Burkhart Grob Flugzeugbau G102 CLUB ASTIR IIIB N633BG accident near Boulder, Colorado

Probable cause

The pilot's incapacitation for undetermined reasons, which resulted in a loss of control.

NTSB analysis

Data obtained from an onboard tracking device depicted the glider maneuvering between 21,000 to 23,800 feet mean sea level (msl) before entering a spiral descent, which is consistent with a loss of control. Several eyewitnesses observed the glider descending in a steep spiral turn toward the ground. The glider collided with high voltage transmission wires and terrain in a sparsely wooded area. An examination of the wreckage did not reveal any preimpact anomalies with the glider. Testing of the glider’s oxygen bottle, regulator, and mask detected no anomalies that would have precluded normal operation of the oxygen system. A review of audio recordings revealed that the pilot’s radio transmissions did not have the muffled sounds normally associated with the use of an oxygen mask, suggesting that the pilot may not have been wearing the oxygen mask during a portion of the flight. According to Federal Aviation Administration guidance, on average, a pilot can operate at 22,000 feet msl for about 5 to 10 minutes before losing consciousness due to a lack of oxygen (hypoxia); the glider was above 22,000 feet msl for 13 minutes 36 seconds. Therefore, it is possible that the pilot became incapacitated due to hypoxia.

Additionally, the pilot, who was diabetic and routinely self-tested his blood sugar levels, recorded a blood sugar level of 58 mg/ml about 1 hour 40 minutes before the flight; according to the U.S. Library of Medicine, a blood sugar level below 70 mg/dl is considered low (hypoglycemia). Although it is likely that the pilot addressed the low blood sugar level by eating or taking glucose tablets, his blood sugar level at the time of the accident is unknown. Hypoxia and hypoglycemia have similar symptoms including headache, blurred vision, tingling or numbness, and unclear thinking, and both conditions can lead to loss of consciousness if they are not promptly remedied. There was no record of the pilot being trained to recognize hypoxia symptoms, and it is possible that he may have mistaken the symptoms of hypoxia for hypoglycemia. It is likely that the pilot became incapacitated and lost control of the glider; however, it could not be determined if the pilot's incapacitation was a result of hypoxia, hypoglycemia, or a combination of the two conditions.

Text from the NTSB. Open the NTSB docket for CEN11FA259 for the full record.