NTSB Identification: FTW00LA010.
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Accident occurred Sunday, October 10, 1999 in GRANITE, OK
Probable Cause Approval Date: 11/22/2000
Aircraft: Piper PA-18-150, registration: N4473A
Injuries: 2 Serious.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The airplane was destroyed upon impact with terrain following a loss of control during the takeoff/initial climb. Severe corrosion was found at two structural tube clusters (one with a windshield pillar) located directly below the right wing forward and aft attach fittings. The portion of the forward structure available for carrying loads prior to the accident was less than 30% of the circumference of the pillar tube. Deformation associated with the separation of the pillar tube indicates that the pillar failed under compression loads. The manufacturer's representative stated that the 'windshield pillar is normally subjected to a tensile load during flight.' It is possible that compression loads generated during the takeoff roll led to a compression failure of the pillar. This failure would have changed the lift characteristics of the wing since the separation point was directly below the right wing forward attach fitting. The most recent annual inspection was performed in January 1999; the maintenance entry for the annual stated, in part, 'treated steel tubing as required.' In June 1999, a major repair was made to a tube cluster in the aft fuselage area 'due to external rust through from past acid spill.'

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

Corrosion failure of the fuselage support tubing below the right wing forward attach fitting due to inadequate maintenance.

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