What happened
On May 9, 2009, at approximately 1435, an Upton Bakeng Duce, registration N86YP, experienced an in-flight breakup while maneuvering. The aircraft entered an uncontrolled descent and impacted an open field approximately 6.4 miles east-southeast of Ramona, California. The flight originated from the Ramona airport at 1424 under visual meteorological conditions with 10 miles of visibility and light winds.
There were 2 fatal injuries, as both the pilot in the rear seat and the pilot in the front seat were killed upon impact. The aircraft, which was an experimental, single-engine, amateur-built airplane, sustained substantial structural damage and fragmentation. No fire occurred following the crash.
While one witness reported observing a midair collision between the red and white aircraft, radar data examined by the Safety Board and FAA indicated only one aircraft was in the vicinity at the time of the accident. Other witnesses reported hearing a sputtering engine and observing the aircraft spiral toward the ground, noting that a wing had separated from the main body during the descent.
The investigation
Investigators examined the wreckage and found the main wreckage in a level field, with fragmented parts located between 0.1 and 0.3 miles away. The aircraft's wings were broken from their fuselage attachment points and lift struts. While the left wing spar was found broken in several locations, the right wing spar was found highly fragmented.
Laboratory analysis of the right wing spar components revealed that the wood grain was not aligned with the longitudinal axis of the spar. The examination showed an average grain deviation of approximately 12 degrees. This misalignment was not visible from the exterior surfaces of the spar because wood strips had been attached to the top and bottom edges, obscuring the defect.
Findings
- The right wing spar strength was significantly compromised due to the 12-degree grain deviation.
- According to FAA guidance, the maximum permissible grain deviation for a wood wing spar is 3.8 degrees.
- A 12-degree deviation reduces the bending strength of the spar to between 30 and 70 percent of its intended capacity.
- The structural failure occurred when the pilot performed a maneuver that generated aerodynamic loads exceeding the degraded strength of the wing spar.