pilot was committed to landing at the field, he realized there were large power lines traversing across the approach end of the field. Once over the power lines, the pilot "slipped" the airplane to lose as much altitude as quickly as possible. He reported the "slip" got the airplane on the ground faster, but also increased the airspeed. Once on the ground, the pilot attempted to apply the brakes on the damp grass but realized the airplane was going too fast and was not going to stop by the end of the field. At the end of the field, the pilot observed various trees and a small driveway size opening. Using the rudder authority he had left, the pilot maneuvered the airplane to the right, placing the fuselage into the opening. As the airplane skid towards the tree line, the airplane traveled through a barbed wire fence, and the right wing impacted a tree, turning the airplane to the right about 90°. The left wing dug into the ground, stopping the forward movement of the airplane. The airplane came to rest on the nose wheel and the left main landing gear, with the empennage elevated in the air as shown below in figure 1. The pilot performed a shutdown and the three occupants egressed without further incident. Figure 1 - Front view of the airplane nose down (courtesy of the pilot). A Federal Aviation Administration (FAA) aviation safety inspector (ASI) responded to the accident site. During a postaccident on scene inspection of the accident airplane, a fuel sample was obtained, and no contamination was found. The FAA ASI reported that both fuel cells in each wing remained intact with no ruptures observed and the fuel quantity was unable to be verified due to the extreme positioning of both wings. The airplane sustained substantial damage to both wings and the fuselage. AIRCRAFT INFORMATIONAn examination of the airplane's maintenance records revealed no evidence of uncorrected mechanical discrepancies with the airframe, engine, and propeller. No evidence of a 400-hour inspection or an overhaul being conducted on the engine during its service history was found listed in the engine maintenance records. On October 8, 2004, the number 3 cylinder was replaced with an unknown overhauled cylinder. On November 16, 2004, the number 3 cylinder was replaced with a new cylinder from Lycoming Engines. During these two maintenance procedures, it was undetermined if the number 3 valve keys (intake as shown below in figure 2 as item #9 and exhaust as shown below in figure 2 as item #10), or the number 3 valve stem cap (exhaust as shown below in figure 2 as item #11) were replaced or if they remained the original parts installed by the manufacturer during the initial production of the engine. Figure 2 - Drawing of the valve assembly and related parts (courtesy of Lycoming Engines). AIRPORT INFORMATIONAn examination of the airplane's maintenance records revealed no evidence of uncorrected mechanical discrepancies with the airframe, engine, and propeller. No evidence of a 400-hour inspection or an overhaul being conducted on the engine during its service history was found listed in the engine maintenance records. On October 8, 2004, the number 3 cylinder was replaced with an unknown overhauled cylinder. On November 16, 2004, the number 3 cylinder was replaced with a new cylinder from Lycoming Engines. During these two maintenance procedures, it was undetermined if the number 3 valve keys (intake as shown below in figure 2 as item #9 and exhaust as shown below in figure 2 as item #10), or the number 3 valve stem cap (exhaust as shown below in figure 2 as item #11) were replaced or if they remained the original parts installed by the manufacturer during the initial production of the engine. Figure 2 - Drawing of the valve assembly and related parts (courtesy of Lycoming Engines). WRECKAGE AND IMPACT INFORMATIONThe wreckage was transported to a secure location and an airframe and engine examination was conducted under the direction of the National Transportation Safety Board investigator-in-charge. No preimpact mechanical malfunctions or failures were noted with the airframe and propeller. All engine accessories were present. The engine case was intact. All engine controls displayed a full range of motion with no binding. Engine continuity was established by rotating the propeller and observing the rocker assembly movement and gear rotation in the rear of the engine. Thumb compression readings were established for cylinders 1-2-4, the number 3 cylinder did not produce compression. The top four sparkplugs were removed for examination with a lighted digital borescope. The number 3 piston area displayed internal impact marks from the piston impacting the exhaust valve as shown below in figure 3. Figure 3 - Borescope image of the number 3 piston impact marks from the exhaust valve. The number 3 exhaust valve was open and displayed impact marks from contact with the piston. The rocker arm cover was removed on the number 3 cylinder and it was observed that fractured pieces of the valve key (exhaust) and valve stem cap (exhaust) were laying on the bottom of the rocker area of the cylinder head as shown below in figure 4 and 5. The number 3 valve spring seat (upper exhaust) had impact damage to the top of the seat as shown below in figure 5. The number 3 exhaust valve assembly moved freely inside the valve seats and was contacting the top of the piston. Figure 4 – View of the fractured valve key (exhaust) and valve stem cap (exhaust) pieces at the bottom of the rocker area of the cylinder head. Figure 5 – View of the fractured valve key (exhaust), fractured valve stem cap (exhaust), and damaged valve spring seat (upper exhaust). ADDITIONAL INFORMATIONReciprocating Engine Overhaul Necessity Aircraft Powerplants by Thomas W. Wild and Michael J. Kroes discusses reciprocating engine overhaul practices. This document discusses the need for an overhaul of a reciprocating engine and states in part: After a certain number of hours of operation, an engine undergoes various changes which make an overhaul necessary. The most important of these changes are as follows: 1. Critical dimensions in the engine are changed as a result of wear and stresses, thus brining about a decrease in performance, an increase in fuel and oil consumption, and an increase in engine vibration. 2. Foreign materials, including sludge, gums, corrosive substances, and abrasive substances, accumulate in the engine. 3. The metal in critical parts of the engine may be crystalized as a result of constant application of recurring stress. 4. One or more parts may actually fail. ORGANIZATIONAL AND MANAGEMENT INFORMATIONFederal Aviation Administration According to FAA Order 8900.1 Flight Standards Information Management System, 14 CFR Part 91 operators are not required to comply with a manufacturer's entire maintenance program and overhauls are not mandatory. The FAA has published Advisory Circular 43-11 Reciprocating Engine Overhaul Terminology and Standards. This document defines what a major overhaul and top overhaul is and states in part: A major overhaul consists of the complete disassembly of an engine. The overhaul facility inspects the engine, repairs it as necessary, reassembles, tests, and approves it for return to service within the fits and limits specified by the manufacturer's overhaul data. This could be to new fits and limits or serviceable limits. The engine owner should clearly understand what fits and limits should be used when the engine is presented for overhaul. The owner should also be aware of any replaced parts, regardless of condition, as a result of a manufacturer's overhaul data, service bulletin, or an Airworthiness Directive (AD). Top overhaul consists of repair to parts outside of the crankcase and can be accomplished without completely disassembling the entire engine. It can include the removal of cylinders, inspection and repair to cylinders, inspection and repair to cylinder walls, pistons, valve-operation mechanisms, valve guides, valve seats, and the replacement of piston and piston rings. All manufacturers do not recommend a top overhaul. Some manufacturers indicate that a powerplant requiring work to this extent should receive a complete overhaul. Lycoming Engines Lycoming Engines has published the O-320 Series Operator's Manual. This FAA-approved document discusses the inspection requirements for the O-320 series engine and states in part: 400-HOUR INSPECTION. In addition to the items listed for daily pre-flight, 50-hour and 100-hour inspections, the following maintenance check should be made after every 400 hours of operation. Valve Inspection – Remove rocker box covers and check for freedom of valve rockers when valves are closed. Look for evidence of abnormal wear or broken parts in the area of the valve tips, valve keeper, springs and spring seats. If any indications are found, the cylinder and all of its components should be removed (including the piston and connecting rod assembly) and inspected for further damage. Replace any parts that do not conform with limits shown in the latest revision of Special Service Publication No. SSP1776.
A partial loss of engine power due to the failure of the No. 3 valve key (exhaust).
Source: NTSB Aviation Accident Database
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