On December 17, 2007, about 1940 Alaska Standard time, the crewmembers of a Beech 1900 airplane, N111AX, aborted their takeoff from Aniak, Alaska, and taxied back to the airport ramp, when they heard a "hissing" sound from the cargo area. Once on the ramp, the captain shut off the engines. The captain and first officer lost consciousness before they could exit the airplane. The airplane was being operated as an instrument flight rules (IFR) cross-country nonscheduled cargo flight under Title 14, CFR Part 135, when the incident occurred. The airplane was operated as Flight 81, by Alaska Central Express, Anchorage, Alaska, and was not damaged. The airline transport certificated pilot was not injured. The airline transport certificated first officer received minor injuries. Visual meteorological conditions prevailed, and an IFR flight plan was filed from Aniak, to Anchorage, but was not activated.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on December 17, the director of operations for the operator reported that he received a call from a ramp agent at Inland Aviation, Aniak. Inland Aviation personnel provided ground handling and fueling for the flight. The agent reported that after the airplane left the ramp, he paid no further attention to it until he received a telephone call from Anchorage Air Traffic Control Center (ARTCC) personnel. ARTCC was inquiring about the airplane, as the crew had not radioed that they were airborne. The agent looked out of the freight building and noticed the airplane sitting on the ramp. The engines were stopped, and the airplane lights were "on" but dim. He opened the door of the airplane, and found the first officer unconscious, inside the door. The captain was unconscious at the controls. He pulled both crewmembers out of the airplane, onto the ramp, and then ran to call for help.

The flight crew regained consciousness while lying on the ramp, and then walked to the freight building. Medical personnel provided care, to include 100 percent oxygen. The flight crew were flown to a hospital in Anchorage where they were released the following day. The crewmembers had no memory of events from the time they stopped the airplane on the ramp, until they woke-up on the airport ramp. The first officer reported that as the captain was taxiing back to the airport ramp, he felt the effects of the gas release, and both crewmembers opened the cockpit windows. Once stopped, he got up to open the forward door, but collapsed at the door.

The director of operations indicated that the airplane initially departed Sheldon Point, Alaska, with an intermediate stop in Aniak for fuel and cargo, before continuing to Anchorage. At Sheldon Point, the crew loaded several large metal cylinders designed to hold compressed gas. Several of the cylinders were empty, but several were full of carbon dioxide. The cylinders have a screw type valve, and a threaded metal safety cap, which is used to protect the valve. Several cylinders were placed in metal racks along the left and right sides of the airplane's cargo compartment. Several others were placed on the floor.

On December 18, a Federal Aviation Administration (FAA) inspector, Anchorage Flight Standards District Office (FSDO), inspected the airplane in Aniak. He reported that the cargo compartment had two tank racks containing five bottles each, standing vertically along each side of the airplane. Two of the cylinders in the left side rack did not have any safety caps installed. Three of the cylinders in the right side rack also did not have any safety caps installed. The caps were found on the floor of the airplane. The inspector found that the middle tank of the three in the right side rack, had a partially open valve. The interior of the airplane had a large amount of frost. The open tank valve was about 1/2 turn open, and was positioned against the interior side-wall of the cargo compartment.

The FAA inspector also indicated that nine carbon dioxide tanks were lying on the floor of the cargo area. They were braced by chocks, but were not strapped down. The crew oxygen masks were not utilized, and the crew oxygen supply tank was full.

An FAA Hazardous Materials Division inspector reported that the cylinders of carbon dioxide are considered hazardous material because they are a pressurized gas in excess of 40 psi. The inspector also indicated that the shipper has a responsibility to properly identify and declare hazardous materials that they are shipping, the carrier has a responsibility to properly train airplane crewmembers to identify and accept hazardous materials, and the flight crew has a responsibility to properly secure hazardous materials during transport.

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