What happened
A cross-country flight from New York to Florida ended in a fatal crash in the Caribbean Sea, north of Jamaica. Approximately 1 hour and 40 minutes into the flight, while cruising at FL280, the pilot notified air traffic control of an abnormal indication and requested a descent to FL180. Although the pilot acknowledged instructions to descend to FL250 and turn, the pilot subsequently failed to comply with further altitude and heading assignments. During radio communications, the pilot exhibited long pauses and signs of confusion, which were consistent with cognitive impairment.
Military aircraft dispatched to intercept the plane approximately 30 minutes after the final radio transmission observed the turboprop flying normally at FL250. However, the crew of the intercepting aircraft noted that both occupants appeared to be unconscious or asleep and were not wearing oxygen masks. Visual inspection from the military aircraft suggested the cabin had depressurized, as the emergency exit door appeared recessed. The aircraft continued on its course until roughly 5 hours and 48 minutes after takeoff, when it descended into the ocean. Wreckage, including the engine and fuselage, was recovered from the seabed four months later.
Findings
Data recovered from the global air system controller (GASC) revealed that several fault codes related to the cabin pressurization system were recorded. An overheat thermal switch (OTSW) had activated, which triggered a shutdown of the engine bleed air supply to the cabin. This loss of bleed air caused the cabin altitude to rise to match the external environment within approximately 4 minutes.
While the pilot likely observed the initial system alerts, the investigation identified several contributing factors. The pilot's cognitive abilities began to decline within 2 minutes and 30 seconds of the initial report, a timeframe consistent with the onset of hypoxia. The investigation noted that the pilot's recent transition training included only a brief segment on pressurization emergencies. Furthermore, the Pilot's Operating Handbook (POH) for the aircraft contained emergency checklists where the instruction to don an oxygen mask was a suggestion rather than a requirement. The investigation also found the cockpit oxygen switch was in the "off" position, though the lack of masks on the occupants rendered this point secondary to the depressurization event. The manufacturer subsequently updated the GASC programming and wiring to prevent unnecessary bleed air shutdowns.