WPR18FA247 · Schempp-Hirth Duo Discus T

The two pilots onboard the motorized glider, one of whom was an owner of the glider, were participating in an informal competition, the goal of which was to fly between two predetermined locations in the shortest time. The takeoff, tow to altitude, and initial stages of the flight appeared uneventful.

A low-resolution track of the glider’s flightpath indicated that the glider was likely flying around 14,500 ft mean sea level (msl) and circling in thermal lift just before the accident. A group of paraglider pilots was preparing to launch from an 8,700-ft-high bluff on the mountain over which the accident glider was circling. Their attention was drawn to the glider, which appeared to be performing a series of tight maneuvers that they described as “loops.” After a couple of “loops,” the glider’s wings began to flex upward almost vertically before one wing broke away and impacted the tail, and the glider descended to the ground. Ballistic trajectory analysis indicated that the glider likely broke up at about 11,500 ft msl while traveling in a steep descent toward the witnesses.

Most of the glider was consumed by fire on impact, with only the separated portion of the left wing and a series of control surface, skin, and cabin components strewn over a 1,500-ft-long debris path escaping thermal damage. Although sections of the horizontal stabilizer broke away during the breakup, the horizontal stabilizer mount and elevator control structure remained attached to the glider at impact.

Examination of the wreckage did not reveal any evidence of anomalies that would have precluded normal operation.

Given that the purpose of the flight was a timed competition and that performing aerobatics would have been out of character for the pilot/owner, it is highly unlikely that the pilots were intentionally performing aerobatic loops as described by the paraglider pilots, particularly over a mountain ridge. Due to the witnesses’ location relative to the glider, it is more likely that the witnesses were observing the glider in either a spin or a spiral dive rather than a loop. The position of the sun, which would have been generally behind the glider and to the right relative to their position, would have presented it in a high-contrast environment and could also have hindered their frame of reference. G-loading calculations revealed that a loop of the radius and period observed by the witnesses was not physically possible because the glider would have been subject to stress significantly beyond its ultimate design limit load and would have experienced structural failure well before completion of the first maneuver.

The published stall recovery technique requires that the pilot firmly ease the control stick forward and, if necessary, apply opposite rudder and aileron. With a more forward center of gravity, as was the case with the two occupants onboard, should the stall develop into a spin, the glider will enter a spiral dive after recovery, which is accompanied by a rapid increase in speed and acceleration that can quickly exceed limitations.

The glider’s airbrakes can be extended at speeds approaching the never-exceed speed (Vne); however, damage to the left airbrake was consistent with an inflight separation, suggesting that the pilot had extended the airbrakes in an attempt to slow the glider and that the glider exceeded Vne. Excessive elevator control input during the attempted recovery from a high-speed dive would have resulted in the witness-observed upward bending of the wings and the subsequent failure of the wings due to overload. Such upward loading of the wings would not have been possible without the presence of the horizontal stabilizer and pilot input via the elevator.

Evidence of electrical arcing was present in wiring that would only have been energized if the retractable engine was in transit. Those wires were protected by circuit breakers and the engine was stowed; therefore, the arcing was likely the result of an electrical short circuit that occurred on impact or as the glider broke apart in flight. Additionally, the glider’s FLARM collision avoidance system operated throughout the flight, further indicating that some form of electrical fire was unlikely.

No evidence of bird strike was observed to any of the recovered components, and radar information revealed no evidence of any conflicting traffic in the immediate vicinity of the accident glider before the accident.

The canopy was closed at impact; although both occupants were wearing parachutes, and one of them was not wearing the shoulder straps of his harness, there were no other obvious indications that they attempted to egress. Postaccident medical evaluation of the two pilots was limited due to the extent of injury. Whether a medical condition of either occupant contributed to the circumstances of the accident could not be determined.

Given the lack of mechanical anomalies, the overload failure of the left wing, and the glider’s rapid descent before the accident, it is likely that the glider entered an inadvertent spin or spiral dive while maneuvering and that the pilot’s delayed recovery resulted in a rapid increase in speed. It is likely that, during recovery, the pilot either used excessive pitch control beyond the glider’s published maneuvering speed (Va) or the pilot exceeded the glider’s Vne during the subsequent steep descent.

Verbatim NTSB analysis and probable cause from the NTSB dataset