Two F-16 Fighting Falcons from the Ohio Air National Guard that scrambled to intercept a non-responsive Cirrus SR22 cruising at 25,000ft over Ohio late on the evening of 30 July discovered what controllers had no doubt suspected -- a pilot "unconcious" at the flight controls.
Once fuel was depleted, the aircraft pitched nose-up and decelerated at 25,000ft "until a sharp, descending left spiral was entered," writes the NTSB in its prelminary report, based on recovered avionics data.
Authorities responding to the crash scene in Ravenswood, West Virginia, found the remains of the lone pilot, though it's unclear whether he died before the final spiral.
The accident smacks of a tragic loss of six aboard a Learjet 35 in October 1999, including professional golfer Payne Stewart. F-16s were called to intercept Stewart's chartered plane for a flight from Orlando to Dallas after all contact was lost. The aircraft, with no visible signs of life onboard, ultimately climbed to more than 46,000ft and cruised Northwestward until it ran out of fuel and crashed near Aberdeen, South Dakota. NTSB ruled that the probable cause of the crash was "incapacitation of the flight crewmembers as a result of their failure to receive supplemental oxygen following a loss of cabin pressurization, for undetermined reasons."
In the Cirrus accident, fhe first clues of a problem began when a controller cleared the pilot to climb from 22,000ft to 23,000ft enroute to Eagle Creek Airpark (EYE) in Indianapolis, Indiana, from the departure airport, York Municipal Airport (JYR) in York, Nebraska. Continuous supplemental oxygen is required by the FAA for pilots flying above 14,000ft. Earlier that day, the accident pilot and the non-pilot owner of the N581DS had flown to JYR from EYE at 25,000ft using supplemental oxygen. "The oxygen system was not serviced prior to departure, but was going to be serviced at EYE on 'Monday'", according to the NTSB's interview with the owner.
NTSB says the pilot acknowledged the instructions to climb to FL230, but the controller noted that his voice "had changed, and had taken on a 'helium / Mickey Mouse' quality".
The situation deteriorated from there, with the pilot later "stepping all over himself" in communications calls with air traffic control. An airline pilot listening in said the pilot sounded "incoherent". For anyone who has taken oxygen deprivation training, such hypoxic-like symptoms, are not at all suprising...
The pilot, perhaps realizing the gravity of the situation, did later ask for a descent to 12,000ft, but he never accomplished the task. His final radio transmission consisted of "labored breathing," according to the NTSB's report. The investigation continues...