Killer gas

 作者:木隈     |      日期:2019-03-07 03:05:00
By Paul Marks An incident in which a plane’s crew almost suffocated at the controls has highlighted a possible risk to pilots of cargo aircraft. America’s aviation safety watchdog now fears that carbon dioxide gas from evaporating dry ice, which is frequently used to cool perishable cargo, could starve the crew on the flight deck of oxygen. The National Transportation Safety Board (NTSB) is now pressing the Federal Aviation Administration (FAA) to undertake tests to find out how much dry ice – solid carbon dioxide – can safely be carried on various types of cargo aircraft. The NTSB believes that current rules might allow a plane to carry more dry ice than its ventilation system can cope with. The NTSB’s concern follows an incident in 1998 at Brownsville, Texas, in which all four crew on a Douglas DC-8 cargo aircraft fell ill as they taxied to the runway for take-off. Their symptoms included dizziness, racing heartbeats and shortness of breath. A fire department analysis of the atmosphere in the plane noted that oxygen levels were low. Dry ice is popular as a refrigerant on cargo planes because it sublimes directly to CO2 gas, rather than melting to form water which could corrode the plane. The DC-8 had been carrying frozen seafood cooled by 435 kilograms of dry ice. But the aircraft maker’s guidelines on carrying dry ice, based on an FAA standard set in 1974, say the plane ought to be able to carry more than 2500 kilograms of dry ice. NTSB investigators now believe the rate at which dry ice sublimes changes markedly depending on how it is packaged. The dry ice on the DC-8 was spread among 198 separate containers. This means that it would sublime far faster than the large lumps of dry ice on which the FAA’s current sublimation rates are calculated. As a result, CO2 emissions could have been “an order of magnitude higher than current guidelines would suggest”, the NTSB says. Paul Schlamm, a spokesman for the NTSB,