What aircraft was involved and where did it happen?

The accident on 2003-09-30 involved a Walter Binder Motorenbau GmbH /, at Flugplatz Büchig, DE.

What was the probable cause of the accident?

The structural failure of the fuselage was caused by aerodynamic loads from a rudder input during a sideslip/spiral dive, a specific load case that was not accounted for in the aircraft's design regulations.

Structural failure causes glider breakup during spin testing

A glider undergoing type certification testing broke apart in mid-air after the pilot applied rudder input to recover from a spiral dive, leading to the loss of the aircraft.

What happened

On September 30, 2003, a Walter Binder Motorenbau GmbH / Eta glider was conducting flight testing at Büchig airfield to establish its certification parameters. The flight involved performing various spin tests. During one specific maneuver, intended to test asymmetric fuel distribution, the pilot climbed to 2,650 meters, retracted the engine, and set the flaps to a specific position.

After initiating a spin, the aircraft entered a spiral dive. As the pilot attempted to end the rotation by applying opposite rudder at an indicated airspeed of 140 km/ . The sudden aerodynamic forces caused the fuselage to break apart approximately 2,300 meters above the ground. The aircraft became uncontrollable and entered an inverted state. The pilot and an observer successfully exited the aircraft using parachutes. The unmanned wreckage continued flying in an inverted position before crashing into a coniferous forest, resulting in heavy damage to the aircraft and minor damage to the surrounding forest.

The investigation

The BFU examined the wreckage and the structural integrity of the composite materials. A material investigation conducted by the DLR (German Aerospace Center) analyzed samples from the fracture site. The investigation focused on the fuselage construction and the aerodynamic loads experienced during the recovery attempt. Investigators also reviewed the design specifications under JAR-22 regulations and analyzed flight data from a GPS logger and flight computer found on board.

Findings

The primary cause of the breakup was the aerodynamic load generated by applying rudder input against the direction of a sideslip.

The existing design regulations (JAR 22) did not require the aircraft to be designed for the specific load case of combining rudder deflection with a sideslip condition.

The aircraft was in a spiral dive, and the rudder input significantly increased the aerodynamic force on the vertical stabilizer beyond what the fuselage structure could withstand.

While investigators found irregularities in the composite layup, including air bubbles, resin-rich areas, and missing 0-degree layers, these structural weaknesses were considered secondary to the excessive aerodynamic loading.

The aircraft was slightly overweight, exceeding the maximum takeoff mass by approximately 2.7% to 4%.

Safety action

The manufacturer has since modified the construction of the Eta to ensure the airframe can safely withstand the combined forces of sideslip and rudder deflection.

The BFU issued a safety recommendation (32/2009) suggesting that EASA include the specific load case of maximum possible aerodynamic force from the combination of rudder deflection and sideslip in the design regulations for glider and motor glider vertical stabilizers.