Accident Details
On October 6, 2019, about 1300 mountain standard time, an experimental Glaser-Dirks DG-500 Elan Trainer glider, registration N600TT, sustained substantial damage when it was involved in an accident near Marana, Arizona. The flight was conducted as a Title 14 Code of Federal Regulations (CFR) Part 91 test flight. The commercial pilot in the rear seat sustained minor injuries, while the private pilot in the front seat was not injured.
Maneuver and Recovery
The rear seat pilot reported that the purpose of the flight was to perform aerobatic maneuvers as part of the glider’s airworthiness program and to evaluate the front seat pilot’s proficiency in aerobatic maneuvers for solo operation. After completing several maneuvers, the front seat pilot began a “Sharks Tooth” maneuver, which consisted of a climb, roll to inverted, and a vertical down line. During this maneuver, the glider rolled inverted, the nose dropped to 20° above the horizon, and the glider stalled.
The rear seat pilot instructed the front seat pilot to pull to vertical, but the elevator input was insufficient. The airspeed increased from about 90 knots to the never-exceed speed (VNE) “almost instantaneously,” exceeding 155 knots. The glider then experienced a high-frequency elevator flutter while the nose remained below the horizon. During the recovery, the right spoiler deployed, and the pilots found the elevator uncontrollable. They decided to make an emergency landing on runway 17R/35L at El Tiro Gliderport (AZ67) with minimal directional control due to the asymmetrically deployed air brake. The glider landed hard, resulting in damage to the empennage.
Postaccident Findings
During a postaccident examination, the rear seat pilot observed that the drive link was disconnected at the rod end of the air brake. He noted that there is a significant structure to force the air brakes open, but the bearing welded to the push/pull tube likely failed after years of movement. He opined that the part may have had cracks or had been hardened by manufacturing methods and years of pulling against the outer ring, eventually causing failure. The additional forces from the wing bending moment at high airspeed and air loads weakened the joint. The pilot stated that the in-flight damage was likely due to the excessive speed reached during the maneuver recovery.
Manufacturer's Comments
According to the manufacturer, similar failures had been reported when the air brakes were extended in negative g-flight, leading to the issuance of technical note TN348-4T in Germany in 1994. The air brake should be examined as part of the annual inspection, as listed in the maintenance manual. The over-center force must be measured and set accordingly. If the rod end on the air brake side breaks off, the stiffness of the air brake control in the wing should be inspected to exclude damage to the air brake control support structure on the root rib.
The manufacturer also reported that it should have been possible for the pilots to deploy the other air brake normally to compensate for the asymmetric behavior and maintain some control over the approach glidepath. While the air brakes are not intended for normal aerobatics, they can be deployed for emergencies. However, no emergency procedure for air brake malfunction was described in the flight manual.