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|Category:||Fire Smoke and Fumes|
Smoke Hoods are protective head coverings which, as a minimum, have a filter system that prevents wearers from breathing in unwanted smoke gases and particulates generated in a fire. They may also incorporate a small oxygen generator by which their useful endurance as a protective device is increased. They are made of a heat resistant material which is able to retain its integrity up to temperatures in excess of 350 degrees Celsius.
The Smoke Hazard
It has long been observed that occupant survival in a major aircraft fire, long enough to effect a successful evacuation, requires not only that a fire does not become a directly debilitating hazard but that the associated smoke does not have a similar effect first. It has also frequently been noted that whilst thermal injuries are responsible for around 20% of fatalities in aircraft fires, the majority of the rest are attributable to the direct or indirect effects of exposure to toxic and/or irritant smoke. Whilst means to reduce overall smoke levels have received much attention, the alternative of finding ways to create a survivable personal environment for individual occupants has also been the subject of much research and debate, and this has led to the development of ‘smoke hoods’.
Smoke Hood Function
The most important basic function of any smoke hood is the ability to convert toxic carbon monoxide to relatively harmless carbon dioxide through a catalytic process. Most smoke hood designs also use some form of activated charcoal filter to screen out corrosive gases, such as those of ammonia and chlorine, and acid gases, such as hydrogen chloride and hydrogen sulphide. The temperature of inhaled gases must also be maintained within acceptable limits.
"Heavy Duty" Smoke Hoods
Within the last twenty years, "heavy duty" smoke hoods have become accepted equipment for Cabin Crew. The purpose is to allow them the opportunity to attempt fire suppression and manage the cabin and its eventual evacuation without becoming affected by smoke gases. A key issue in the design of all smoke hoods is the assumed respiration rate which will vary according to the extent of physical activity by a factor of approximately 3. Because minimum-use duration requirements specified by Regulators for Cabin Crew Smoke Hoods are usually 20 minutes when ‘physically active’, they also incorporate a potassium dioxide cartridge which functions as an oxygen generator as well as filters. They also have a higher quality visor and speech capability such that it is possible to use a megaphone or crew interphone whilst wearing them. Overall, they are of much more substantial construction and provide greater all-round protection for the wearer than the lightweight ‘basic’ smoke hoods which have been persistently proposed for passenger use. Consequentially, they are much heavier and the ability to safely and effectively use them requires specific classroom familiarisation which is routinely included in the initial and recurrent training of aircrew who operate aircraft in which they are carried. Cabin Crew Hoods are carried in sealed containers and, once donned, allow a neck seal to be made and are secured by tapes. The oxygen generator must be manually activated.
Passenger Smoke Hoods
Smoke Hoods are now routinely provided for Cabin Crew but, despite years of debate dating back to as long ago as an NPRM (Notice of Proposed Rule Making) issued by the FAA in 1969, they have not yet been approved for passenger use.
Should smoke hoods ever be approved for passenger use, they will clearly have a lower performance requirement, which will reduce their weight and complexity so that their rare use by people with only the guidance of a cabin safety briefing and instruction cards is likely to be practicable. It is mainly concerns at the range of passenger abilities to use such equipment, and awareness of experience with passenger oxygen mask use during cabin de-pressurisations, which has prevented the adoption of smoke hoods for passengers so far.