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CONTENTS 1. BACKGROUND 2. MATERIALS USED 3. HAZARDS FOR THE FIREFIGHTER 4. CONDITIONS UNDER WHICH
COMPOSITE MATERIALS ARE HAZARDOUS 5. POST CRASH SITE MANAGEMENT 6. CONCLUSION 7. RECOMMENDATIONS 8. APPENDIX ‘A’ - LIST
OF CONSIDERATIONS 9. APPENDIX ‘B’ - SAMPLE
RISK ASSESSMENT 10. REFERENCES |
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COMPOSITE
MATERIALS IN MODERN AIRCRAFT:
THE RISK TO
FIREFIGHTERS
DIVISIONAL OFFICER GRAEME DAY
WEST SUSSEX FIRE BRIGADE
GATWICK LIAISON OFFICER
Composite materials are increasingly being used in the construction of modern civil and military aircraft. The materials are used in various parts of modern aircraft in a variety of applications. They are also retro-fitted to older aircraft following repair, (as was discovered at the Coventry Airport crash).
As composite materials become more commonplace in aircraft construction, awareness of the problems related to these materials is increasing.
During the 1970s, the United States Government embarked on a lengthy, multi-agency research programme (which included NASA). The programme sought to standardise procedures and regulations used by aircraft accident rescue and recovery teams and crash investigators. It drew heavily on the experience of staff based at the advanced composites section of the US Air Force’s Air Logistics Centre. The programme also assessed the affects of broken-down composite materials on humans.
In the UK, the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health Regulations 1994, require that exposure to substances hazardous to health be prevented or where this is not reasonably practical that the hazard be controlled.
The Civil Aviation Authority has just begun a three year research project into the hazards posed by composite materials, and the Air Accidents Investigation Branch is actively involved in examining the operational problems caused by such materials. The fire service has been given additional guidance in the form of a Dear Chief Officers’ Letter (10/1991).
It is the Royal Air Force however, certainly in the UK, that have the most experience in dealing with composite materials in aircraft. Their awareness of the problem stems from dealing with the aftermath of a Harrier GR5 crash that occurred on 17th October 1990. The aircraft, based at RAF Wittering, crashed in a field on the island of Moers in Denmark following an engine failure. The pilot ejected safely and the RAF’s Repair and Salvage Unit (RSU) covering the area duly responded. There had not been a crash in the area for some fifteen months and as a result, for quite a few team members it was their first experience of an actual crash.
Wreckage was spread over a distance of 250 metres with strands of composite material spread over a cone shaped area of 70 metres. There was a vast amount of airborne dust and a considerable amount of burnt composite material within the crater.
The RSU crew originally wore goggles, ori-nasal masks and filters to give respiratory protection. However, after their personnel began to suffer respiratory problems, sore throats, eye and skin irritation, the decision was made to obtain full length chemical protection suits and to up-grade the respirators in use (there was also conflicting information on site regarding the type of filter to use). From that point on anyone entering the inner cordon wore full protective clothing and there were no significant problems with respiratory ailments as a result.
Almost all modern civil aircraft use composites in their construction and it is therefore essential that defined operational procedures and a practical approach allied to the efficient use of personal protective equipment be established to ensure the safety of firefighters at aircraft accidents.
The major types of composite material used on modern aircraft include a wide range of materials e.g. Carbon, Glass, Aramid (Kevlar), Graphite, Boron and hybrids of these. There is also a wide variety of resins which are used to bond the materials together. Hybrids may also contain metals such as aluminium and can be found in such areas as engine cowlings e.g. Airbus A340. The term “composite” is applied to any material utilising two or more substances in its basic construction, e.g. Carbon Fibre/Epoxy Resin; Carbon Fibre/Epoxy Resin/Honey Comb Matrix. Composites are classified according to their matrix phase, e.g. polymer matrix composites (PMC), ceramic matrix composites (CMC) and metal matrix composites (MMC).
The feature common to all composite processes is the combining of a resin, a curing agent, some type of reinforcing fibre and in some cases a solvent. Usually, heat and pressure are used to “cure” the mixture into a finished part. Resins are used to hold the fibres together and protect them and to transfer the load to the fibres in the fabricated composite part.
The popularity of composite materials in the world of aviation construction is due to a number of factors. They possess excellent strength to weight ratios, do not fatigue like metals, are lightweight, can be moulded into a variety of shapes and accept a multitude of coatings. They are also corrosion resistant and, when these properties are combined, the composites are known as “advanced composites”. They do however, burn well because of their resin content and the oxygen in the spaces of the honeycomb-like construction. Carbon will resist higher temperatures than aluminium.
They are used in such areas as engine cowlings, flaps, floor panels and beams, undercarriage doors, leading and trailing edges of wings and nose gear doors. These are all fairly small parts in relation to the size of the aircraft and with the exception of the floor panels, are located at some distance from the fuselage. However, tailplanes and fins are likely these days to be completely constructed from composite materials.
Approximately 20% of the structure of modern civil airlines is constructed from composite materials (e.g. MD 11 = 30%, A320 = 16%). Military aircraft are a different proposition though, for example, composites account for 32% of the structure of the Harrier GR5 and for 70% of the Euro Fighter.
There are also many smaller aircraft made almost entirely from composites such as gliders and homebuilts. The early types of which were made primarily from glass reinforced plastic. More recent types however, contain a significant amount of carbon fibre. The difference with this material is that the fibres are silicon not carbon and they are often arranged somewhat randomly in the resin, whereas carbon fibres are arranged in a particular way depending on the structural property required.
Temperature resistant composites can also be found in engine components (often containing metal fibres in a polymer base) and in brake components.
Highly toxic fumes are given off by the resins and bonding agents used in the manufacture of composite materials. Many resins contain hydrogen, chlorine, nitrogen and oxygen. As a result Hydrogen Cyanide, Hydrogen Chloride & Nitrogen Dioxide are given off which are extremely toxic. Other poisonous products of combustion include Formaldehyde, Ammonia, Toluene and Carbon Monoxide and isocyanates from polyurethane based resins. Whilst it can be argued that the majority of these gases will be taken away in the smoke plume of the main fire, they may still be present in the post crash scenario where they pose a severe threat to firefighters.
The main threat posed by the composite materials themselves can be grouped as follows:
· Respiratory problems
· Dermatitis and skin irritation
Respiratory Problems:
The sharp particles in composite material dust, which can easily become airborne, may be ingested into the respiratory system. These particles may cause long term damage to body tissues although research into this area is ongoing. Inhaled fibres can also cut and irritate the internal linings of the nasal passages. Respiratory problems will also be caused by the fumes from the range of resins used in composite material construction.
Dermatitis and Skin Irritation:
Airborne dust and fibre particles can cause minor skin and eye irritation as can contact with the epoxy resins used for bonding composite materials. Composite material dust will also absorb aviation fuel and residues of other substances which will add to the dermatitic reaction suffered by anyone who comes into contact with it.
Skin irritation is caused when broken filaments become embedded in the skin. This causes itching and/or irritation. Well designed protective clothing (which does not induce perspiration) will help to prevent ingress of fibres into normal clothing. The situation will also be improved by having effective procedures for site working.
The hazards presented to firefighters by composite materials vary under different conditions. When the material has been subjected to impact damage only, it will shatter releasing fibres that have a mean diameter not less than the manufactured size which is generally 6-12 microns. It has been shown that only fibres with a mean diameter of less than 3 microns can cause deep respiratory problems i.e. damage to the lungs. Thus the main concern is for fibres of a size which can penetrate deep into the lungs thus reaching the alveoli, i.e. fibres with a mean diameter of 3 microns and an aspect ratio (length to diameter) of 3 : 1. However, high concentrations of fibres and dusts, irrespective of size, could pose additional risks. Additionally the substances that are absorbed by dust and fibres (including airborne dust and fibres) must not be ignored.
Ripped panels of composite material often produce edges that are extremely sharp and are quite capable of penetrating leather gloves. Care must therefore be taken when handling the material in this form. Puncture wounds are a real risk from impact damaged composite material and blood can be subsequently contaminated.
When composites are subjected to fire damage only, hazards manifest themselves in a different manner. Large amounts of toxic gases are released as the resins begin to burn away at between 400 - 500ºC. The mean diameter of some composites, e.g. Kevlar, may be reduced. Fire damage will significantly reduce the structural strength of composite materials. They may look as if they are intact but they will not support the weight of a firefighter in this condition. This is significant in aircraft where certain floor panels are made from composite materials.
Whilst impact or fire damaged materials pose a certain degree of hazard from dust and fibres, it is when composites are damaged by a combination of fire and impact that the risks are at their highest. Composite material dust poses a severe respiratory hazard to all personnel who may be in the area.
Although only 20% of the structure of modern civil airliners may be composed from composite materials and the hazards posed by them are not fully understood it is essential that a post aircraft accident site is properly managed. The post crash scenario will not need to be dealt with in the urgent manner required when dealing with the rescue and firefighting phases of the operation. It will however pose serious health threats to firefighters and therefore needs to be strictly controlled.
The type of aircraft accident i.e. high or low speed, will initially determine the level of risk. A low speed crash i.e. take off or landing may result in a compact crash site where damage is restricted to certain areas. Other variables including gross weight of the aircraft, fuel load and angle of impact will affect the extent to which composite materials are involved in the incident.
A dynamic risk assessment approach must be taken by the officer-in-charge in relation to the tasks to be undertaken and the resources available. All firefighters committed to the area of special hazard must be protected by full fire gear, including flash hoods and positive pressure breathing apparatus. Chemical protection suits may also be an option if the tasks to be undertaken are simple.
Decontamination zones must be nominated early and all non-essential personnel must remain upwind of the incident. Wet decontamination procedures can be employed, but it is essential that all contaminated clothing is secured in polythene bags and subsequently dealt with by a specialist cleaning company.
The risk of spread of composite material fibres and particles can be reduced in the following ways:
·
The application of a fine water spray to the area.
·
The application of a water based suppressant. This will consolidate the top 3 - 4 mm of
the crash site debris.
·
Application of a foam blanket. This does have the disadvantage of covering
holes in aircraft floors and other obstructions.
·
Small areas can be covered by polythene sheeting.
·
Sections of composite materials may be separated from
other debris and secured in polythene bags for future disposal.
NB Removal of crash debris would only be considered in the post crash scenario and having consulted the AAIB first.
The risk to firefighters and other personnel may also be reduced by strict cordon management which would restrict the passage of personnel through the crash site. (This will also assist with Post Crash Investigation). The movement of fixed and rotary wing aircraft must also be restricted to prevent the spread of composite material particles.
Constant liaison with AAIB and Environmental Health personnel is recommended. This will enable a complete picture to be built up regarding the risk posed to firefighters from composite materials at a particular site. The environmental health officer will also need to be included as top soil may have to be removed as low level contaminated waste.
There may also be a limited risk to electrical apparatus from airborne composite materials downwind from the incident. Research continues into this subject, but NASA’s experiments showed that a fibre concentration of 1O6 FIBRES/M3/SEC. would have to be produced before electrical apparatus would be affected. It is a factor however, along with the evacuation of nearby buildings, that could be considered by the officer-in-charge.
Composite materials are widely used in modern manufacturing processes. Their use is not restricted to aircraft construction and they can be found in many modern vehicles and modern railway carriages. Although composite materials are only one of the risks posed by modern aircraft to firefighters (fuel, hydraulic oils, plastics, oxygen cylinders and metals are examples of other hazards facing firefighters), their hazards are well documented.
It is therefore essential that levels of awareness of the composite material hazard are raised among all personnel likely to attend an aircraft accident. Manufacturers must be encouraged to be more open about the types and extent of use of composite materials in modern aircraft. This can also be achieved by liaison between the emergency services, aircraft manufacturers and aviation authorities.
Whilst composites account for only 20% of aircraft construction today, in 5 years time we can expect to see aircraft with wings made completely from the material and within 10 years aircraft could be operating with fuselages made from composites too. It is of paramount importance that the risks posed by composites are understood and catered for before these developments take place.
1. Education regarding the risks posed by composite materials should be provided for:
· The emergency services
· Emergency planning officers
· Airport officials
· Technicians
· Environmental health officers
2. Produce an aircraft hazard information data base.
3. Review procedures in the light of the future publication of the CAA research project which is assessing the risks posed by composites.
4. Encourage a common approach for emergency service personnel in dealing with this hazard. The influence of CACFOA, the Home Office and ICAO (via Annex 13) could bring this about.