On October 6, 2019, about 1300 mountain standard time, an experimental Glaser-Dirks DG-500 Elan Trainer glider, N600TT, sustained substantial damage when it was involved in an accident near Marana, Arizona. The commercial pilot seated in the rear seat sustained minor injuries and the private pilot seated in the front seat was not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 test flight.
The rear seat pilot stated that the purpose of the flight was to perform aerobatic maneuvers as part of the experimental glider’s airworthiness program. The rear seat pilot was also evaluating the front seat pilot’s proficiency in aerobatic maneuvers so that the front seat pilot could fly the glider solo as part of the test program. After completing several maneuvers, the front seat pilot was performing a “Sharks Tooth” maneuver, which comprised a climb, roll to inverted, and a vertical down line. During the maneuver, the front seat pilot rolled inverted, the glider’s nose dropped to 20° above the horizon, and the glider stalled.
The rear seat pilot called for the front seat pilot to pull to vertical; however, the front seat pilot did not apply sufficient elevator, and the airspeed increased from around 90 kts to the never-exceed speed (VNE) “almost instantaneously.” The airspeed increased past 155 kts and the glider experienced a high frequency elevator flutter with the nose still below the horizon. During the recovery, the right spoiler deployed, and the pilots felt that the elevator was not controllable. They opted to make an emergency landing to runway 17R/35L at El Tiro Gliderport (AZ67) with minimal directional control from the asymmetrically deployed air brake. The glider landed hard, resulting in damage to the empennage.
During a postaccident examination, the rear seat pilot noted that the drive link was disconnected at the rod end of the air brake. He stated that there is significant amount of structure to force the air brakes open but the bearing that was welded to the push/pull tube likely failed after so many years of movement on that bearing. He opined that the part likely had cracks or had been hardened by the manufacturing methods and years of pulling against the outer ring flexed it until failure. The additional forces of the wing bending moment from the high airspeed, and the air loads weakened the joint to fail. The pilot stated that he thought the damage to the glider in flight was because of the excessive speed they reached during the maneuver recovery.
According to the manufacturer, similar failures had been reported when the air brakes were extended in negative g-flight. This led to technical note TN348-4T being issued in Germany in 1994. The air brake should be examined as part of the annual inspection and as such, is listed in the maintenance manual. The over center force has to be measured and set accordingly and if the rod end on the air brake side broke off, the stiffness of the air brake control in the wing should be inspected to exclude any damage to the air brake control support structure on the root rib.
The manufacturer reported that it should have been possible for the pilots to deploy the other air brake normally to compensate for the asymmetric behavior and have some (reduced) control over the approach glidepath. The air brakes (spoilers) should not be used for normal aerobatics but can be deployed for emergencies. There was no emergency procedure described in the flight manual for air brake malfunction.