N781FECessna 208B · 1991

On November 23, 2018, about 1733 Pacific standard time, the pilot of a Cessna 208B, N781FE, became incapacitated while taxiing near Bakersfield, California. The airline transport pilot received minor injuries and the airplane was not damaged. The airplane was owned by FedEx Corporation and operated by West Air, Inc as a Title 14 Code of Federal Regulations Part 135 on-demand cargo flight.

According to the pilot, while loading the airplane, he counted a total of 68 boxes, 41 of which displayed labels indicating they contained 2 lbs of dry ice in each box. A FedEx dangerous goods representative approved the shipment but did not check the contents of the boxes nor was he required to. According to the operator's internal policy, a maximum of 168 lbs was permitted in the cabin. The reported dry ice weight furnished by the shipper was within the company’s operating limitations.

After the loading was completed, the pilot finished his paperwork and started the airplane at 1729. While taxing to the runway, he felt "strong sleepiness" accompanied by difficulty breathing. He stopped the airplane at the runway run-up area and closed his eyes.

After the pilot failed to respond to air traffic control for 25 minutes, the tower controller alerted the fire department, and a firefighter illuminated the cockpit and observed the pilot with his head tipped back and his mouth open. The firefighter placed wheel chocks in front of the main landing gear tires and boarded the airplane. Following several unsuccessful attempts to get the pilot's attention, the firefighter administered a sternal rub, which caused the pilot to move. The firefighter engaged the fuel cutoff to shut down the engine and subsequently disengaged the electrical system. During this time, the pilot became more conscious, but his speech was unintelligible. Moments later, the pilot was able to demonstrate to the firefighter that he was coherent by answering a series of questions. The pilot exited the airplane on his own and was transported to the hospital by an ambulance.

The pilot reported that he carried dry ice only on occasion and usually a total of about 10 boxes (also 2 lbs each), located in the back of the airplane at zones 5 or 6. In this incident, the boxes were loaded into zones 1, 2, and 3, located directly behind the cockpit. The pilot did not report any medical conditions or use of medications at the time he was issued his most recent airman medical certificate on January 20, 2018. Several months later, he was diagnosed with and treated for Valley Fever, a fungal lung infection, with the drug fluconazole. The medication is not generally considered cognitively impairing. The pilot stated that he had not experienced any side effects while taking the medication apart from drowsiness experienced during the first night he took the medication, which took place several months prior to the incident.

Postincident medical treatment records showed that the pilot was given a CT scan of the head/brain that was unremarkable, a cardiac catheterization that showed no evidence of coronary artery disease, an echocardiogram that showed no abnormalities, and extensive laboratory testing that did not identify any abnormalities. A few weeks after the incident, the pilot received an outpatient review with his cardiologist who concluded that the signs and symptoms the pilot experienced as a result of the medical testing were consistent with carbon dioxide (CO2) poisoning from the dry ice on the incident airplane. Excess inhaled CO2 is rapidly exhaled (within minutes) down to normal levels.

Review of the shipper's procedures revealed that the company used a standard of one scoop of dry ice that equaled 2 lbs per box, a standard practice they had used for years. The dry ice was not measured on a scale or by any other means nor was there a procedure to verify the weight of the dry ice used in packaging. According to a representative of the company, each package should contain only one scoop of dry ice. The company did not employ anyone to directly oversee the packaging process, but a supervisor on the floor was available during packaging.

The shipper stated that the incident shipment was an extremely high-volume shipment and irregular for their normal air freight operation. At the time the order was placed, they did not have their usual staff to fill the order, and they reassigned an employee who had not packaged hazardous materials in 17 months or completed hazardous materials training since September 2015 to complete the order.

The employee who distributed the dry ice for the incident flight stated that he normally used 2 scoops using the provided 64 oz scoop for larger boxes. On the day of the incident, he used between 1.5 to 2 scoops of dry ice for each box and then marked "0.9 kg" (2 lbs) on each box label because that is the standard they always used. Following the incident, the employee reported that the shipper measured the full scoops and determined that one scoop is equal to 2.3 lbs.

Based on the 2 lbs per scoop per box weight, the declared total shipment of dry ice would have been 81 lbs for the 41 boxes. Factoring in the weights supplied and adding 1.5 to 2 scoops per box, the computations showed that the net mass of the dry ice in the shipment would have been between 122 lbs and 162 lbs.

FAA Advisory Circular (AC) 91-76A describes the hazards associated with the sublimation of dry ice aboard aircraft. According to the AC, dry ice is generally carried aboard aircraft to keep food, medicine, or biological materials frozen or in a chilled condition and is considered a hazardous material. Dry ice sublimates into gaseous CO2 at aircraft environment temperatures and may lead to aircrew incapacitation at excessive levels of exposure. Tests performed by the FAA demonstrated that for small, insulated shipping packages containing 4.6 to 5.3 lbs of dry ice, such as those onboard the incident airplane, the sublimation rate averaged 2% per hour.

The AC states that exposures to CO2 should not exceed a concentration of 0.5% (5,000 parts per million [ppm]). The signs and symptoms of CO2 poisoning are similar to hypoxia and include headache, dizziness, muscular weakness, drowsiness and ringing in the ears. Removal from the exposure results in rapid recovery. Exposure can be mitigated by maintaining adequate circulation of fresh air. The pilot reported that at the time of the incident, he had closed off both the overhead and cockpit vents because it was cold outside, and he was in light clothing. The operator did not have any policies requiring pilots to keep the cabin ventilated with dry ice onboard at the time of the incident.

A sublimation study performed by the NTSB Vehicle Performance division showed that the cabin CO2 concentration in an unvented configuration would have reached 1% in 5 minutes, twice the FAA and Occupational Safety and Health Administration standard. By 30 minutes, the concentration would have reached 6.37%; this concentration is consistent with loss of consciousness, coma, and eventually, death. For additional information, see the dry ice sublimation study in the public docket for this accident.

At the correct weight of 81 lbs (37 kg) of dry ice, the cabin would have a reached CO2 concentration level of 1% in about 10 minutes and would have likely been above 4% when the firefighter opened the door.

The NTSB medical review stated that as CO2 levels rise above 1,000 ppm the person will begin to experience drowsiness, poor concentration, along with other symptoms consistent with low oxygen. As inhaled levels rise above 5,000 ppm, "coma and eventually death will ensue. At higher inhaled levels, negative effects occur more rapidly."

At the time of the incident, the operator's policy for carrying dry ice included a chart that stated a maximum of 168 lbs of dry ice could be carried with both the overhead and side vents closed. The operator distributed CO2 detectors to many of its pilots following the incident. The pilot did not have a CO2 detector with him at the time of the incident.

On March 5, 2018, the pilot experienced similar symptoms as he did on the day of his loss of consciousness. He picked up a shipment of dry ice and after he closed the door, started the engine, and began taxiing, the pilot had difficulty breathing and keeping his eyes open. The pilot reported that he did not have any of the cockpit vents open at that time. The CO2 detector issued to him by the operator indicated 4,000 ppm after he had only taxied about 200 ft. An irregularity report filed by the pilot stated that he opened 3 air vents after the detector indicated levels above 5,000 ppm. The pilot turned the airplane around to return to the ramp and, as he approached, his symptoms grew worse, and the detector showed 7,800 ppm. According to the irregularity report, he then opened the door and the detector immediately decreased to 6,450 ppm.