Kailua-Kona, HI, USA
N8406J
Robinson Helicopter Company R22A
The helicopter was ditched in the ocean after a complete loss of engine power. Post-accident examination of the engine revealed that the #3 cylinder's exhaust valve had failed. Because of mechanical damage and oxidation to the fracture surfaces, the specific cause of the exhaust valve failure could not be precisely determined. However, it was most likely related to overheating and non-uniform seating loads due to the severe wear and bell-mouthing of the valve guide. The bent pushrods found during the examination also indicated some valve sticking in the guides prior to failure that caused excessive loads in the pushrods. Valve stems and guides were normal wear items. Mutual wear increased the stem to guide clearances and exhaust valves, particularly the sodium filled valves utilized in the accident engine, rely on proper clearance to the guide to maintain good heat transfer properties. The most likely scenario for this failure was that the valve-to-guide clearance became so great as to impede heat transfer from the valve to the head through the guide. The valve then overheated as indicated by the decreased stem hardness and radial head cracks found during the metallurgical examination. Off center valve closure loads, due to the bell mouthing, then fractured the overheated and lower strength valve head. In May 1992, the engine manufacturer issued a service bulletin (SB) recommending engine inspections to determine the wear condition of exhaust valve and guides. The SB details procedures to assess the wear on the engine by wobbling the valve in the head. The SB recommended performing this inspection every 300 hours on helicopter engines. Engine logbooks only showed that the SB was complied with twice, once in January 2005, at 1,921.8 hours, and the last time in March 2005, at 2,204 hours total time. The last inspection was only 133.3 hours prior to the accident. The engine manufacturer reported that if performed properly this inspection method would have been successful in detecting excess valve wear. The presence of extensive lead deposits on the valve stem and in the valve spring keeper groove indicated that stem to guide clearances were relatively large allowing the passage of combustion products for an extended period of time. This suggests that the SB inspections were not performed properly.
HISTORY OF FLIGHT On April 23, 2005, at 1700 Hawaii standard time, a Robinson Helicopter Company R22A single engine helicopter, N8406J, sustained substantial damage during an autorotational ditching in the ocean 50 yards off shore near Kailua-Kona, Hawaii. A total loss of engine power preceded the autorotation. The flight instructor and private pilot receiving instrument instruction were not injured. The helicopter was operated by Hawaii Pacific Aviation, Inc., d.b.a. Mauna Loa Helicopters, Kailua-Kona, under the provisions of 14 CFR Part 91 as an instructional flight. Visual meteorological conditions prevailed, and a flight plan had not been filed for the local training flight. The local flight departed the Kailua-Kona Airport (KOA) approximately 10 minutes prior to the accident. According to the operator, the helicopter departed and was climbing through 1,300 feet for 1,500 feet when the engine "suddenly quit." The instructor initiated an autorotation and the helicopter touched down on the ocean. Upon contacting the water, the helicopter rolled to the right and came to rest on its right side submerged in approximately 8 feet of water. The instructor and student exited the helicopter and swam to shore. The instructor reported that after engine start and run-up, they began to taxi to the north ramp for fuel. During the taxi, the instructor noticed a strange vibration for about 20 seconds, but it then went away. No other anomalies were noted. The instructor and student's written statements both indicated that during the climb out the engine rpm dropped to zero and the engine stopped. The student attempted to restart the engine to no avail. The instructor was the flying pilot throughout the flight and autorotation. AIRCRAFT INFORMATION The accident helicopter was equipped with a 160-horsepower Lycoming O-320-B2C engine, serial number L-12940-39A. In May 1992, Lycoming issued Mandatory Service Bulletin (SB) No. 388B, which called for the inspection of the valve stem and valve guide clearance on a number of their engines. According to the SB, failure to comply with the SB could result in engine failure due to excessive carbon build-up between the valve guide and valve stem. The SB noted that the outcome of this build-up was sticking exhaust valves or broken exhaust valves, which result from excessive wear (bell-mouthing) of the exhaust valve guide. In November 2004, the SB was revised to 388C, which, in essence, combined SB No. 388B and a supplement to the SB, and added dimensions for fabrication of a tool to measure valve wobble. The SB recommended that the valve guide clearance be inspected every 300 hours for helicopters, or sooner if valve sticking was suspected. The helicopter accumulated an airframe and engine total time of 2,337.4 hours at the time of the accident. On May 11, 2000, at a total time of 1,678 hours, an entry in the logbook noted that a mechanic "complied with Lycoming SB 388B and reamed exhaust guides to 0.500 on cylinders 2, 3, 4." The last annual inspection was completed on September 30, 2004, at a total time of 1,636.6 hours. On January 26, 2005, at a total time of 1,921.8 hours, the engine underwent a 100-hour inspection; Lycoming Service Bulletin (SB) 388B was noted as having been complied with. On March 25, 2005, at a total time of 2,204.1 hours, 133.1 hours prior to the accident, the engine underwent another 100-hour inspection and the endorsement for that inspection indicated that SB 388C had been complied with. The last inspection, also a 100-hour inspection, took place on April 15, 2005, at a total time of 2,303.4 hours. WRECKAGE AND IMPACT INFORMATION On April 24, 2005, the helicopter was recovered from the ocean to a ramp located at the KOA airport. On April 26, 2005, the National Transportation Safety Board investigator-in-charge (IIC) examined the engine on the ramp at KOA. The Safety Board IIC removed the bottom ignition leads and spark plugs in an attempt to obtain thumb compression on the cylinders. The removal of the #3 cylinder spark plug resulted in metallic debris falling from the spark plug hole. A borescope inspection of the #3 cylinder revealed that the exhaust valve head was not in place and had separated from the valve stem. Chunks of metal (one of which resembled the distorted remains of the valve head) were located within the cylinder head. The #3 piston displayed impact damage on its head. The Safety Board IIC removed the #3 cylinder and metallic debris and retained them for further examination. TESTS AND RESEARCH The #3 cylinder, debris, and pushrods were shipped to the Safety Board Materials Laboratory in Washington, DC, for a more detailed examination. According to the laboratory's factual report, the exhaust valve was fractured with the stem remaining in the cylinder head. The outboard end of the cylinder, the intake valve, springs, seats, keepers, and rocker arm were intact and appeared assembled normally. Although the exhaust valve was fractured, the springs, spring seats, keepers, rocker arm, and rotator cap remained assembled in their normal positions and appeared undamaged. The valve contacting faces of the rocker arm were polished and worn in the normal contact regions. The rocker arm contact face of the exhaust valve rotator cap was worn and dished in the center, removing no more than 0.005-inch of material. Both valve push rods were intact but closer examinations determined the exhaust push rod was noticeably bent while the intake push rod was deformed only slightly. Though the combustion chamber was covered with heavy layers of dark deposits and was locally dented and deformed, it was determined that the mechanical damage appeared to be consistent with repeated contact with the separated exhaust valve head. The intake valve was in place and undamaged as was the exhaust valve seat. The dark deposits were determined by energy dispersive analysis (EDS) to be primarily lead based with significant amounts of bromide, consistent with combustion products. With the springs removed, significant play was present between the exhaust valve stem and the valve guide in the head. The play was notable in all directions but noticeably greater parallel to the long axis of the rocker arm. The exhaust valve assembly was disassembled with little difficulty and the valve stem was easily removed from the head for closer examination. Measurements of the inner diameter of the valve guide established that it was ovalized (bell-mouthed) parallel to the long axis of the rocker arm, which measured 0.5200-inch at the inboard end (combustion chamber side) and 0.5122-inch at the outboard end (rocker arm side). Ninety degrees to the long rocker arm axis the inboard side of the valve guide measured 0.5047-inch and the outboard end measured 0.5033-inch. Because of the valve fracture and the lack of specialized fixtures, the condition of the valve and guide could not be determined in the manner recommended in Lycoming SB No. 388C. The valve stem had a minimum outer diameter of 0.4907-inch measured at 1.8 inches from the rocker arm end. The outer diameter of the stem increased to a maximum of about 0.4935-inch as the stem progressed toward the rocker arm end. The as-manufactured stem diameter was specified to be between 0.4938- and 0.4945-inch. The cylinder and valve guide did not contain the marking and features that would indicate that "improved Hi-Chrome" valve guides had been installed. The stem of the exhaust valve had faint markings "74541" on the outer diameter surface above the keeper slot. Lycoming reports that "74541" was an obsolete exhaust valve part number that was superceded by improved valves in 1984 (part number 19001). The manufacturer specified that "74541" was a sodium filled exhaust valve with a chromium plated stem. The stem and head material consisted of a high temperature stainless steel material with a nickel base chromium tungsten alloy weld overlay on the valve face. The exhaust valve was fractured through the stem adjacent to the head, and the stem surfaces were darkened from the fracture to the keeper groove near the tip. Thick deposits were present on the stem adjacent to the fracture with thinner, rubbed deposits and some corrosion apparent on the rest of the stem. Energy dispersive spectrometry (EDS) established that the deposits were primarily lead based with significant amounts of bromide, very similar to those found in a combustion chamber. The stem base material was consistent with that specified by the manufacturer. However, no visual, scanning electron microscope (SEM), or EDS evidence of the required chromium plating was found on the stem. Hardness testing on a flat around the length of the valve stem showed a pronounced softening in the stem at the fractured head end when compared to the manufacturer's specified hardness values. The hardness value dropped rapidly near the valve fracture area. The valve head was further fragmented into seven pieces, with the outside diameter edge of the head pieces containing a series of radial cracks all in the seat weld overlay region of the valve seat. A metallographic section through the edge of the valve head revealed an extensive network of large cracks in the stem side (seat area) of the head. The large crack network was mostly confined to the seat overlay material with some penetration into the base material. EDS spectra of the weld overlay regions on the seat and valve face were consistent with the specified materials. The fracture through the stem was adjacent to the head in the tubular area that forms the sodium chamber. Both fracture faces were severely distorted and mechanically deformed in addition to being covered by combustion deposits. SEM examination of both fracture faces found extensive mechanical damage that completely obliterated the head side fracture face with no original facture surfaces remaining. The stem side fracture displayed severe oxidation with no recognizable fracture features. SEM viewing also uncovered globules of resolidified lead on the fracture face and the adjacent outer diameter surface of the stem. ADDITIONAL DATA The purpose of the valve guides is to support the side forces operating on the valve stem. The guide also centers the valve on the valve seat and conducts a part of the heat from the valve head via the valve stem to the cylinder head. Valve stems and guides are normal wear items. Mutual wear increases the stem to guide clearances and exhaust valves, particularly sodium filled valves, rely on proper clearance to the guide to maintain good heat transfer properties. According to a Lycoming newsletter, "the engine will almost always provide a warning by running very rough at start up. As the engine warms up it may then smooth out after a few seconds and run normally, but the initial roughness is a warning that preventive maintenance action is required." The newsletter also provides recommended maintenance and operational procedures to avoid sticking valves. The failed valve is at present an obsolete design that was replaced in production and spares with an improved design in 1984. Maintenance records indicate that the engine was manufactured in February 1983; therefore, the inclusion of this part number valve was reasonable at the time. The records further show that the engine had not been overhauled since new, having accumulated 2,328.1 hours since new in the last 22 years. On average, the engine was operated 106 hours per year, however, from October 2004 (total time of 1,636 hours) to the accident date (7 months), the engine had been used almost 100 hours per month. Lycoming Service Instruction (SI) 1009AR, issued June 22, 2004, recommends overhauling O-320-B series of engines at least every 2,000 hours. The maintenance records for this engine denote the recommended overhaul period to be 2,200 hours. The SI 1009AR also recommends overhauling engines every 12 years if the time between overhaul (TBO) service hours are not met in that time. Under this guidance, this engine would be approaching its second overhaul.
a total loss of engine power due to the failure of the engine's #3 exhaust valve due to an excessive valve guide clearance. Contributing to the accident was the failure of maintenance personnel to adequately check the valve guide clearance during the last valve guide inspection.
Source: NTSB Aviation Accident Database
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