Airlines should take several extraordinary safety measures when preparing to transport Covid-19 vaccines at temperatures of minus 70 deg C, according to recommendations outlined in a new Safety Alert for Operators (SAFO) issued by the FAA.
Much of the alert addresses special considerations centering on the sublimation rate of dry ice, or the rate at which the material transitions directly from a solid to a gas, at temperatures higher than minus 78 deg C under normal atmospheric pressure. A potential risk associated with dry ice sublimation centers on the possibility of gaseous CO2 replacing oxygen in aircraft compartments and interfering with the ability of occupants to breathe. High levels of CO2 gas can lead to unrecognized loss of cognition and present asphyxiation hazards to ground crew who handle the loading and unloading of cargo containers, for example. The hazards increase with the amount of dry ice carried, its sublimation rate, and any aircraft ventilation deficiencies.
Decreased pressure such as an 8,000-foot cabin altitude will increase the sublimation rate, notes the FAA. Reducing cabin pressure will draw CO2 from its packages, increasing its concentration in the compartment. Therefore, operators shouldn’t use existing smoke/fire/fume procedures unless they modify them to address the phenomenon.
The hazards have prompted the FAA to recommend aircraft manufacturers to provide information on maximum dry ice quantities the aircraft’s ventilation can accommodate depending on the sublimation rate. That will determine the maximum amount of dry ice an aircraft can safely transport and require dispatch with fully operational environmental control systems, including all air conditioning packs and the auxiliary power unit to ensure proper ventilation for ground and in-flight operation.
Other considerations include the loss of weight that occurs as dry ice sublimates, affecting the aircraft’s center of gravity. The FAA also recommends the installation or carriage of CO2 sensors in the aircraft or worn by the pilots and other crewmembers to help them recognize dangerous levels of the gas. Training on specific conditions and procedures can improve pilot decision-making in case of a CO2 alert or other system abnormalities, the SAFO adds.
Finally, operators need to consider the fact that at the end of the flight, CO2 concentrations will have increased in the cargo compartment and can take several minutes to dissipate. Once crewmembers open the door, the area immediately outside will also contain high levels of CO2 for several minutes, concludes the SAFO.