Serious incident Douglas DC-8-51 (F) N507DC, Wednesday 29 April 1998

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Date:	Wednesday 29 April 1998
Time:	22:00 LT
Type:	Douglas DC-8-51 (F)
Owner/operator:	Fine Airlines
Registration:	N507DC
MSN:	45855/281
Year of manufacture:	1966
Total airframe hrs:	3636 hours
Engine model:	P&W JT3D-3B
Fatalities:	Fatalities: 0 / Occupants: 4
Other fatalities:	0
Aircraft damage:	None
Category:	Serious incident
Location:	Brownsville, TX - United States of America
Phase:	Taxi
Nature:	Unknown
Departure airport:	(KBRO)
Destination airport:	Laredo, TX (KLRD)
Investigating agency:	NTSB
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
While taxiing to takeoff, all four occupants of the DC-8 freighter became short of breath. They donned oxygen masks, and the flight returned to the ramp. Cargo included 960 pounds of dry ice (solid carbon dioxide) in 198 separate 4.85-pound blocks, used as a refrigerant for frozen shrimp. Sublimation of dry ice to the gaseous state occurs constantly. When the carbon dioxide (CO2) gas concentration reaches 0.5% shortness of breath may occur, at 2%, depth of respiration increases, and above 4%, breathing becomes rapid and very deep. At sea level and 70 degrees F (incident conditions were 23 feet msl and 66 degrees F), 0.4 pounds of CO2 gas in the cockpit of a DC-8 produces a 0.5% concentration and 3.3 pounds produces a 4% concentration. In 10 minutes, at sublimation rates of 1% and 14% per hour, 960 pounds of dry ice produces 1.6 and 22.4 pounds of CO2 gas, respectively. The 1% sublimation rate is commonly used when calculating amounts of dry ice that can safely be carried on aircraft; however, it was determined for bulk shipments of dry ice and may not be applicable to dry ice used as a refrigerant. The 14% rate was determined using a single 5 pound block of ice.

Probable Cause: The flightcrew experienced physical impairment resulting from an accumulation in the cockpit of carbon dioxide fumes produced by dry ice, a hazardous material, carried in the main cargo compartment.

Accident investigation: