Staunton

The pilot and three passengers departed on a cross-country flight. Shortly after the airplane climbed through 26,600 ft, the pilot stopped responding to air traffic control instructions. According to ADS-B data, the airplane continued climbing to 34,000 ft, then flew at that altitude along its flight plan waypoints, turning southwest to overfly the intended destination about 1 hour later. The airplane continued flying for about another hour along a relatively constant track and altitude before entering a spiraling descent and impacting terrain. United States Air Force (USAF) pilots intercepted the airplane about 2 minutes before it began the spiraling descent. They observed no breaches of the airplane structure or doors, no smoke in the cockpit or passenger cabin, and no oxygen masks deployed in the cabin. One occupant was observed slumped over in the pilot seat and no movement or other occupants were observed in the cabin. Based on the lack of response to air traffic control communications, ADS-B data showing the airplane following its flight plan waypoints at the altitude last assigned by air traffic control, and the USAF pilot observations, it is likely that the pilot of the accident airplane became incapacitated during the climb to cruise altitude. It is also likely the airplane trajectory was then directed by the autopilot until a point at which it was no longer able to maintain control. The pilot had medical conditions, including high blood pressure and high cholesterol, that represented some increased risk of an impairing or incapacitating cardiovascular event. In addition, the pilot had prior prescriptions for medications that could be impairing if used too recently before flight. However, there was no evidence of the pilot being at exceptionally high incapacitation risk, or of using medications inappropriately. Based on the accident circumstances, it is likely that all the airplane occupants were incapacitated due to a common environmental condition, such as loss of cabin pressurization. Maintenance records indicated that, at the time of the accident flight, five items were overdue for inspection, including the co-pilot oxygen mask. About 4 weeks before the accident flight, maintenance personnel noted 26 discrepancies that the owner declined to address, including several related to the pressurization and environmental control system. Furthermore, 2 days before the accident flight, maintenance personnel noted that the pilot-side oxygen mask was not installed, and the supplementary oxygen was at its minimum serviceable level. At that level, oxygen would not have been available to the airplane occupants and passenger oxygen masks would not have deployed in the event of a loss of pressurization. No evidence was found to indicate that the oxygen system was serviced or that the pilot-side oxygen mask was reinstalled before the accident flight. Altitude-related hypoxia, although not verifiable from forensic medical evidence, likely explains the incapacitation of the airplane occupants. According to the FAA Pilot’s Handbook of Aeronautical Knowledge, impairing effects from hypoxia are often vague and are experienced differently by different individuals; they include confusion, disorientation, diminished judgment and reactions, worsened motor coordination, difficulty communicating and performing simple tasks, a false sense of well-being, diminished consciousness, and, if conditions aren’t remedied or mitigated, death. Between 30,000 and 35,000 ft, the time of useful consciousness for a pilot to take protective action against hypoxia, including donning an oxygen mask and descending, is about 1/2 to 2 minutes. These times depend on multiple variables, including medical factors, with substantial variation among individuals. The times are decreased by about half when depressurization is rapid. However, gradual depressurization can be as dangerous or more dangerous than rapid depressurization because of its potential to insidiously impair a pilot’s ability to recognize and respond to the developing emergency until the pilot is no longer effectively able to do so. Cognitive impairment from hypoxia makes it harder for affected individuals to recognize their own impairment. Based on the available information, it is likely that the airplane occupants became hypoxic due to a lack of oxygen during the flight and became incapacitated. However, the reason for the loss of pressurization, and whether it was rapid or progressed over time, could not be determined.

The airplane was operated by a company pilot. A noncompany pilot, who had not attended or completed a training course or received a checkout for Raytheon Aircraft Company Beech King Air 200 airplanes, was asked by the pilot to accompany him on the flight so that the noncompany pilot could accumulate flight time. The flight only required one pilot. While the airplane was in cruise flight (27,000 feet mean sea level), the cockpit voice recorder (CVR) recorded the sound of the windshield fracturing. The CVR transcript indicated that the company pilot was not in the cockpit when the windshield fractured because he was emptying trash in the cabin. This action showed poor judgment considering the noncompany pilot was not qualified in the airplane. Although the windshield stayed in place, the company pilot stated that “within seconds” after it fractured, he depressurized the airplane because he was unsure about the windshield’s “integrity.” However, the Beech King Air Airplane Flight Manual (AFM) states to maintain cabin pressurization in the event of a fractured windshield and further states that the airplane can continue flight for up to 25 hours with the windshield fractured. During the on-scene examinations, an unapproved document (not derived from the AFM) that contained several checklists was found on the airplane. The company pilot stated that he used this document and that it “came with the airplane.” The document did not include a checklist addressing a cracked or shattered windshield. The company pilot most likely was not aware that the airplane should not have been depressurized nor that it could operate for 25 hours after the fracture occurred and, therefore, that the fractured windshield did not present an in-flight emergency. The CVR transcript revealed that, after depressurizing the airplane, the pilots attempted to use the oxygen masks but were unable to receive any oxygen. (The pilots most likely did not turn the oxygen on once they needed it because they either forgot as a result of the emergency or because they did not have time to do so before they lost consciousness.) According to the company pilot, during his preflight inspection of the airplane, the oxygen system was functional. He stated that, after the inspection, he turned the oxygen system ready switch to the OFF position because he wanted to “save” the oxygen, which was not in accordance with the Before Start checklist in the AFM. Post accident functional testing of the oxygen system revealed normal operation. The unapproved checklists document did not include the instruction to leave the oxygen system on. Regardless, the pilot stated that he knew the approved checklist stated to leave the oxygen system on but that he still chose to turn it off. The pilot exhibited poor judgment by using an unapproved, incomplete checklists document and by knowingly deviating from approved preflight procedures. About 1 minute after the pilots tried to get oxygen, the CVR recorded the last comment by either pilot. For about the next 7 minutes until it stopped recording, the CVR recorded the sounds of increased engine propeller noise, the landing gear and overspeed warning horns, and altitude alerts indicating that the airplane had entered an uncontrolled descent. (The CVR’s 4-g impact switch was found in the open position during the on-scene examination, indicating that the airplane experienced at least 4 acceleration of gravity forces.) Further, a plot of two radar data points, recorded after the last pilot comment, showed that the airplane descended from 25,400 feet to 7,800 feet within 5 minutes. Shortly thereafter, the pilots regained consciousness and recovered from the uncontrolled descent. The airplane was substantially damaged by the acceleration forces incurred during the uncontrolled descent and subsequent recovery. Examination of the windshield revealed that a dense network of fractures was located on the inner glass ply; however, the windshield did not lose significant pieces of glass and maintained its structural integrity. Therefore, the fractures did not preclude safe continued flight. Post accident examinations revealed evidence that the fracture initiated due to a design deficiency in the glass. The manufacturer redesigned the windshield in 2001 (the accident airplane was manufactured in 1998), and no known similar fractures have occurred in the newly designed windshield. The manufacturer chose not to issue a service bulletin for a retrofit of the new windshield design in airplanes manufactured before 2001 because the fracture of one pane of glass is not a safety-of-flight issue.

Henson Airlines flight 1517 was cleared for an instrument approach to the Shenandoah Valley Airport, Weyers Cave, Virginia, at 0959 on September 23, 1985, after a routine flight from Baltimore-Washington International Airport, Baltimore, Maryland. Instrument meteorological conditions prevailed at Shenandoah Valley Airport. there were 12 passengers and 2 crew members aboard the scheduled domestic passenger flight operating under 14 CFR 135. Radar service was terminated at 1003. The crew of flight 1517 subsequently contacted the Henson station agent and Shenandoah unicom. The last recorded radar return was at 1011, at which time the airplane was east of the localizer course at 2,700 feet mean sea level and on a magnetic track of about 075°. At 1014 the pilot said, '..we're showing a little west of course...' and at 1015 he asked if he was east of course. At 1017, the controller suggested a missed approach if the airplane was not established on the localizer course. There was no response from the crew of flight 1517 whose last recorded transmission was at 1016. The wreckage of flight 1517 was located about 1842 approximately six miles east of the airport. Both crew members and all 12 passengers were fatally injured.