History of Flight
On April 1, 2011, at approximately 1715 mountain daylight time, a Burkhart Grob Flugzeugbau G102 Club Astir IIIB glider, registration N633BG, was substantially damaged after colliding with high-voltage transmission wires and terrain near Boulder, Colorado. The private glider pilot was fatally injured. The glider was registered to Mile High Youth Gliding Association, Inc., and operated by Mile High Gliding under 14 CFR Part 91 as a personal flight. Visual meteorological conditions prevailed, and the flight operated without a flight plan. The glider departed from Boulder Municipal Airport (BDU) at 1628.
The tow plane pilot stated that the glider released from the tow at 13,900 feet mean sea level and described the air conditions as bumpy with updrafts of 1,000 feet per minute in several areas. Several eyewitnesses observed the glider in a spiral turn toward the ground with a steep bank angle. One witness reported the glider leveling off momentarily, then pitching nose up and climbing, then pitching nose down and descending.
Personnel Information
The pilot, aged 46, held a private glider pilot certificate issued on September 24, 2010. A review of the pilot's logbook showed 104.4 total flight hours, most logged in the accident glider. The pilot had a history of diabetes and monitored his blood sugar level. The last recorded reading, taken on the accident day at 1450, was 58 mg/dl. The pilot had reported his condition to Mile High Gliding and was required to carry glucose tablets.
Aircraft Information
The single-seat, low-wing glider, serial number 5633CD, was manufactured in 1985 with composite construction. A review of the maintenance logbook indicated an annual inspection on May 1, 2010, at 2,113.4 airframe hours. On March 19, 2011, the wing spar spigots were inspected per Airworthiness Directive 2011-01-03 and found free of cracks.
Meteorological Information
The National Weather Service (NWS) Boulder released a soaring forecast on April 1, 2011, based on a 0600 upper air sounding from Denver. The thermal soaring index was forecast as “excellent” with expected climbs of 1,122 feet per minute between 13,800 and 18,100 feet msl. Above this layer, mountain wave turbulence was possible between 18,000 and 20,000 feet msl. An 1800 sounding was consistent, with winds at 20,000 feet from 310 degrees at 50 knots. Satellite imagery showed clouds near the mountain range characteristic of mountain wave activity. At 1712, BDU reported winds from 300 at 17 knots gusting to 21, visibility 10 statute miles, and clear skies.
Communications
FAA inspectors reviewed audio recordings of the pilot’s radio transmissions and noted the absence of muffled sounds normally associated with oxygen mask use.
Wreckage and Impact Information
The accident site was in a sparsely wooded area. Initial impact signatures included white paint transfers and fiberglass deposits on two parallel high-voltage transmission wires approximately 75 feet above ground level. The debris path followed a 075-degree heading. The right aileron was found directly under the west set of wires. Metal rods from both ailerons, bent with signatures of a wire strike and thermally damaged with arcing evidence, were found in scorched terrain. The main wreckage, located 495 feet east of the wires, consisted of the inverted main fuselage, 15.5 feet of left wing, and 2 feet of right wing. The empennage had separated about 4 feet aft of the centerline tire and was fractured in multiple places. Flight controls were fractured in multiple places with overload signatures or melting. The A-14 diluter-demand oxygen regulator separated from the fuselage and was found in the debris path; its dial indicated an altitude just outside the “normal” placard setting, and the dilutor control was in “normal” position. The oxygen bottle was in the cockpit with the valve near full open. Testing of the oxygen system revealed no preimpact anomalies. Examination of the airframe found no preimpact anomalies.
Medical and Pathological Information
An autopsy performed on April 2, 2011, by the Boulder County Coroner’s medical examiner ruled the manner of death an accident. Forensic toxicology by the FAA Bioaeronautical Sciences Research Laboratory detected chlorpheniramine, dextromethorphan, dextrorphan, doxylamine, and pseudoephedrine in various specimens. According to the U.S. National Library of Medicine, these drugs are used for allergy, cold, and flu symptoms.
Additional Information
The glider was equipped with a Volkslogger GPS data recorder that recorded the flight from 1607 at three-second intervals. Data showed the glider was air-towed from BDU at 1628 to a point 3.5 nautical miles west of the airport, maneuvered clear of the tow at 1642, then turned toward Jamestown and back toward Boulder. The glider ascended through 20,000 feet msl at 1651:50 and through 22,000 feet msl at 1654:20, maneuvering between 21,000 and 23,800 feet before descending in spiral-like turns beginning at 1707:56.
The FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25A, 2008) describes hypoxic hypoxia and its symptoms. There is no record of the pilot having received hypoxia training. The handbook includes times of useful consciousness: at 20,000 feet, 30 minutes or more; at 22,000 feet, 5 to 10 minutes; at 25,000 feet, 3 to 5 minutes.
Prior to flight, the pilot recorded a blood sugar level of 58 mg/dl. According to the U.S. National Library of Medicine, blood sugar below 70 mg/dl is low, with symptoms including blurred vision, headache, shaking, tingling, numbness, weakness, and unclear thinking. Some symptoms of hypoglycemia can mimic hypoxia.