West Air Cessna 208 B incident

The pilot loaded the cargo onboard the airplane, closed the door, and started the engine with the cabin air vents closed, as it was cold outside. While taxiing the airplane to the run-up area, the pilot became sleepy, had difficulty breathing, and subsequently became unconscious and unresponsive to tower controllers. About 37 minutes after the pilot closed the door of the unvented cockpit, a firefighter responded to the airplane, which was still parked in the run-up area, opened the airplane door, and the pilot regained consciousness. The airplane was not damaged. The shipper of the boxes had grossly underreported the amount of dry ice contained in the boxes due to an improperly trained employee who had deposited nearly twice the amount of dry ice in each box. Dry ice sublimates into carbon dioxide (CO2), which can lead to loss of consciousness and death at certain exposure levels. Within 5 minutes, the airplane's unvented configuration led to a CO2 concentration that was twice the Federal Aviation Administration and Occupational Safety and Health Administration standards and, within 30 minutes, the concentration reached a level consistent with loss of consciousness. Based on the CO2 concentrations at the time of the incident, the cause of the pilot's loss of consciousness was CO2 poisoning from the sublimation of dry ice in an unventilated space. The pilot may still have suffered symptoms from CO2 poisoning and possibly incapacitation had the unventilated cabin been loaded with the dry ice weight that was reported on the label; however, proper ventilation would have decreased the CO2 concentration in the cabin and may have prevented the pilot's loss of consciousness altogether. At the time of the incident, publicly available literature included an advisory circular to pilots flying cargo loads of dry ice to maintain an adequate circulation of fresh air. Despite the availability of this information, the pilot chose to fly the airplane in an unventilated configuration, and the operator did not have any guidance to encourage ventilation, as its policy permitted operation with both the overhead and side vents closed.

Verbatim NTSB analysis and probable cause from the NTSB dataset