Investigation Summary
The investigation centered on the Aeroproducts propeller assembly of the No. 2 (right) engine. Investigators discovered that during the manufacturing of the torque pistons, ten units were diverted from the standard production line for a statistical study on part growth. These parts were intended to be reintroduced into the normal flow, which included a nitriding furnace treatment to harden the steel. However, due to a breakdown in production tracking, the pistons bypassed the nitriding stage. Because nitrided and non-nitrided steel look identical after cleaning, the error was not visually apparent. Furthermore, quality control checks, including Rockwell hardness and magnaflux tests, failed to detect the soft steel before the parts were installed in engines. In the case of N73130, the soft splines on the torque piston of the right engine wore away over 16,216 flight hours until the piston could no longer hold the blades in the commanded pitch. The blades flattened into a low-pitch state so rapidly that the mechanical pitch lock could not engage, leading to a massive overspeed and centrifugal separation of all four blades. One of these blades sliced through the fuselage, severing control cables and causing an in-flight breakup.
Final Conclusions
The NTSB determined the probable cause was the failure of the right propeller due to the omission of the torque piston nitriding process during manufacture, compounded by the failure of manufacturing quality control to detect the omission. The separation of the propeller blades caused direct physical damage to the fuselage and severed critical control cables, rendering the aircraft uncontrollable.
Video Analysis
Lake Central Airlines Flight 527 Flight Path Animation — Official FAA animation showing the flight path and sequence of events leading to the structural failure of Flight 527 near Marseilles, Ohio.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Systems & Failure Modes
Propeller Torque Piston
A steel component with helical splines that controls the pitch of the propeller blades.
Nitriding Process
A heat-treatment process that diffuses nitrogen into the surface of a metal to create a case-hardened surface.
Propeller Pitch Lock
A safety mechanism designed to prevent the blades from moving to a dangerously low pitch in the event of hydraulic pressure loss.
Interesting Facts
- 01The accident remains the deadliest aviation disaster in the history of the state of Ohio.
- 02The aircraft was a Convair 340-31 (MSN 23, manufactured in 1952) that had been converted to a Convair 580 using Allison 501-D13D turboprop engines.
- 03The cockpit voice recorder captured a sound described as 'the first few seconds of an air raid siren' just before power was lost, indicating the propeller overspeed.
- 04Debris was found in a trail 1.5 miles long and 0.5 miles wide, confirming an in-flight breakup.
- 05The failed torque piston was one of ten units that bypassed the nitriding process during a manufacturing study.
Safety Actions & Advisories
Manufacturing Process Control
The investigation highlighted the dangers of 'special' production flows. Procedures were tightened to ensure parts removed for testing or studies are strictly tracked to ensure they complete all required hardening and inspection steps before re-entering the supply chain.