Pilot Debrief

STEMME GMBH & CO S10-VT near Mackinaw City, MI — 2013-08-31

Final reportCEN13LA524
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Date
2013-08-31
Location
Mackinaw City, MI, USA
Aircraft
STEMME GMBH & CO S10-VT
Registration
N389BD
Category
Glider
Highest injury
Serious
Fatalities
0
Phase of flight
Approach

Probable cause

The mechanic’s failure to inspect the tightness of the bolts that connected the flexible disk to the driveshaft, which resulted in the failure of these components.

NTSB narrative

The pilot reported that the motor-glider was on a long, straight-in visual approach to land when he heard a very loud "bang" in the front of the motor-glider followed by a severe vibration. The pilot shut down the engine and made a forced landing to a field. A postaccident examination of the aircraft revealed that the flexible disk on the front of the driveshaft had disconnected. Only one of the three bolts that connected the disk to the driveshaft remained partially installed. No evidence of Loctite or torque paint was found on the bolt. The bolt's threads were rusted, and the bolt's head was digging into the gearbox housing. The second bolt was sheared off, and the third bolt was missing. A review of maintenance records revealed that the motor-glider's most recent annual inspection was completed about 1 month before the accident. According to the manufacturer's maintenance manual, a mechanic was required to check the tightness of the bolt connections of the flexible disk on the front side of the drive shaft. It is likely that the mechanic improperly inspected the bolts that connected the flexible joint to the front side of the driveshaft during the last annual inspection, which resulted in the ultimate failure of these components. The maintenance records also revealed that, during the last annual inspection, Federal Aviation Administration Airworthiness Directive (AD) 2006-19-08, which required inspection for deformations and cracks in the exhaust pipes and the replacement of any damaged exhaust pipes found, had not been properly complied with when a weld repair was made to the No. 1 exhaust pipe. The mechanic's failure to not properly comply with the AD and the operator's failure to ensure that the AD was complied with were not causal to the accident; however, because the AD was not complied with, the motor-glider was not considered airworthy.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Properly inspect and secure the flexible disk during the last annual inspection.

NTSB coding

Evidence available

  • 7 docket documents
View NTSB final reportView NTSB docket

Docket documents7

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