Incident Overview
A commercial pilot and his wife departed New York in their turboprop airplane for a cross-country flight to Florida. About 1 hour 40 minutes into the flight, while cruising at flight level (FL) 280, the pilot notified air traffic control (ATC) of an abnormal indication and requested a descent to FL180. The controller instructed a descent to FL250 and a left 30° turn, which the pilot acknowledged and executed. He then again requested a lower altitude. The pilot declined emergency handling and did not specify the problem, but the controller independently suspected a pressurization issue and coordinated with another facility to clear traffic. The controller then instructed the pilot to descend to FL200 and change course; however, the pilot acknowledged multiple times but did not comply.
Evidence of Hypoxia
The pilot's failure to comply, long microphone pauses after concluding statements, and confusion were consistent with cognitive impairment due to hypoxia. Transmissions to ATC showed impairment within 2 minutes 30 seconds of reporting the abnormal indication, within the FAA's published time of useful consciousness ranges for hypoxia. Military airplanes were dispatched about 30 minutes after the pilot's final transmission. They reported the airplane flying normally at FL250, both occupants appearing asleep or unconscious, and neither wearing an oxygen mask. Photographs showed the emergency exit door recessed into the fuselage, consistent with a depressurized cabin. The military escorted the airplane until it approached Cuban airspace. Radar data indicated the airplane continued on a constant course and altitude until about 5 hours 48 minutes after takeoff, when it descended to impact in the Caribbean Sea north of Jamaica.
Wreckage and System Data
Some wreckage, including fuselage and engine components, was recovered from the ocean floor about 4 months after the accident. Data from the global air system controller (GASC) nonvolatile memory showed several fault codes associated with the cabin pressurization system. The overheat thermal switch (OTSW) had activated, shutting off engine bleed air to the pressurization system. Without bleed air, cabin altitude would increase to outside altitude over about 4 minutes. The faults and associated alerts/warnings would have been displayed to the pilot as crew alerting system (CAS) messages and master warning/caution annunciations.
Pilot Actions and Checklist Deficiencies
The pilot was known to routinely monitor cabin altitude. He likely observed CAS messages at their onset, triggering his call to ATC. He had not recently trained in an altitude chamber but should have been familiar with pressurization emergency and oxygen mask donning procedures from a transition course. However, the pressurization segment was only about 90 minutes of a 36-hour course. It is unknown if he retained enough to recognize the CAS messages' significance. Given his reported diligence with checklists, he likely would have attempted to review emergency procedures in the Pilot's Operating Handbook (POH). A review of the 656-page POH found that only one of four emergency checklists for pressurization-related CAS messages included a step to don an oxygen mask, and it was a suggestion, not mandatory. This lack of mandatory guidance, combined with the rapid cabin altitude increase, significantly increased hypoxia risk.
Cognitive Decline and Missed Opportunities
The pilot initially requested a descent to FL180 instead of 10,000 ft as prescribed, accepted FL250, and declined priority handling—either early signs of hypoxia or misinterpretation of CAS messages. Although the cabin bleed-down took about 4 minutes, cognitive deterioration was evident within 2 minutes 30 seconds of the initial report. The pilot had less than 4 minutes to detect the failure, report to ATC, locate procedures in a voluminous POH, and execute them while suffering from a mentally impairing condition.
Post-Accident Modifications
Following the accident, the airplane manufacturer revised emergency procedures for new airplanes to require oxygen mask donning as the first checklist item in relevant emergency checklists, with plans to extend to previous models in 2017. The manufacturer also changed GASC programming after finding that the pressurization system design forced the GASC to stop bleed air if temperature exceeded an initial threshold and did not fall below a secondary threshold within 30 seconds. Although many OTSW replacements had been documented, those tested were normal; the accident OTSW passed functional tests despite internal corrosion from sea water.