# NTSB CEN21LA213: Beech C23, N793Y, Merrill, WI, May 11, 2021 - Type: NTSB accident investigation - Classification: Accident · Minor injuries · Class 3 · Limited · Final report - Date: May 11, 2021, 15:00 local (20:00 UTC) - Place: Merrill, WI - Coordinates: 45.242042, -89.613289 - Nearby events: https://openairdocs.org/map.md?lat=45.2420&lon=-89.6133&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Defining event: [Enroute-descent: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) - Highest injury: Minor injuries - Injuries: 2 minor - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Central Region - Parties: Federal Aviation Administration, Lycoming - NTSB case key (mKey): 103071 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN21LA213 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/103071/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=103071 - Page: https://openairdocs.org/ntsb/CEN21LA213 ## Probable cause The loss of engine power due to tappet failure. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Engine (reciprocating): recip eng cyl section](https://openairdocs.org/ntsb-findings/01058530): Aircraft › Aircraft power plant · Failure; cases with it: [Beech C23, 1 case](https://openairdocs.org/ntsb-findings/01058530/cases.md?model=1151242), [All aircraft, 373 cases](https://openairdocs.org/ntsb-findings/01058530/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Enroute-descent: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) (defining event); cases with it: [Beech C23, 7 cases](https://openairdocs.org/ntsb-events/342/sequence.md?model=1151242), [All aircraft, 1,858 cases](https://openairdocs.org/ntsb-events/342/sequence.md) ## Analysis The pilot reported that he was returning to the airport after a 1-hour flight when the engine started to run rough and gradually lost power. The pilot switched fuel tanks and cycled the magnetos, with no change in the power loss. He executed an off airport forced landing. The landing gear collapsed during the landing and the airplane slid to a stop coming to rest upright. The airplane sustained substantial damage to both wings and fuselage. Examination of the engine found that the oil filter contained metallic particles that were determined to be consistent with tappet material. Further examination revealed 7 of the 8 tappets were destroyed. A metallurgical examination of the tappets revealed that the tappets fractured from overstress. The fracture started along the heads facing their corresponding camshaft lobes, which proceeded towards the back surface. While there were no indications of pre-existing cracks, such as from fatigue, there was evidence of pitting and surface wear on many of the tappet head faces that contact the camshaft. Of note was the No.2 exhaust tappet, the only tappet that underwent wear on the tappet face significant enough to change its geometry. The engine was overhauled 19 years before the accident and had accumulated 838 hours since the overhaul. The engine manufacturer recommends overhaul of the engine at 2,000 hours, or 12 years in service, whichever comes first. No record of tappet replacement was found in the maintenance records. ## Factual narrative On May 11, 2021, at 1500 central daylight time, a Beech C23, N793Y, was substantially damaged when it was involved in an accident near Merrill, Wisconsin. The pilot and passenger received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he was returning to the airport after a 1-hour flight. When the airplane was about 5 miles from the airport and descending through about 1,300 ft above ground level, the engine started to run rough and gradually lost power. The pilot switched fuel tanks and cycled the magnetos, with no change in the engine power. He executed an off-airport forced landing. The airplane touched down, the landing gear collapsed, and the airplane slid to a stop coming to rest upright. The airplane sustained substantial damage to both wings and fuselage. Examination of the engine found that the oil filter contained metallic particles consistent with tappet material. Further examination revealed 7 of the 8 tappets were destroyed. The tappets were removed for further examination. Examination of the tappets by the National Transportation Safety Board Materials Laboratory revealed that the tappets fractured from overstress. The fracture started along the heads facing their corresponding camshaft lobes, which proceeded towards the back surface. While there were no indications of pre-existing cracks, such as from fatigue, there was evidence of pitting and surface wear on many of the tappet head faces that contact the camshaft. Of note was the No. 2 exhaust tappet, the only tappet that underwent wear on the tappet face significant enough to change its geometry. Figure 1 shows the No. 2 exhaust tappet face, which was reflective and consistent with polishing from repeated contact with the mating camshaft lobe. The depression at the center of the tappet head corresponding to the stem exhibited circular wear marks. The fracture surface on the bottom of the figure was circular, conforming to the geometry of the stem radius. Figure 2 shows the widespread spalling of the remaining surface, demonstrating the degree of material loss on this part. Review of the engine logbooks revealed that the engine was overhauled on February 19, 2002, and installed on the accident airplane on February 1, 2019. The last annual inspection was completed on February 24, 2021. At the time of the accident, the engine had about 838 hours of operation since overhaul. No record of tappet replacement was found in the maintenance records. The engine manufacturer recommends overhaul of the engine at 2,000 hours, or 12 years in service, whichever comes first. An oil sample collected on May 11, 2021, before the accident flight, was sent in for analysis. The vendor who conducted the oil sample analysis provided an oil sample report. Nothing remarkable was found in the oil sample report. Figure 1. View of the #2 exhaust tappet head. Figure 2. Worn areas of the #2 exhaust tappet head, showingspalling and pitting. ## Aircraft - Registration: [N793Y](https://openairdocs.org/aircraft/N793Y), registered now, S/N M-2184, built 1979 (OpenAirDocs) - Reported make and model: BEECH C23 - Registry model: [Beech C23](https://openairdocs.org/model/beech-c23) (registry code 1151242) (OpenAirDocs) - Registry engine: [Lycoming O&VO-360 series](https://openairdocs.org/engine/lycoming-o-vo-360-series) (registry code 41514, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: M-2184 - Year built: 1979 - Airframe hours: 4419.3 - Engines: Lycoming O-360-A2G reciprocating - Damage: Substantial damage - Flight: Personal, Part 91 - Registered owner: Park City Flight Training - Airworthiness certificate: Standard, Normal - Last inspection: 100 hour, February 24, 2021, 55.7 hours before - Airport: RRL, Merrill Municipal Airport - Equipment: Electronic primary flight display, Auto-pilot, ADS-B, Stall warning system ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 60; injuries: none; private; airplane ratings: single-engine land; medical: class 3, with waivers/limitations; last flight review April 15, 2021; seat: left; restraint: 3-point; flight hours: 233 total, 136 in this make and model, 77 as pilot in command, 25 in the last 90 days, 9 in the last 30 days, 1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 20° at 3 knots, gusting 14 - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 55 °F, dew point 25 °F - Altimeter: 30.31 inHg - Observed at: RRL, 5 nm away ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20210511008299A](https://openairdocs.org/incident/20210511008299A), May 11, 2021: Beech C23, Merrill, WI, Substantial damage. On may 11, 2021 at approximately 1530 local time a Beech aircraft C23 sundowner registration number N793Y with the pilot and one passenger departed on a part… ## Docket (8 files) The files NTSB released with the investigation on August 15, 2023, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=103071. 57.1 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 9 pages, 5.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12317053&FileExtension=pdf&FileName=6120_Redacted-Rel.pdf - On scene photos (item 2, PDF, 3 pages, 1.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14235820&FileExtension=pdf&FileName=on%20scene%20photos-Rel.pdf - Engine examination summary (item 3, PDF, 3 pages, 108 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15823068&FileExtension=pdf&FileName=CEN21LA213%20Engine%20Field%20Notes-Rel.pdf - Materials laboratory factual report (item 4, PDF, 26 pages, 30 photos, 6.8 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15245567&FileExtension=pdf&FileName=23-026%20-%20Materials%20Lab%20Factual%20Report%20(CEN21LA213)-Rel.pdf - Maintenance records (item 5, PDF, 3 pages, 7.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15311028&FileExtension=pdf&FileName=2002Overhaul_Redacted-Rel.pdf - Maintenance 2-year history (item 6, PDF, 15 pages, 35.8 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15311286&FileExtension=pdf&FileName=2yearsMX_Redacted-Rel.pdf - Memorandum for record a�� telephone interview, maintenance (item 7, PDF, 1 page, 110 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15713905&FileExtension=pdf&FileName=Record%20of%20Conversation_Redacted-Rel.pdf - Oil sample report (item 8, PDF, 1 page, 192 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15748446&FileExtension=pdf&FileName=N793Y%20Oil%20Sample%20Report_Redacted-Rel.pdf From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.