Holden, UT, USA
N204BF
Czech Sport Aircraft Sportcruiser
During a night flight, the airplane was at 11,000 ft mean sea level when the engine sputtered and lost total power. The propeller would not windmill, and the engine would not restart. Due to the dark night conditions, the pilot could not identify a safe forced landing area, so she deployed the ballistic recovery system (BRS) about 700 ft above ground level. The left-rear steel suspension cable, one of the two cables and two fabric straps attaching the BRS parachute to the airplane, separated in overload when it caught on a bolt during the BRS deployment. This resulted in an improper deployment of the BRS before the airplane touched down in a left-wing low attitude which substantially damaged the wing. Examination of the engine revealed the No. 2 intake valve spring retainer was broken, and the intake valve had fallen into the No. 2 combustion chamber, which resulted in the loss of power. The No. 2 valve spring shim exhibited a worn appearance with a groove visible around the shim. According to the engine manufacturer, worn valve spring shims are a clear sign of an engine operating with air in the oil system. This valve spring retainer failure was not the first one with a Rotax 900 series engine; in 2017, at the end of a cross-country flight, an airplane powered with the same series engine experienced a total loss of engine power, and the pilot performed a forced landing during which the airplane sustained substantial damage. Examination of that engine revealed the presence of a broken valve spring retainer that had resulted in the loss of power. Additionally, it was discovered that the valve spring retainer displayed evidence of metal fatigue. During 2019 and 2020, in addition to this accident, three more cases of broken valve spring retainers on the Rotax 900 engine series occurred in the United States. All the engines had differing hours of operation. Extensive metallurgical examination of the engine components from these four engines revealed that they met their specifications, and the fractured surfaces on the valve spring retainers revealed the presence of fatigue with pronounced vibration stripes, which was the same pattern that was observed on the valve spring retainer from the 2017 accident. Review of the engine manufacturer’s published guidance revealed that air could be introduced into the oil lubrication system through several means, including exceedance of the maximum bank angle of 40º, poorly or insufficiently vented hydraulic valve tappets, lack of proper oil system purging, spinning the propeller in the reverse direction from normal rotation, or opening portions of the oil system during maintenance or servicing. Testing of an exemplar engine with air introduced into the lubrication system revealed that with air trapped in the hydraulic tappets, it took about 6.5 minutes of engine operation at 2,538 rpm for air to be purged from the tappets allowing them to work as designed. This indicated that with air trapped in the hydraulic tappets, the valve train could be overloaded, which could lead to a fatigue crack and breakage of a valve spring retainer; this was likely the reason for the fatigue cracking of the valve spring retainers in the 2017 accident, in this accident, and in the other four 2019-2020 engine failures. During the investigation, the engine manufacturer reviewed its records and found a total of 18 production engine failures due to broken valve spring retainers. The engines were installed on multiple types of aircraft with a large spread in operating hours from as low as 7 hours to as high as 1,936.6 hours. All the components examined met their specifications, and not all the engines were affected by service bulletins that had been issued due to deviations in the manufacturing process of the valve push-rod assembly, which could result in partial wear on the rocker arm ball socket and lead to rocker arm cracking leading to a malfunction of the valve train. These engine failures indicated that valve train failure could occur for reasons other than the push-rod manufacturing issue such as air being introduced into the lubrication system. Additionally, after the engine manufacturer’s record review, an engine in an airplane that was produced in 2021, which should have had all changes included in Rotax guidance materials incorporated before it was placed into service, experienced a valve spring retainer failure, confirming that valve train failure could occur for reasons such as air being introduced into the lubrication system.
HISTORY OF FLIGHTOn October 22, 2019, about 1950 mountain daylight time, a Czech Sport Aircraft Sportcruiser airplane, N204BF, was substantially damaged when it was involved in an accident near Holden, Utah. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that during a night flight, the airplane was at 11,000 ft mean sea level when the engine sputtered and then experienced a total loss of power. She noted that the propeller was not windmilling, and her attempts to restart the engine were unsuccessful. Due to the dark night conditions, the pilot could not identify a safe forced landing area, so about 700 ft above ground level, she deployed the airplane's ballistic recovery system (BRS). She stated that the parachute jolted the airplane up and to the left and then to the right before impact with the ground. AIRCRAFT INFORMATIONThe airplane was a single-engine, all metal, low-wing monoplane of semi-monocoque construction with two side-by-side seats. It was equipped with a fixed tricycle undercarriage with a castering nosewheel. The airplane was powered by an American Society for Testing and Materials-compliant, 4-cylinder, horizontally opposed, 100-horsepower, Rotax 912 ULS 2 engine. The engine used a single central camshaft with hydraulic tappets. The cylinder heads were liquid cooled, and the cylinders were ram air cooled. The oil system was a dry sump, forced lubrication system. The engine used a reduction gearbox to drive the three-bladed, ground-adjustable, composite Sensenich propeller. According to maintenance records, the airplane was manufactured in 2017. The airplane’s most recent 100-hour inspection was completed on October 4, 2019. At the time of the inspection, the airplane had accrued 1,689.8 hours of operation. During the 100-hour inspection, the oil filter was opened and examined; no contaminants were found in the oil filter. AIRPORT INFORMATIONThe airplane was a single-engine, all metal, low-wing monoplane of semi-monocoque construction with two side-by-side seats. It was equipped with a fixed tricycle undercarriage with a castering nosewheel. The airplane was powered by an American Society for Testing and Materials-compliant, 4-cylinder, horizontally opposed, 100-horsepower, Rotax 912 ULS 2 engine. The engine used a single central camshaft with hydraulic tappets. The cylinder heads were liquid cooled, and the cylinders were ram air cooled. The oil system was a dry sump, forced lubrication system. The engine used a reduction gearbox to drive the three-bladed, ground-adjustable, composite Sensenich propeller. According to maintenance records, the airplane was manufactured in 2017. The airplane’s most recent 100-hour inspection was completed on October 4, 2019. At the time of the inspection, the airplane had accrued 1,689.8 hours of operation. During the 100-hour inspection, the oil filter was opened and examined; no contaminants were found in the oil filter. WRECKAGE AND IMPACT INFORMATIONExamination revealed that the airplane sustained substantial damage to the left wing and left aileron. The left-rear metal suspension cable for the BRS parachute fractured and separated near the cockpit canopy’s left hinge retention bolt. The cable separation exhibited a frayed or broom-straw appearance consistent with overload separation from contact with the hinge retention bolt. The right-rear metal suspension cable and the two forward suspension straps remained intact. According to the manufacturer, the BRS performed as expected. No other anomalies were found with the airframe that would have precluded normal operation. Examination of the engine, serial number (S/N) 9569290, revealed the No. 2 cylinder intake valve spring retainer had fractured into two pieces, which were found in the rocker box along with fragments of the intake valve guide. The No. 2 valve spring shim exhibited a worn appearance with a groove visible around the shim. The No. 2 intake valve had fallen into the cylinder, and it was bent into an “S”shape and embedded in the top of the cylinder head. The top of the No. 2 piston was damaged. The No. 2 piston wrist pin had separated from the piston and lodged between the crankshaft and the No. 2 cylinder bore. According to the engine manufacturer, BRP-Rotax GmbH & Co. KG (BRP-Rotax), worn valve spring shims are a clear sign of an engine operating with air in the oil system. Figure 1 is an illustration of the valvetrain components. Figure 1. Illustration of components of the engine valvetrain. Examination with an electron microscope by BRP-Rotax revealed the fracture area of the No. 2 intake valve spring retainer had a fatigue break with pronounced vibration stripes. The breakage exit area was close to the upper edge (outer bore), but the exact breakage exit was in a destroyed or damaged condition. See Figure 2. Figure 2. Image showing the number two valve spring retainer and the worn valve spring shim after removal from the engine. ADDITIONAL INFORMATIONAt the request of the NTSB, BRP Rotax reviewed its records and advised that they had identified a total of 18 production engine failures due to broken valve spring retainers for 900 series engines produced between February 2015 and February 2019. The failures occurred with engines installed on multiple types of aircraft, and the failures occurred over a large spread in operating hours from as low as 7 hours to as high as 1936.6 hours. All components examined at the Rotax factory met their specifications. Not all the engines were affected by or complied with Service Bulletin SB-912 i-008 R1 / SB-912-070-R1 / SB-914-052 R1, which was originally issued due to deviations in the manufacturing process of the valve push-rod assembly that could result in partial wear on the rocker arm ball socket. This wear could lead to rocker arm cracking / fracture and subsequent malfunction of the valve train. Icon Airplane Valve Spring Retainer Failure On August 10, 2021, the NTSB was notified of another valve spring retainer failure on a Rotax 912S engine (S/N 7705135) that was installed in an Icon A5 airplane, N639BA. The engine was manufactured in 2021 and should have had all changes that were addressed in previous Rotax guidance materials complied with before being placed into service. The airplane was in cruise flight at a power setting of about 5,350 rpm when the pilot felt the engine vibrating. The exhaust gas temperature (EGT) for cylinder No. 1 began to steeply drop, and the engine rpm dropped to 4,820 rpm without throttle reduction by the pilot. About 2 seconds later, the EGTs for cylinders Nos. 2 and 4 began to drop. Shortly thereafter, the engine lost total power. The pilot then tried twice to restart the engine without success. The pilot made an uneventful forced landing. Post incident examination revealed that the No. 1 cylinder exhaust valve spring retainer was broken in half. Half of the valve spring retainer was discovered in the rocker box cover, and the other half was found jammed between the cylinder head and the exhaust rocker arm. The No.1 exhaust valve was found severed, and the No.1 piston was impact-damaged. Corrective Actions As a result of these occurrences, to increase safety, these organizations took the following actions: BRP Rotax o Revised Service Bulletin SB-912 i-008 R1/ SB-912-070 R1/ SB-914-052 R1 to include a specific venting procedure for the oil system. (Now SB-912 i-008R2/ SB-912-070R2/ SB-914-052R2.) o Revised Service Instruction SI-915 i-003/ SI-912 i-004R1/ SI-912-018R2/ SI-914-020R2 to help preclude lack of proper oil purging after an engine had been first installed and /or an engine had been re-worked, and to help to prevent engine failures in the field, as air could be trapped in the valve tappets and cause valve train failure. (Now SI-916 i B-003/ SI-915 i-003R1/ SI-912 i-004R2/ SI-912-018R3/ SI-914-020R3.) o All future instructions for continued airworthiness (service bulletins, service instructions, and alert service bulletins) will provide direct references to instructions found in other documents that pertain to the required procedures. o Notified their distributers of the publication of Service Instruction SI-916 i B-003/ SI-915 i-003R1/ SI-912 i-004R2/ SI-912-018R3/ SI-914-020R3 and encouraged them to inform their customers proactively and to encourage original equipment manufacturers (OEMs) to also distribute the information relating to air in the lubrication system in documents issued by the OEM to significantly improve the chance to reach the end customer with the information. They also asked that their distributors ensure that all OEMs in their regions understand the importance of the revised service instructions, check their relevant instructions for continued airworthiness (ICAs) for possible checks and required changes, and have their aircraft customers, operators, and maintenance technicians made aware and informed about it. Additionally, they further asked their distributers to transmit the relevant ICAs to all their service centers, OEMs, retail sellers, flying schools, flying clubs, authorities, and press, for accomplishment or information. o Began a process to improve the materials and dimensions of the valve spring retainers and cotters to make the valve train system more robust. Rotech Flight Safety o Distributed Service Bulletin SB-912 i-008R2/ SB-912-070R2/ SB-914-052R2 on the Rotax-owner website, advising that the new revisions included instructions on purging the oil system after the work was completed. A video clarifying purging of the lubrication system was also included. o Distributed Service Instruction SI-916 i-003R1 / SI-915 i-003R2 / SI-912 i-004R3 / SI-912-018R4 /SI-914-020R4 on the Rotax-owner website to provide further guidance for the lubrication system with respect to purging and venting and to avoid air in the lubrication system. They also advised that the service instruction should help to avoid engine failures in the field, as air can be trapped in the valve tappets and cause valve train failure, and it is very important to complete these instructions in their entirety. Icon Aircraft o Issued Service Letter SL-081221-A to provide awareness that air entering the engine lubrication system could lead to potential failure of valvetrain components, and that following the correct procedures when performing any installation, maintenance, repair, and overhaul activities on the engine has been shown to minimize the occurrence of this situation. Additionally, the service letter advised that certain uncoordinated or unloaded flight maneuvers should be avoided as they can lead to air entering the lubrication system, and that one such incident in an Icon A5 resulted in loss of engine power inflight and an emergency landing. TESTS AND RESEARCHAccident with N561TU The National Transportation Safety Board (NTSB) first became aware of valve spring retainer fracturing issues with Rotax 900 series engines in 2017 due to an accident that occurred in Stevensville, Maryland, with a Tecnam P92 airplane, N561TU, that was powered by a Rotax 912 ULS2-01 engine, S/N 9569084 (NTSB Case No. ERA17LA246). In this accident, the airplane experienced a total loss of engine power at the end of a cross country flight, and the pilot performed a forced landing during which the airplane sustained substantial damage. The airplane had recently been purchased, and the engine had 13.2 hours total operating time. Review of onboard data indicated that the fuel pressure, cylinder head temperature, and oil temperature remained relatively steady until the loss of power occurred, which indicated that the engine failure likely did not involve the fuel system, cooling system, or lubrication system. Examination of the engine revealed that there was no oil in the oil line between the oil thermostat and oil pump. The oil pump drive pin also displayed excessive wear in relation to the operating hours of the engine, and the magnetic plug was covered in metallic particles, although the oil filter was clean. Further examination of the engine revealed that the No. 1 cylinder was damaged, and evidence of bluing was present. The cylinder’s exhaust valve spring retainer was fractured in half, and one half of the cotter was fractured. A small ridge could be felt on the exhaust valve spring retainer and galling (a rough surface) was visible on the exhaust valve bore in the cylinder head. The hydraulic lifter for the exhaust valve displayed a small indentation on the edge of the lifter, and when the hydraulic lifters were manually depressed, the lifter for the exhaust valve was easier to depress than the lifter for the intake valve. The pushrod for the exhaust valve was straight but displayed a ridge on the rocker arm side of the pushrod, and the rocker arm displayed impact damage on the valve connection face. The exhaust valve was found in the combustion chamber. It was chipped, and bent, and deformed into an “S” shape. A hole was visible in the piston as the result of the piston face striking the exhaust valve after it dropped into the cylinder. A small amount of oil captured from the hydraulic tappets indicated that the oil contained significantly elevated levels of nickel, which could have come from manganese-containing alloys, as they occur in high-alloy hardened steels, e.g., for camshafts, valves, or valve shafts. Examination of the fractured surface on the exhaust valve spring retainer revealed the presence of fatigue with pronounced vibration stripes when viewed with an electron microscope; however, the heat treatment corresponded to the target specifications, as did the statistical process control value. According to the NTSB’s final report on the accident, the root cause of the failure could not be determined based on the available information. Additional Valve Spring Retainer Fractures In 2019 and 2020, another four valve spring retainer fractures occurred in the United States involving the following aircraft: N1PJ, N204BF (this case), N117B, and N562TU (NTSB Case No. ERA20LA341). Examinations of the damaged engines revealed: o S/N 4421750 (N1PJ), intake valve failure, broken valve spring retainer cylinder No. 2 o S/N 9569290 (N204BF), intake valve failure, broken valve spring retainer, cylinder No. 2 o S/N 9569271 (N117BF), intake valve failure, broken valve spring retainer, cylinder No. 2 o S/N 9569181 (N562TU), exhaust valve failure, broken valve spring retainer, cylinder No. 1 All the engines had differing hours of operation; however, all experienced a valve spring retainer failure during engine operation. At the request of the NTSB, numerous components from the four engines were shipped by Rotech Flight Safety to the Austrian Federal Safety Investigations Authority (BMK) for examination and testing at the engine manufacturer’s factory in Gunskirchen, Austria. Extensive metallurgical examination of the intake and exhaust valves, valve spring retainers, valve springs, valve tappets, pushrod assemblies, pistons, cylinder heads, valve cotters, and camshafts was conducted. The results of the examinations were similar to those from the examination of the engine components from the 2017 accident with N561TU. All the parts met their specifications, and the fractured surfaces on the exhaust valve spring retainers revealed the presence of fatigue with pronounced vibration stripes. Review of Published Guidance Review of Rotax 900 series operators manuals indicated that the dry sump lubrication system would provide sufficient lubrication up to a maximum bank angle of 40º. The engines were also limited to a maximum of 5 seconds of operation at -0.5 G. A limited review revealed that about 463 aircraft models used Rotax 900 series engines. These included plans-built aircraft, kit aircraft, and certificated manufactured aircraft. Review of published guidance materials from some of these manufacturers revealed that the Rotax engine bank angle G limitations were not published in the flight manuals or pilot’s operating handbooks, and in many cases, the maximum published bank angle limitation for the aircraft was 60º, which exceeded the Rotax published limitation. Review of the Rot
The fatigue failure of the No. 2 cylinder intake valve spring retainer due to air trapped in the lubrication system, which resulted in a total loss of engine power. Contributing to the severity of the damage was the improper deployment of the ballistic recovery system.
Source: NTSB Aviation Accident Database
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