WPR18LA163

The hang glider's encounter with a developing vortex or dust devil while maneuvering during an airshow routine, which resulted in a loss of control at an altitude that precluded recovery. Contributing to the accident was the pilot's preexisting and worsening arthritis in his left shoulder, which inhibited his range of motion and resulted in a reduced ability to control and recover the hang glider during the encounter.

The hang glider pilot was performing a routine in an airshow that included the hang glider being towed to altitude by a truck-driven winch trailer. The truck and hang glider typically needed to make three passes along the runway in order to gain the altitude required for the routine; however, at the beginning of the third pass, the hang glider appeared to descend and then made an aggressive turning maneuver. The hang glider then stabilized for a few seconds before rolling to a  nose-down attitude and striking the ground. Postaccident examination did not reveal any anomalies with the airframe or the tow system that would have precluded normal operation. Multiple witnesses reported seeing at least one dust devil on the airfield about the time of the accident. Review of video footage confirmed the presence of a dust devil about the time of the accident along with evidence of low-level wind shift activity. A grass fire on the airfield earlier in the day had resulted in the temporary suspension of the airshow. By the time the accident routine had begun, the fire had been extinguished; however, smoldering areas and hot spots were still present and were being monitored by the fire department. All of the criteria for the development of dust devils were present at the time of the accident except for temperatures above 90°F; however, the burn area from the fire would have resulted in locally warmer temperatures in the vicinity of the accident site and increased the likelihood of eddy or vortices in the area of the accident site. Additionally, while the routine was progressing, a military fighter jet was idling on the ramp with its hot exhaust gases facing the runway toward the location of the ground impact. The jet began to taxi during the hang glider routine, and it is possible that the exhaust gases further exacerbated the conditions and created sufficient eddies to trigger a dust devil event. Based on the available evidence, it is likely that the hang glider encountered dust devil activity while being towed to altitude. The 62-year-old pilot had been paralyzed from the shoulders down due an accident about 40 years prior. He had accumulated extensive hang gliding experience since then and had performed the airshow routine many times before. However, the pilot had been suffering from arthritis in his left shoulder, which in recent months had resulted in a significant decrease in its range of movement. As such, he was experiencing difficulties getting into the hang glider harness, operating his wheelchair, and performing routine daily tasks, and he had discussed the possibility of retiring at the end of the airshow season. Although the autopsy identified evidence of mild coronary artery disease, there was no evidence he ever had any symptoms of ischemia, and it is unlikely that his heart disease contributed to the accident. The pilot's recently degraded shoulder function due to arthritis, likely reduced his ability to perform the control inputs required to recover from the dust devil encounter or release the hang gliders airframe parachute.

The 62-year-old pilot had broken his neck in a hang-gliding accident in 1981 and had been a C7 quadriplegic, paralyzed from the shoulders down, since then. He was issued a commercial pilot certificate with a glider rating in February 1996, with limitations for "aero tow" and "hand controls" only. Federal Aviation Administration (FAA) records indicated that the pilot had undergone an FAA medical examination in March 1996, and subsequently was scheduled for a medical flight test in May of that year, but it was not clear if he completed the test or if a medical certificate was ultimately issued. FAA regulations do not require the pilot of a hang glider to hold a pilot or medical certificate. According to the autopsy performed at the request of the Elmore County Coroner's Office, Mountain Home, Idaho, the pilot's cause of death was traumatic blunt force injuries. Mild coronary artery disease was also found. Toxicology testing performed by the FAA Forensic Sciences Laboratory did not identify any tested-for substances. The pilot's personal medical records indicated that, during the week before the accident, he had visited an orthopedic specialist for an evaluation after several months of significantly decreased range of motion and limited function of his left shoulder that made it harder for him to operate his wheelchair. The diagnosis was primarily arthritis, and they discussed a future joint replacement, but no definitive treatment was performed. According to the winch operator, who helped the pilot with day-to-day operations, the pilot had longstanding issues with mobility in his left shoulder, which was progressively deteriorating. As such, the pilot was finding it harder to perform basic tasks such as showering or getting in and out of his truck, and the winch operator had been helping him more with physical tasks. Although the last practice flight occurred 2 days before the accident, the pilot had decided to curtail future flights due to the pain he endured getting in and out of the harness and in order to reduce wear and tear on his shoulder. The pilot stated to the winch operator that his goal was to get through the 2018 show season and decide the future of the operation at the end of the year. The hang glider was composed of a North Wing Pulse 10M wing and a Moyes Delta Gliders Contour model harness. The system was configured with a "softpack" style BRS Aerospace aircraft parachute rescue system mounted to the harness. The harness was connected to the tow line using a two-point strap, which incorporated a barrel-type quick release on the right side next to the pilot's chest. Operation of the quick release would have required pulling the barrel toward the harness while the strap was still under tension. The strap was connected to the tow line with a weak link, and a small parachute was connected to the end of the tow line. The winch system included a spool of ultra-high molecular weight polyethylene cord (tow line), which was driven by an electrical motor. The speed of the spool during the towing process was maintained by a hydraulically-controlled automotive disk brake system, the pressure of which was governed by the tow operator. On June 2, 2018, about 1338 mountain daylight time, an experimental, amateur-built North Wing Pulse 10M hang glider, N62073, collided with the ground while participating in an airshow at Mountain Home Air Force Base (KMUO), Mountain Home, Idaho. The commercial pilot sustained fatal injuries and the hang glider sustained substantial damage. The hang glider was registered to and operated by the pilot as a Title 14 Code of Federal Regulations Part 91 flight. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which departed KMUO about 1332. The premise of the pilot's airshow routine was a comedy act in which he would simulate inadvertently flying the hang glider into an active airshow, interrupting another airplane that was performing an aerobatic routine. The plan was for the hang glider, which was equipped with smoke canisters and a series of trailing streamers, to be towed past show center while the aerobatic airplane made a series of close passes, eventually cutting the streamers. Throughout the routine, the rehearsed radio interactions between the hang glider pilot, aerobatic pilot, and the show announcer were transmitted over the public address system. During a typical airshow, the hang glider was launched by a moving tow-launch system composed of a winch trailer pulled by a truck. The truck and trailer then travelled back and forth down the runway spinning out the tow line while the winch operator maintained line tension as the hang glider climbed to the desired altitude (usually 1,500 ft). The pilot typically released the line once the truck had turned around after the third pass. The pilot would then begin to perform a series of gliding maneuvers down to the ground, while the line, which was equipped with a parachute, was rewound back into the winch spool as it descended. Most of the accident sequence was recorded on a security camera located on the KMUO control tower in the center of the airport, about 2,100 ft south of runway 12/30, and directly across from show center. Footage from the camera was provided by the United States Air Force (USAF). On the day of the accident, the performance began and progressed uneventfully until the end of the truck and winch trailer's second runway pass. After being given the all-clear by the hang glider pilot, the truck and winch trailer turned around as planned and began to accelerate along runway 12 in anticipation of the pilot releasing the line. (See figure 1.) The winch operator stated that, a few seconds after the turn, he looked up and noticed that the hang glider was about 500 ft below the altitude at which he would typically expect to see it. The hang glider then performed an aggressive turning maneuver (see figure 2) and descended another 500 ft. The winch operator then released the winch pressure to supply the hang glider with more slack and prevent it from being impeded by line tension. The hang glider then entered a climb, rolled to the left, descended, and impacted the ground in a nose-down attitude. (See figures 3 and 4.) Figure 1. Hang glider (red circle) and tow truck (green circle) beginning the final pass along runway 12 Figure 2. Hang glider performing aggressive turning maneuver Figure 3. Hang glider rolling left Figure 4. Hang glider striking the ground Multiple witnesses recounted a similar sequence of events, stating that the maneuvers after the second pass were completely unconventional and not part of the routine. The airshow coordinator (air boss), who had seen the routine performed many times before, stated that the performance appeared to be going well and the streamers were cut during the second lap as planned. However, during the final turn, the hang glider was a little lower than normal and it performed a sudden circling maneuver, which he described as a "pinwheel." He commented to one of the crew that something was not right. The hang glider appeared to stabilize and descend, and then performed another pinwheel maneuver before it impacted the ground in a nose-down attitude. A witness saw the hang glider suddenly pitch up before the impact, and based on his experience, assumed that it had encountered a strong wind. Neither the air boss nor the winch operator saw the line parachute deploy, and when the winch operator arrived at the accident site, the parachute and line were in the immediate vicinity of the main wreckage, appearing to have detached on impact. Multiple airshow attendees reported to news media that the tow line was cut by the aerobatic airplane during the routine, however, examination revealed that the line was intact and undamaged. The hang glider came to rest about 250 ft southwest of the right edge of