ERA23FA256

Pilot incapacitation due to loss of cabin pressure for undetermined reasons. Contributing to the accident was the pilot’s and owner/operator’s decision to operate the airplane without supplemental oxygen.

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.

On June 4, 2023, at 1523 eastern daylight time, a Cessna Citation 560 airplane, N611VG, was destroyed when it was involved in an accident near Montebello, Virginia. The airline transport pilot and three passengers were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to flight track records and individuals familiar with the airplane’s flight activity the day of the accident, the airplane’s first flight of the day originated from its home airport of Melbourne International Airport (MLB), Melbourne, Florida, at 1059. The accident pilot conducted the flight as a single pilot without passengers and subsequently landed at Elizabethton Municipal Airport (0A9), Elizabethton, Tennessee, at 1231. While at 0A9, 300 gallons of fuel were added to the airplane and the three passengers were picked up. The accident flight departed 0A9 at 1313, destined for Long Island MacArthur Airport (ISP), Ronkonkoma, New York. Review of FAA air traffic control audio recordings revealed that the pilot contacted the Atlanta Air Route Traffic Control Center shortly after takeoff and reported climbing through 9,300 ft mean sea level (msl) to 10,000 ft msl. The controller subsequently cleared the airplane to 23,000 ft pressure altitude and the pilot read back the clearance. At 1322, the pilot was handed off to another controller within Atlanta Center. The pilot contacted the controller, advising that the airplane was maintaining 23,000 ft. The controller cleared the airplane to 29,000 ft and the pilot read back the clearance. At 1325, the controller cleared the airplane to 34,000 ft and the pilot read back the clearance. According to ADS-B data, the airplane was climbing through 26,600 ft at this time. At 1328, the controller amended the previous altitude clearance, instructing the pilot to stop the climb at 33,000 ft for crossing air traffic. The pilot did not respond to the amended clearance, and the airplane continued the climb to 34,000 ft and leveled off. No further radio transmissions from the pilot were received for the remainder of the flight, despite repeated attempts to contact the pilot. Review of ADS-B data found that the airplane’s flight track to the destination airport was consistent with the filed flight plan route. The airplane arrived overhead of ISP at 1432, maintaining 34,000 ft throughout the flight. The airplane then continued flying southwest on a ground track of about 240°. The flight path showed little deviation in track angle or altitude until 1522, when the airplane entered a rapidly descending right spiral descent into terrain. The figure depicts the ADS-B flight track, the filed flight plan waypoints, and selected time/altitude labels. Figure. Overview of ADS-B flight track, flight plan waypoints, and accident location, with selected time and altitude labels. According to a North American Aerospace Defense Command statement, about 1520, the airplane was intercepted by USAF fighter aircraft. The USAF pilots stated that there was nothing remarkable about the exterior of the airplane, such as holes or missing windows or doors. They did not observe any airframe icing, frost on the cockpit or passenger windows, or smoke in the cabin. They observed a person seated in the left cockpit seat, who was slumped completely over into the right seat and who remained motionless throughout their observations. The person was unresponsive to several radio transmissions, intercept flight maneuvers, and flare deployments. The USAF pilots could not see whether this person was wearing a headset or an oxygen mask, and they could not see whether there were any lights flashing in the cockpit. The passenger window shades were open. No movement was observed in the cabin area, and the USAF pilots did not see any shapes that resembled a person in the cabin area. The airplane impacted mountainous and forested terrain about 1.5 miles north of the Montebello VOR, a short distance from where the airplane’s last ADS-B-derived position was observed. The small area of the debris field and the angle that tree limbs surrounding the debris field were severed were consistent with a high-velocity, near-vertical descent. The wreckage was fragmented and scattered around a main crater, and evidence of a postimpact fire was observed. One engine was generally intact and was located about 100 ft downhill from the impact crater. The other engine was fragmented and located in the debris field. Blades/vanes on both engines were bent opposite the direction of rotation. Flight control continuity could not be established due to the fragmentation of the wreckage. A cockpit voice recorder was not located or recovered from the accident site. The oxygen high pressure relief valve overboard discharge indicator was found intact. A bleed air flow valve and sections of ducting were also located in the debris. Two passenger oxygen masks were located, which remained attached to their respective box assemblies. One oxygen mask stowage/dropout box assembly and its oxygen shutoff valve were found capped with a dust cap rather than an approved AN cap. No other major portions of the oxygen or pressurization system were located in the debris. No evidence of any preaccident mechanical failures or malfunctions were found that would have precluded normal operation of the airplane or its subsystems. According to the airframe manufacturer, the airplane’s environmental control system used engine bleed air to pressurize and air condition the cabin, and to defog the cabin and cockpit windows. Flight crew controls for this system were located on the Pressurization – Environmental Control Panel at the front of the center pedestal. This panel included gauges for pressure differential (between the cabin and outside atmosphere), cabin altitude, and cabin altitude rate of change. Also included were controls for the desired cabin altitude, cabin altitude rate of change, cabin temperature, airflow distribution (between the passenger cabin and cockpit), and pressurization source. Flight crew could also use a guarded emergency dump switch to open the pressurization outflow valves and equalize the cabin altitude with airplane altitude up to about 13,000 ft (± 1,500 ft). The cockpit annunciator panel included a red “CAB ALT 10,000 FEET” light that would illuminate when the cabin pressure altitude was above 10,000 ft. This light would also trigger the master warning system, illuminating the master warning light. Supplementary oxygen was provided to flight crew via sweep-on masks stowed in retainers below each cockpit side window and to passengers via continuous flow masks stowed in the cabin overhead panel at each seat. The passenger system oxygen flow was controlled by an electrically operated solenoid valve, which was actuated by an altitude pressure switch. In the event of a possible decompression, the altitude pressure switch was designed to actuate the solenoid valve between 12,900 to 14,100 ft cabin altitude. When the door actuators were energized with pressure from the supplementary oxygen tank, the stowage/dropout boxes would then open, releasing the masks. Flight crew controls for the oxygen system were located on the left console. An oxygen control valve switch could be used to direct the flow of oxygen to flight crew only or to both flight crew and passengers; this switch could also be used to manually drop the passenger oxygen masks if they did not drop automatically. Flight crew and passenger supplementary oxygen was provided from a single 76 cubic ft bottle located in the tailcone compartment. When fully charged, it would provide about one hour of oxygen for two flight crewmembers and six passengers. An oxygen pressure gauge was located on the right instrument panel. According to Continuous Airworthiness Maintenance Program records for the airplane, 5 maintenance items were overdue on the date of the accident, including an in