Pilot Debrief

STINSON L 5 near Thomson, GA — 2018-06-14

Final reportERA18LA168
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Date
2018-06-14
Location
Thomson, GA, USA
Aircraft
STINSON L 5
Registration
N45TX
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Cruise

Probable cause

Failure of maintenance personnel to inspect the crankshaft after two previous propeller strikes, which resulted in failure of the crankshaft and a total loss of engine power. Contributing to the crankshaft failure was insufficient torque applied to the thrust nut.

NTSB narrative

While in cruise flight, the pilot noticed an increase in oil temperature and loss of oil pressure. Shortly thereafter, the engine lost total power, and the propeller stopped rotating. The pilot performed a forced landing to a lake, which resulted in substantial damage. Examination revealed that the propeller could not be rotated by hand, and the crankshaft was fractured at the No. 2 crank pin. Review of maintenance logbooks revealed that the most recent engine overhaul was performed about 24 years and 711 flight hours before the accident. Since that time, the propeller had been removed and replaced on two separate occasions due to propeller strikes. There was no record of any crankshaft inspections following either of the propeller strikes, contrary to manufacturer guidance. Additional damage found on the Nos. 1 and 2 cylinders was caused by the crankshaft failure and pistons that impacted their respective rods and skirts. In addition to the main fracture, a separate crack was discovered slightly aft of the No. 1 main bearing and had propagated nearly the width of the crankshaft, but had not fully failed. The thrust nut, which required 375 ft-lbs of torque, was easily removed without effort or special tools, indicating that it was torqued to a value significantly below specification. This could cause axial play along the crankshaft and contribute to the progression of the crankshaft failure that was likely initiated during the previous 2 propeller strikes. The crankshaft likely failed after its structural integrity was compromised during previous propeller strikes. The improper torque on the thrust nut likely exacerbated the failure of the crankshaft.

Analysis

Primary failure mode
Engine power loss
First missed decision gate
Performing a thorough crankshaft inspection before the flight.

NTSB coding

Evidence available

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

Docket documents5

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