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Cuando el humo se mete en  los ojos.

Smoke is beginning to fill the passenger cabin of the smal commuter aircraft that has just slid to halt on its belly after the undecarriage collapsed on landing

 

Assuming the passengers are conscious, are their chances of getting out alive greater if they put on smoke hoods before making their way to the easily accessible and well it emergency exits, or will delay cost them their lives?. This is one of the questions that the UK CAA is trying to answer in its research into the use of smoke hoods before deciding wheter to make the mandatory on public transport aircraft. The conclusion of an interin report released in june is that more research is need in specific areas before a decision is made and a final specification issued . The authority expects to reach a decision by the end of this year. In producing its interin report, the CAA put a series of questions to a variety of interested parts including airframe and equipment manufacturers, regulatory authorities, passenger representative bodies, research establishments and operators, and invited their comments.

 

One interesting view held by several of those who commented was that, whatever else any cost/benefit study in the area would reveal, smoke hoods would be of greater benefit to passengers in fixed-wing aircraft than the life jackets that these aircraft are currently required to carry. A certain amount of cynism showed itself when one commentor said that cost/benefit studies were of little value and that the final decision should be based on public opinion and wheter or not passengers would be willing to pay for the equipment to be carried. Opinions deffered again on whether lives would be lost as people delay the evacuation while trying to put the equipment on. Some commentators quoted a US FAA report that said the use of smoke hoods alone did not seem to have a significant effect upon evacuation rates, while others recalled that the FAA had whitdrawn are commendation for their use because AIA test showed that they did delay evacuation.

 

Evacuation tests

 

To try resolve this question the CAA co-operated with Linacre College, Oxford, in a programme of test to measure these effects and study the behaviour of passengers wearing smoke hoods. A fully furnished Trident was provided for the purpose. Each test included evacuation of 85 volunteer passengers through two exit doors from the rear of the cabin. The evacuations were carried out under conditions of both clear air and dense theatrical smoke hoods. Video cameras recorded the tests. Observers were present and passengers filled in a debriefing questionaire afterwards. No passenger took part in more that one test, and none had any previous expierence. The information needs to be studied further, but some interesting results have emerged from preliminary analysis; Virtually all the passengers aceepted the hoods evacuation was measurably slower when they used, but it was also more orderly. The significance of this for safety will be difficult to determine because the delay caused by donning and wearing of the hoods may or may not be compensated for by the protection from fumes that they provide. This could not be properly tested as theatrical smoke is not poisonous and there would be legal insurance complications if the real stuff was used not to mention a shortage of volunteers. If smoke protection equipment is to be used the basic choice is between systems that filter smoke particles and take fumes out atmosphere for the wearer to breath, and those which supply a breathable gas, simple wet towels or plastic bags are not acceptable says the CAA. There are already examples of both filter and breathable gas sytems on the market. UK company Life Support Engineering is maketing its Vivat smoke hood. This has an polycarbonate multifilament mesh screen to stop particles down to one micron (one millionth of a meter) in diameter. Behind this mesh is a piece of activated charcoal cloth that absorbs all the poisonous gases likely to be found in a cabin fire except carbon monoxide (CO). The hood itself is made of a flame retardant plastic to protect the user's head and eyes from flames and hot gases. Is it secured at the neck with a neoprene/lated seal that is said to be effective on all but the smallest children. Weighing less than 70 g it is very light and, at 18.5 cm x 14 cm x 0,5 cm, is compact enough to fit into a small bag. The company does not quote a maximum peiord of protection, but does give performance figures for the filtration of several gases over a 10 minute period with a breathing rate of 30 litres/minute. Under these conditions a maximun of 20 parts per million (ppm) of HCN in a 500 ppm atmospheric concentration will get past the filter, for HCL the figure is 50 ppm in a 1000 ppm concentration and 1 ppm of acroleing will break through in a 20 ppm concentration.

 

 

Lethal fumes

 

HCN is better known as hydrogen cyanide and HCL hydrochloric acid. Carbon monoxide, sulphur dioxide (SO2) and various oxides of nitrogen also add their own distinctive flavours to the gaseous cocktail that a cabin fire offers the passengers. Most victims of fires are poisoned or asphyxiated, or both, rather than burned to death. Of 55 people who lost their lives in the British Airtours Boeing 737 that caught fire at Britain's Manchester Airport in August 1985, 48 were killed by smoke and fumes. This is a provisional conclusion drawn by the CAA's Accident Investigation Branch. Something like the vivat hood could probably have saved most of them. The manufacturer emphasizes that this hood is designed to provide protection long enough only for one escape. It is not intended for use in fighting fire or when deliberately entering does not supply extra oxygen in a deficient atmosphere and the filter does not stop carbon monoxide. (Life Support Engineering can be contacted in the UK on 44 (9066) 2322, telex 877295). German company Drager has been producing protective breathing apparatus since the beginning of this century, and is now developing a new range of systems for use by cabin crew and passengers. The Drager Passenger Emergency system offers emergency oxygen in case of cabin decompression, smoke protection in case of an in-flight fire (they can be used together) and smoke protection for evacuation from a cabin fire on the ground. In the latter case the smoke hood is separated from the oxygen supply. The oxygen is supplied by a chemical oxygen generator based on a potassium dioxide (KO2) the smoke hood has an integral filter element, a constant flow mask and breathing bag. It connects to the oxygen supply via flexible tubing and is stored in the seat armrest. When activated the masks are presented to the passenger and the flow started. In the case of a cabin decompression the mask is used the way a normal oxygen mask would be, the smoke hood remaining packed around the mask. In smoke the passenger pulls the automatically unfolding mas over his/her head, giving protection from the smoke and a supply of oxygen. For evacuation the tubing is uncoupled from the oxygen and the passenger is free to leave the aircraft wearing the hood. Drager is proposing this system as a component of the Keiper Recaro' Aircraft Seating System of the nineties'. And is also being developed for installation in the cointainers used for standard passenger emergency oxygen systems. For cabin crew who have to tend to passenger in airborne emergencies and organise the evacuation of an aircraft on fire on the ground Drager is developing its oxycrew breathing system. This is a fully self contained, portable breathing device that has an integral oxygen generator whose output is governed by the wearer's demand and a closed loop breathing system in which exhaled CO2 is absorbed. The workload contolled oxygen generation syustem offers, says Drager, maximun performance for all emergency scenarios: high workload passenger management and evacuation on the ground, or low workload rest periods. The oxycrew has an integrated speech transmitter to counteract the muffling effect on the voice that wearing a plastic bag might otherwise have. This allows cabin crew to use a megaphone and the aircraft's handsets. The hood and visor protect against flame, radiant heat, and dripping plastic. Without its container, the oxycrew weighs 1,7 kg (Dragerwerk aktiengesellschaft, product group aerospace can be contacted at POB 13 39. D 2400 Lubeck 1, Germany)

 

Protection Levels

 

 

The different approaches of Life Support Engineering and Drager embody a debate currently being held by the CAA into the level of protection that should be demanded by any legislation that emerges from their inquiry, Drager's designs would fit into the type 1 category, in that they provide protection from fumes both in flight and during an evacuation, whereas the Vivat smoke hood fits into the type 2 category, as it is intended only to give passengers extra time for evacuation, the duration of protection being too short for prolonged use. Comment came out generally in favour of the type 1 standard, with many people saying that to limit the required protection to a ground fire emergency would be unacceptable and unnecesarry. Many believed that equipment could be developed that would meet both scenarios without significant penally from either weight or cost. But then several people went on to say that it might be necessary to limit the legislation in its application to existing aircraft, requiring them only to carry type 2 hoods on the grounds that to install type 1 equipment would involve expensive modification of aircraft oxygen systems. This questions has yet not be resolved. Limiting the size of aircraft required to carry smoke protection equipment was also considered in the CAA interin report. The grounds for this were that only aircraft certificated to carry 20 or more passengers need to carry a flight attendant and only those that carry 44 or more have to have an evacuation demonstration. Also, small aircraft can typically be evacuated in 30 seconds or less. The majority feeling on this issue was that aircraft size, number of passengers and speed of evactuation were not really relevant, as type equipment would be carried to cope with an in flight fire as well as ground emergency. An alternative would be to issue parachutes, but these are bulkier and more complex to use than smoke hoods. Further work is now being establish realistic workloads for smoke hoods to ensure that users will not be suffocated by the hood while trying to escape. In other words researchers have to find out how hard a person has to breathe under the strees of escaping from a smoke filled cabin, and specify a hood that can cope with filtering the quantity of aire needed. Toxic gas inhalation limits have to be reviewed in deciding whether to specify a filter system or one that provides a supply of breathable gas. A representative standard of fire severity must be established for use in tests. The issues raised by the study into the use of just one piece of equipment shows just what a can of worms the whole question of aviation safety is. It is impossible to consider any single issue in isolation and draw any meaningful conclusions. Concern was expressed by several of those who commented on the CAA's discussion paper that concentrating on protective breathing equipment should not detract from efforts to improve safety in other areas such as cabin materials and escape provisions and facilities.

 

 

Facing backwards

 

in the introduction, this article asks the reader to assume the passengers are conscious after a crash. This is far from a safe assumption, as people are often knocked out in a survivable accident when the head hits the back of the seat in front and injured by the jack-knife effect when the seat folds up on them. If this happens, people would be in no condition to make use of protective breathing or any other type of safety equipment no matter how easy to reach or simple to use it is. If the seats faced the rear of the aircraft this would not happen. This has been known for years and it is an issue that will not go away. Turning the seats round is probably the simplest and cheapest way on airline can mike its aircraft considerably safer. The sticking point here has been that airlines assume that passengers will not accept rearward facing seats. Very little research has been done in this area, but at least one survey found that 80% of passengers thought this was a good idea if it enhaced safety. The UK Royal Air Force has always had rear facing seats on its transport aircraft and that would seem to be as good and indication as any of their value. Studies have shown that the first few seconds after impact in a survivable crash are vital if people are to get out before fire takes hold. Therefore it helps if they remain conscious. Overhead luggage bins that burst open and shower the cabin with what has effectively become shrapnel constitute another avoidable hazard. Thoughtful redesign of cabin interiors would place the luggage bins under the floor or seats, increasing headroom without compromising space. Setting safe limits for the inhalation of toxic gas is and odd way of approaching a safety issue as poisonous gas need not and should not get into a passengers cabin anyway. The fumes come from burning seat cushions, most of which are made of polyurethane foam or related materials, which have no place in an aircraft cabin. This view was also expressed in the CAA report. Filter type smoke hoods could be made much simpler and cheaper if the range of noxious vapours that they have to neutralize is reduced. For the result of the discussion, the CAA assumed that between 50 and 100 passengers a year worldwide would benefit from the provision of smoke protection equipment. Many commentators believed that this way a low estimate ,citing an incident with a Saudia Tristar in which 301 lives were lost. One conclusion that has emerged so far is that more research needs to be done in this area. A thoughtful and comitted approach, regulatory bodies and industry is needed to determine the nature and scale of the problems and find solutions

 

 

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