CEN13LA225

The pilot reported that, while practicing aerobatics, he pulled the control stick aft and that something in the elevator control system then "broke," and the airplane's nose subsequently started to drop. The pilot established that he still had aileron and rudder control; however, he was not able to control the airplane's pitch. The pilot chose to parachute from the airplane and subsequently landed in a field. The airplane crashed into a field south of the pilot's location and was substantially damaged during the impact. An examination of the flight control system revealed several points of separation along the elevator control tube and elevator torque tube assembly. The weld area between the left side elevator torque tube and the left elevator horn was fractured, and the fracture surfaces showed large areas of preexisting weld anomalies, including lack of fusion and incomplete penetration. Similar weld anomalies were present between the right elevator components. The examinations determined that incorrect weld filler metal was used. The loads that caused the separation of the left elevator torque tube were likely high but within the design specifications, and the separation likely resulted from the loads being applied to the poor quality welds, which appeared to have occurred during original manufacture.

On April 12, 2013, about 1500 central daylight time, an Atkins Acro-Sport 1 airplane, N5897, was substantially damaged when it impacted terrain 1 mile south of the Le Gros Memorial Airport (3R2), Crowley, Louisiana. The pilot had minor injuries. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 without a flight plan. Visual meteorological conditions prevailed for the personal flight. The local flight originated from 3R2 approximately 1435.According to the pilot, he was practicing aerobatic maneuvers in an aerobatic box near 3R2. The airplane was at 3,000 feet mean sea level when the pilot "pulled aft control stick to start the vertical upline for the shark's tooth aerobatic maneuver." The pilot stated that something in the elevator control system broke and the nose started to fall. The pilot established that he still had aileron and rudder control; however, he was not able to control the pitch of the airplane. The pilot elected to egress from the airplane, during which he injured his ankle. The pilot parachuted without further incident to a muddy rice field. The airplane crashed into a grass field south of the pilot's location and was substantially damaged during the impact. A postaccident examination of the flight control system revealed several points of separation along the elevator control tube and elevator torque tube assembly. An examination of the remaining systems revealed no anomalies. The separated portions of the elevator control tube and elevator torque tube with horns were sent to the Materials Laboratory in Washington, D.C., for further examination. The control tube was fractured through the threaded shank of the rod end fitting. The torque tube components were separated at three locations. The control tube fitting fracture and two of the three fractures in the torque tube exhibited features and deformation patterns consistent with overstress and no evidence of preexisting cracking or corrosion were noted on these fractures. The remaining fracture in the torque tube was completely contained within the weld bead between the left side tube and the left elevator horn. Magnified examinations of these fracture surfaces, showed large areas of pre-existing weld anomalies including lack of fusion and incomplete joint penetration. Lack of fusion and incomplete joint penetration accounted for large portions of the tube separation area. The remaining separation was consistent with overstress separation except in areas displaying post fracture mechanical damage. Further examination of the microstructures of the tube and horn were both fine pearlite, typical of normalized alloy steel. The weld fusion zone had a microstructure of predominately ferrite with a small amount of pearlite consistent with a very low carbon steel. The metallographic section also highlighted the lack of fusion and incomplete joint penetration. A metallographic section cut through a randomlocation on the right side elevator horn to torque tube weld also showed incomplete penetration and lack of fusion. Similar to the left side, the microstructure of the tube and horn were fine pearlite consistent with normalized alloy steels while the weld fusion zone was ferrite plus pearlite consistent with low carbon steel.