Pilot Debrief

PIPER PA-28-235 near San Luis Obispo, CA — 2011-01-20

Final reportWPR11LA102
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Date
2011-01-20
Location
San Luis Obispo, CA, USA
Airport
SBP
Aircraft
PIPER PA-28-235
Registration
N8608W
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Approach

Probable cause

The loss of engine power due to the fatigue fracture of one attachment bolt for the left idler gear shaft, which resulted from an insufficient preload on both attachment bolts.

NTSB narrative

The pilot reported that, while on approach to the runway, the vacuum pump failed, then the engine lost power. The pilot initiated a forced landing to a field, and the airplane impacted multiple metal poles. Postaccident examination of the engine revealed that the left magneto drive gear and the left idler gear shaft were separated from their mount. Both of the attachment bolts for the left idler gear shaft were separated from the crankcase, and their respective bolt bores were elongated. One of the attachment bolts for the left idler gear shaft was fractured, and metallurgical examination revealed that the fracture surface was consistent with fatigue. Metallurgical analysis determined that the fatigue was due to a lack of sufficient preload on the bolts. Maintenance records showed that the last logged maintenance activity in which the bolts could have been manipulated was performed about 20 years before the accident. The engine had accumulated about 1,400 operating hours since that time.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot could have aborted approach after vacuum pump failure.

NTSB coding

Evidence available

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

Docket documents8

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