Boeing 737-4Q8 N771AS incident near Sitka, Alaska

Probable cause

The placement of operating equipment in close proximity to an external heating unit's intake by ground personnel, resulting in the contamination of the airplane's cabin with carbon monoxide.

NTSB analysis

After departure the cabin crew reported to the captain that they were becoming nauseated. The captain noted that the departure was the first flight of the day and that the airplane had been sitting on the ramp overnight connected to an external heating unit. He said that prior to flight the first officer and cabin crew went to prepare the airplane while he remained in the operations center with paperwork. He said the first officer requested the ground crew shutdown the external heater because of exhaust fumes in the airplane's cabin. The first officer started the airplane's auxiliary power unit to provide power and heat to the airplane. The captain said that about 15 minutes later he did not detect any unusual smells or odors, and wiped his hand across seats and surfaces to detect contamination. Finding none, he felt it was safe to continue boarding. He reported that the boarding process was normal, and none of the crew or passengers reported any unusual smells or odors. When the cabin crew reported feeling nauseated, the captain determined that they were 20 minutes from their destination and decided to continue the flight rather than turn back. The captain reported that he and the first officer were symptom free, and that the first officer remained on oxygen for the remainder of the flight as a precaution. Upon arrival the flight crew all tested positive for elevated levels of carbon monoxide. The cabin of the airplane was examined and no traces of carbon monoxide were found. Tests of the ground heating unit found no anomalies, and the heat output hose was tested for CO with negative results. According to the director of safety for the operator, the contamination probably occurred when the morning ground crew started the push-back unit in the proximity of the heater's fresh air intake. As a result, the heating unit has been repositioned away from other operating equipment, and a 20-foot exhaust hose attached. Carbon monoxide detectors are now placed in the operator's aircraft during extended external heater usage.

Text from the NTSB. Open the NTSB docket for ANC09IA015 for the full record.