Test Flight Details
The accident occurred during a test flight aimed at expanding the aircraft's speed envelope from 370 to 380 knots indicated airspeed (KIAS). A previous test flight at 350 KIAS had recorded side load forces on the vertical stabilizers reaching a company-imposed limit of 40 percent of the ultimate failure load determined by the designer. During the run between 375 and 380 KIAS, the aircraft pitched violently nose-up, followed by a general breakup.
Background on Vertical Stabilizer Design
The company noted that the original Bede-built BD-10 prototype had sustained a vertical stabilizer failure during flights at the 1994 Reno Air Races. In response, Bede subsequently designed a fix that strengthened the vertical tails. Ground substantiation load tests to failure were conducted by Bede, and the resulting yield failure load limit was provided to the company. The company used these numbers to establish the 40 percent flight test limit. The new fix was incorporated into the accident aircraft.
Wreckage Examination
Examination of the wreckage revealed that the airplane's vertical tails sustained structural overloads and separated from the aircraft in flight. The intact left vertical tail assembly was found early in the wreckage path and exhibited a clean, almost instantaneous failure load with right bending at the spar-to-fuselage attachment points.
Production Configuration Testing
A production configuration left side empennage was built, placed in a test fixture, and the vertical tail was loaded to failure. The failure mode and separation point matched those observed on the accident aircraft's left vertical tail assembly. The test showed that the vertical stabilizer spars began to yield at 40 percent of the failure load limit supplied by Bede, and spar failure occurred at 65 percent.