# NTSB CEN24LA038: Bede BD-4, N931AS, Lowell, IN, November 14, 2023 - Type: NTSB accident investigation - Classification: Accident · Minor injuries · Class 3 · Limited · Final report - Date: November 14, 2023, 13:53 local (19:53 UTC) - Place: Lowell, IN - Coordinates: 41.17, -87.25 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=41.1700&lon=-87.2500&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Defining event: [Enroute-cruise: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338) - Highest injury: Minor injuries - Injuries: 1 minor - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Central Region - Parties: Civil Aviation Authority Accidents and Incidents Investigations, FAA - NTSB case key (mKey): 193402 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN24LA038 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/193402/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=193402 - Page: https://openairdocs.org/ntsb/CEN24LA038 ## Probable cause Fatigue cracking of the propeller blade root, which resulted in the in-flight separation of the propeller blade from its root. Contributing to the accident was the improper machining of the blade root threads, which resulted in chatter/ratchet marks that weakened the strength of the blade root. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Propeller system: propeller blade section](https://openairdocs.org/ntsb-findings/01046111): Aircraft › Aircraft propeller/rotor · Failure (probable cause); cases with it: [Alfred C Skinnell BD-4 EXPERIMENTAL CL, 1 case](https://openairdocs.org/ntsb-findings/01046111/cases.md?model=05632A2), [All aircraft, 51 cases](https://openairdocs.org/ntsb-findings/01046111/cases.md) - [Propeller system: propeller blade section](https://openairdocs.org/ntsb-findings/01046111): Aircraft › Aircraft propeller/rotor · Fatigue/wear/corrosion (probable cause); cases with it: [Alfred C Skinnell BD-4 EXPERIMENTAL CL, 1 case](https://openairdocs.org/ntsb-findings/01046111/cases.md?model=05632A2), [All aircraft, 51 cases](https://openairdocs.org/ntsb-findings/01046111/cases.md) - [Maintenance: fabrication](https://openairdocs.org/ntsb-findings/02062025): Personnel issues › Task performance · Owner/builder (probable cause); cases with it: [Alfred C Skinnell BD-4 EXPERIMENTAL CL, 1 case](https://openairdocs.org/ntsb-findings/02062025/cases.md?model=05632A2), [All aircraft, 30 cases](https://openairdocs.org/ntsb-findings/02062025/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Enroute-cruise: Powerplant sys/comp malf/fail](https://openairdocs.org/ntsb-events/340); cases with it: [Alfred C Skinnell BD-4 EXPERIMENTAL CL, 1 case](https://openairdocs.org/ntsb-events/340/sequence.md?model=05632A2), [All aircraft, 820 cases](https://openairdocs.org/ntsb-events/340/sequence.md) - [Enroute-cruise: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338) (defining event); cases with it: [Alfred C Skinnell BD-4 EXPERIMENTAL CL, 1 case](https://openairdocs.org/ntsb-events/338/sequence.md?model=05632A2), [All aircraft, 563 cases](https://openairdocs.org/ntsb-events/338/sequence.md) - [Landing: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [Alfred C Skinnell BD-4 EXPERIMENTAL CL, 1 case](https://openairdocs.org/ntsb-events/440/sequence.md?model=05632A2), [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md) - [Landing: Roll over](https://openairdocs.org/ntsb-events/097); cases with it: [Alfred C Skinnell BD-4 EXPERIMENTAL CL, 1 case](https://openairdocs.org/ntsb-events/097/sequence.md?model=05632A2), [All aircraft, 261 cases](https://openairdocs.org/ntsb-events/097/sequence.md) ## Analysis About 25 minutes after takeoff, the engine started to run rough and shake violently. The pilot performed an off-airport landing in a soybean field. Upon touchdown, the airplane flipped inverted, which resulted in substantial damage to the fuselage. When the pilot exited the airplane, he discovered one of the propeller blades was missing. The pilot had installed the propeller about 18 months before the accident but had to replace the blades after 12 hours of service because the propeller blades were backing out of the hub. The manufacturer sent the pilot three new blades, which contained retention pins. Metallurgical examination of the blade root of the missing propeller blade revealed multiple fatigue cracks that initiated at the inner surface of the blade root where thread roots intersected a radial hole for the retention pin; the radial hole for the retention pin was located where the composite section of the propeller blade integrated with the metallic root. The darker appearance of the thumbnail-shaped region and curved bands of progression marks emanating from the cracks suggest they had initiated and had been growing before the accident flight. Chatter and ratchet marks, most likely a result of machining, were seen on the threads of the retention pin holes. A cross-section examination of the remaining two propeller blade roots revealed similar fatigue cracks emanating from the retention pin holes. The cracks were not visible from the surface and therefore would not be seen during routine propeller inspections. It is likely that the combination of the chatter/ratchet marks and operational vibrations resulted in fatigue cracking and ultimately overstress failure, which caused one of the propeller blades to separate at the blade root. ## Factual narrative On November 14, 2023, about 1353 central standard time, a Bede Corp BD-4 C, N931AS, was substantially damaged when it was involved in an accident near Lowell, Indiana. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. About 25 minutes after departure, the engine began to run rough and shake violently. The pilot performed an off-airport landing in a soybean field. Upon touchdown, the airplane flipped over, resulting in structural damage to the fuselage. After exiting the airplane, the pilot discovered that one of the propeller blades was missing. The root of the missing propeller blade was still inside the hub. The missing blade was never found. The airplane was equipped with an AXSport Aviation AX3-70 propeller. The propeller was advertised as an experimental 3-blade electric constant speed/variable pitch propeller designed for non-type-certified aircraft. The blades were carbon fiber reinforced epoxy composite structures integrated with an aluminum alloy root section that were assembled into an aluminum alloy hub. The root of the missing propeller blade, propeller hub, and remaining 2 propeller blades were examined postaccident. The metallurgical examination of the propeller blade root from the missing propeller revealed dark gray thumbnail-shaped regions and cracks that originated at the holes for the retention pins—specifically, the inner surface where the thread roots intersected the retention pin holes. Chatter marks, wavy surface imperfections usually caused by vibrations transferred from the tool-work interface during the machining process, were observed on the flanks (straight side) of the threads. Ratchet marks, slight vertical steps within a fracture, correlated with the presence of chatter marks. One of the remaining intact propeller blades was not damaged on the surface, and the other propeller blade exhibited partial chordwise cracks and spanwise separation consistent with ground impact. Visual inspection of both blade roots did not reveal any visible cracks. However, both blade roots were cross-sectioned during the metallurgical examination and several flat crack profiles were observed, extending toward the outer surface from the threads of the retention pin holes. Maintenance logbook records revealed that the propeller was installed in May of 2023. In August of 2023, all three propeller blades were replaced after only 12 hours of service. The owner stated that the original blades did not have retention pins, and the blades were backing out of the hub. The manufacturer sent the owner three new blades that contained retention pins. The new blades were installed, and the propeller was balanced, in accordance with the operator’s manual, to a value of 0.02 in per second. The blade failure occurred after about 33 hours of service. ## Aircraft - Registration: [N931AS](https://openairdocs.org/aircraft/N931AS), registered now, S/N 1359B, built 2015 (OpenAirDocs) - Reported make and model: Bede Corp BD-4 (C) - Registry model: [Alfred C Skinnell BD-4 EXPERIMENTAL CL](https://openairdocs.org/model/alfred-c-skinnell-bd-4-experimental-cl) (registry code 05632A2) (OpenAirDocs) - Registry engine: [Lycoming O-360-A1A](https://openairdocs.org/engine/lycoming-o-360-a1a) (registry code 41594, 4 cycle) (OpenAirDocs) - Category: Airplane, amateur-built - Serial number: 1359B - Year built: 2015 - Airframe hours: 234 - Engines: Lycoming O-360-A1A 180 horsepower reciprocating - Damage: Substantial damage - Flight: Personal, Part 91 - Airworthiness certificate: Special, Experimental (Special) - Last inspection: Condition, July 1, 2023 - Departed: KIGQ, Lansing, IL - Destination: 5011, Bainbridge, IN - Airport: n/a - Equipment: ADS-B, Electronic flight bag or handheld device ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 67; injuries: minor; private; airplane ratings: single-engine land; medical: basicmed, with waivers/limitations; last flight review August 15, 2022; seat: left; restraint: 4-point; flight hours: 408 total, 200 in this make and model, 293 as pilot in command, 32 in the last 90 days, 15 in the last 30 days, 3 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 170° at 10 knots, gusting 14 - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 61 °F, dew point 23 °F - Altimeter: 30.32 inHg - Observed at: KRZL, 14 nm away - METAR: KRZL 141955Z AUTO 17010G14KT 10SM CLR 16/M05 A3032 RMK AO2 ## 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 20231114021899A](https://openairdocs.org/incident/20231114021899A), Nov 14, 2023: Alfred C Skinnell BD-4 EXPERIMENTAL CL, Lowell, IN, Substantial damage. On November 14, 2023, an experimental BD-4 single engine airplane operated by a pilot in command (PIC) as the sole occupant, departed lansing municipal airport… ## Docket (6 files) The files NTSB released with the investigation on April 23, 2025, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=193402. 11.5 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 11 pages, 2.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=18207538&FileExtension=pdf&FileName=N931AS%20accident%2011-14-23_Redacted-Rel.pdf - Maintenance records (item 2, PDF, 1 page, 572 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=18207548&FileExtension=pdf&FileName=Maintenance%20Records_Redacted-Rel.pdf - Photos (item 3, PDF, 3 pages, 699 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=18208495&FileExtension=pdf&FileName=Photos-Rel.pdf - Excerpt from operator's manual (item 4, PDF, 1 page, 50 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=18208309&FileExtension=pdf&FileName=Excerpt%20from%20Operator%27s%20Manual-Rel.pdf - Memorandum for record a��pilot (item 5, PDF, 1 page, 64 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=18205353&FileExtension=pdf&FileName=MOR-Rel.pdf - Materials laboratory factual report 24-063 (CEN24LA038) (item 6, PDF, 18 pages, 15 photos, 8.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=18300538&FileExtension=pdf&FileName=Materials%20Laboratory%20Factual%20Report%2024-063%20(CEN24LA038)-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.