Cirrus SR-22 N581DS accident near Ravenswood, West Virginia

Probable cause

The pilot's improper modification of the certified, on-board oxygen system, which resulted in incapacitation due to hypoxia, and the airplane's subsequent uncontrolled descent into terrain.

NTSB analysis

The pilot of the single-engine, non-pressurized airplane in cruise flight at 25,000 feet above mean sea level requested and was issued a descent clearance to 12,000 feet. The pilot acknowledged the clearance, but the airplane did not descend. Air traffic control (ATC) noted that the pilot sounded "in distress and out of breath." The pilot was issued the clearance multiple times, but the airplane never descended. The last radio transmission received from the airplane was the pilot's labored breathing. Approximately 1 hour later, the airplane crossed directly over the destination airport at 25,000 feet, and maintained its on-course heading. National Guard aircraft scrambled to intercept the airplane were unable to gain the pilot's attention. The intercepting pilots observed an "unresponsive individual who appeared to be unconscious." The airplane continued in cruise flight at 25,000 feet for another hour after passing the destination airport before it slowed, departed controlled flight, and descended into terrain. All major components of the airplane were accounted for at the accident site. Examination of non-volatile memory from the accident airplane revealed that the onboard oxygen system had 29 percent of its total oxygen capacity remaining when the accident occurred. The airplane was equipped with a factory-installed oxygen system that the pilot had augmented by installing a supplemental pulse-demand oxygen system several months prior to the accident. The manufacturers of both systems explicitly advised against the use of non-original components with their respective systems. The pilot routinely used masks from the airplane's original oxygen system with components from the supplemental system he installed, and even noted the occurrence of a previous encounter with hypoxia in his pilot logbook as a result of this practice.

Text from the NTSB. Open the NTSB docket for ERA09FA429 for the full record.