Aeronca 7AC N82694

Summary

On 11 June 2016 at about 17:36 local time, a Aeronca 7AC registered N82694 was involved in an accident near East St. Louis, Illinois, United States. 2 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

11 June 2016 at 17:36

Classification

Accident

Location

East St. Louis, Illinois, United States

Coordinates

38.6167, -90.1333

Aircraft

Aeronca 7AC

Registration

N82694

Category

Airplane

Year built

1946

Operator

Not recorded

Operating rule

Part 91: General Aviation

Purpose of flight

Personal

Phase of flight

Not recorded

Route

Columbia → Columbia

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

CEN16LA218

Nobody was injured.

A total loss of engine power due to fracture of the piston pin, which led to fatigue cracking and eventual fracture of the adjacent connecting rod.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

The airline transport pilot and passenger departed on a local flight from an uncontrolled airport. The pilot flew for about 15 minutes when he heard "a loud noise, a rattle," and the engine lost total power. He chose to conduct a forced landing on a section of closed highway; however, the pilot was unable to slow the airplane and impacted barricades at the end of the road closure. Postaccident examination of the engine revealed two holes through the top of the crankcase, and cylinders No. 1 and No. 2 were separated from their mounts on the crankcase. Inside the No. 2 cylinder, the connecting rod was found fractured just below the piston head. Upon further examination of the connecting rod, the piston pin, which was manufactured from resulfurized steel, was also found fractured inside the connecting rod bore. The No. 2 cylinder failed due to a fractured piston pin that retained the connecting rod inside the piston crown underside. The fracture of the piston pin led to misalignment of the connecting rod, which eventually failed from fatigue in reverse bending. The fatigue stresses on the connecting rod were relatively low, consistent with most of the fracture surface exhibiting fatigue propagating from multiple crack initiation sites. The piston pin likely fractured first, in part exacerbated by intergranular cracking in the outer, case-hardened layer. Resulfurized steels, designed for better machinability, exhibit reduced notch toughness and ductility. Carburizing or carbonitriding of these steels is generally not recommended, as the surface treatment can increase susceptibility to intergranular fracture in the hardened condition. In addition, the degree of the surface case hardening was deeper than typical for this alloy. It is likely that a stress high enough to fracture the outer layer would lead to through-fracture of the pin. However, the location of the pin fracture was such that immediate failure of the engine did not occur until the connecting rod subsequently fractured from fatigue. The pin had been partially embrittled by the surface case hardening, which sufficiently lowered the fracture toughness to cause the pin to fracture prematurely.

Quoted verbatim from the NTSB record.