Pilot Debrief

Cessna A185 near Saginaw, MI — 2019-08-03

Final reportCEN19LA249
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Date
2019-08-03
Location
Saginaw, MI, USA
Aircraft
Cessna A185
Registration
N5806J
Category
Airplane
Highest injury
Minor
Fatalities
0
Phase of flight
Cruise

Probable cause

Maintenance personnel’s failure to properly align the crankshaft during an engine reassembly which led to a fatigue failure of the crankshaft and the loss of all engine power.

NTSB narrative

The pilot was in cruise flight in a single-engine airplane when the engine lost power. He selected a road for the forced landing, during which the airplane impacted a power pole and electrical wires. The airplane came to rest inverted and incurred substantial damage to the airplane’s fuselage and wings. Postaccident examination revealed that the engine’s crankshaft had failed due to fatigue. The accident crankshaft was subject to a mandatory service bulletin (MSB) and subsequent Federal Aviation Administration airworthiness directive (AD) requiring inspection of the crankshaft material for metallurgical anomalies; however, this inspection was not performed on the accident airplane’s crankshaft. A detailed metallurgical examination of the crankshaft revealed that a fatigue crack had progressed through most of the crankshaft web cross section, consistent with relatively low-stress, high cycle fatigue. Because no indications of intergranular fracture or large areas of overstress fracture with the planar dimpled rupture features indicative of thermal damage on the fracture surface were noted, it is unlikely that manufacturing issues resulted in the fatigue cracking even though metallurgical testing revealed that the crankshaft material did not meet specification. Rather, the cracking likely initiated due to stray or excessive wear to the forward journal to web radius. It is further likely that a misalignment during a previous reassembly of the engine that occurred about two years prior when two cylinders were replaced resulted in the failure. Given this information, it is likely that maintenance personnel’s failure to properly align the affected crankshaft during the reassembly resulted in a low-stress high cycle fatigue failure of the crankshaft and the subsequent loss of all engine power.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Failure to comply with MSB-00-5 could have prevented crankshaft failure.

NTSB coding

Evidence available

  • Photos
  • 8 docket documents
View NTSB final reportView NTSB docket

Docket documents8

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