WPR14FA369 · Backovich George C G P 5

The one-of-a-kind airplane designed specifically for air racing was participating in a practice air race session. Photographic and video evidence showed that the airplane entered the race course while accelerating and descending as the pilot was banking left around the back side of the race course. While passing one of the race pylons, the outboard portion of the right wing failed, and the airplane began rolling to the right followed by the empennage fragmenting and separating from the fuselage as the airplane descended into the ground.

Based on the available evidence, the outboard right wing separation was the initiating failure and the empennage separation was a secondary failure. The outboard portion of right wing was reconstructed using the pieces found in the initial debris path. The reconstruction revealed that the right wing forward box spar fractured at the inboard end of a splice repair to the right wing spar in an area where the upper and lower spar caps transitioned from a singular rectangular cross section to two smaller rectangular finger sections. The scarf joints on each finger were also fractured through the adhesive with no evidence of wood grain failure on either side of the scarf areas. Typically, failure of a bonded wood joint should occur within the wood grain adjacent to the bond area if the adhesive has been properly prepared and applied.

The accident airplane was involved in a landing accident about 30 months before the accident that substantially damaged the right wing. The wing was reportedly removed from the airplane and repaired by the original designer. No information existed on the repair, no logbook entry was made for the repair, and no requirement for documenting any repairs to the airplane existed in the operating limitations. It is likely that the right wing spar repair was not done in accordance with the approved methods in the Federal Aviation Administration Advisory Circular (AC) 43.13-1B, thus the strength of the wing in the area of the repair was reduced.

Aircraft maintenance records contained entries related to cracking in the right and left wing lower skins. Personnel working on the airplane during the air races reported that the cracking was located in the area aft of the forward spar near the main landing gear trunnions, which was well inboard of the area where the right wing failed. The investigation determined that these cracks did not contribute to the accident.

The morning of the accident, the pilot attempted a practice flight; however, he pulled off the course shortly after entering the course due to an excessive vibration. The race crew determined that the pilot had encountered the rev-limiting function on the engine control unit. The pilot reported to another pilot that he thought the airplane was going to shake itself apart during the event. It is possible that the airframe vibrations induced by the engine could have affected the spar repair on the right wing. Further, the airplane's high speed, high g-loads, and left bank produced wing loads sufficient to fail the right wing forward spar at the location of a previous repair. Even though the airplane was likely not being operated outside the original design envelope at the time of the right wing failure, the reduced strength of the repair led to the wing's failure.

Verbatim NTSB analysis and probable cause from the NTSB dataset