NTSB Identification: DEN07IA066.
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Incident occurred Tuesday, March 06, 2007 in Pueblo, CO
Probable Cause Approval Date: 03/31/2008
Aircraft: Diamond Aircraft Industries DA-20-C1, registration: N921DA
Injuries: 2 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
The flight instructor reported that abnormal nose-up trim was required while in flight. The flight concluded without additional or further incidence. After the training flight a 2 to 3-inch gap between the horizontal and vertical stabilizer was noted. Examination of the airplane revealed two bolts, securing the front horizontal stabilizer mounting bracket to the vertical stabilizer, had failed. Examination of the bolts revealed failure due to undetected fatigue. On the failed bolts, the sizes of the final overstress regions were very small indicating that peak and mean loads on the bolts were also small. This would indicate that only a small or marginal preload was present on the bolts. Insufficient preload could be the result of low installation torque or the loss of preload during service. The off center wear on the washers suggests that the fastened surfaces may not have been in alignment allowing the installation preload to quickly degrade.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The failure of the horizontal stabilizer forward bracket bolts due to insufficent preload during factory installation. Full narrative available
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