ANALYSIS: How the risk of fire in airliners is increasing

London
Source:
This story is sourced from Pro
See more Pro news »

Fire on board aircraft has caused fewer fatalities in recent years than it once did, but the risk of fire-caused incidents and accidents is increasing.

That is the conclusion of international experts assembled by the Royal Aeronautical Society, who recently published the first part of a study called "Smoke, fire and fumes in transport aircraft" (SAFITA).

Capt John Cox - president of US-based safety consultancy Safe Operating Systems (SOS), and one of the experts on the SAFITA team - says that as aircraft technology and construction materials change, so does the fire risk profile - and almost certainly not for the better. Meanwhile, the US Federal Aviation Administration and the UK Civil Aviation Authority have just jointly launched a fire safety awareness campaign which includes an instructional video. In a particularly chilling statement at the campaign launch, the CAA said: "Of particular concern is the threat of fires breaking out in hidden areas of the aircraft, which cabin crew are unable to access and bring under control in-flight."

The CAA statement continues: "The importance of reducing fire risks was highlighted with the recent significant fire on the Ethiopian Airlines Boeing 787 on the ground at London Heathrow [12 July 2013]. As the film points out, an in-flight fire that is out of control will, on average, lead to flight crew losing control of the aircraft within 15 minutes." The vulnerability of aircraft to fire is on the CAA's "Significant Seven" list of the greatest threats to aviation safety.

 Fire on an Ethiopian Airlines 787 highlighted the importance of reducing risks

Cox observes that two of the principal reasons for the change in the fire risk profile is the proliferation of lithium batteries on aircraft - both batteries installed in the aircraft and those carried by crew and passengers in personal electrical/electronic devices - and also the increasing use of composite materials in aircraft hulls. "Composites in the vicinity of lithium batteries - how is this going to play out?" he asks.

In this question he is not only acknowledging the kind of risk that the Ethiopian 787 fire demonstrated, but also expressing concern about the fact that the industry has little experience of the behaviour of composite materials in the presence of heat and fire. Aluminium, for all its limitations, is a known quantity.

One of the reasons for the fewer fatalities in recent years is that the fire-related accidents in the last decade have mostly involved freighters. Since 2011, two Boeing 747Fs - one operated by South Korean carrier Asiana, one by US package shipper UPS - have been brought down by fire with the loss of the entire crew. In both cases the fire was believed to have started in pallets of lithium batteries carried as cargo.

In the UPS case, said the investigators in their official report, the time between the fire warning being triggered and the first failures in a cascading loss of aircraft systems was 2min. The flightdeck filled with smoke and captain left his seat when the supply of oxygen to his mask unaccountably failed. The co-pilot soon could not see his instruments nor see out of the windscreen, nor see even to change the radio frequency, so was totally incapacitated. The report and its conclusions are a nightmare to read.

 

 AirTreamImages

 Fire on a UPS 747F started in pallets of lithium batteries 

There is no indication, according to the RAeS, that passenger aircraft are safe from similar events and, if a single event like the loss of Swissair Flight 111 were to take place now, it would reverse the interpretation of medium-term statistics. Swissair 111 was a McDonnell Douglas MD-11 that crashed into the sea near Halifax, Canada in 1998 when a short-circuit in a damaged wiring bundle generated a fierce fire in the ceiling just aft of the cockpit. The spreading fire caused a sequence of cascading system faults, eventually wiping out the primary flight instruments. Finally smoke blinded the pilots so they could not fly.

The SAFITA report contains this general conclusion with a number of individual ones: "While the number of fatalities caused by aviation accidents has decreased, the risk of future fire-related incidents or accidents has increased due to the proliferation of lithium batteries and other risks. The importance of continued research, improved regulation, improved manufacturing standards, adoption of technology to mitigate in-flight smoke and fire, and oversight by safety professionals is proven in this document."

Both the recent 747F losses have been attributed to fires that began in cargoes of lithium-ion batteries. The Ethiopian Airlines 787 fire at Heathrow, according to initial examination by the UK Air Accident Investigation Branch, involved a lithium-ion-battery-powered emergency locator transmitter, and an extensive area of the composite fuselage crown just ahead of the fin suffered heat damage. When fire crews attended, halon extinguishant directed to that area from within the cabin failed to bring the fire under control, says the AAIB's initial factual report. But eventually, water was effective in dissipating the heat and stopping the fire's progress.

There has always been a concern that cabin crews are poorly equipped and trained to handle cabin fires that start behind the wall and ceiling panels, and the Ethiopian event just highlights this. Barring the lavatories, there are no heat or smoke detectors anywhere in the cabin area to provide early warning or to indicate the location of a fire, nor do the means exist to direct extinguishant into the space behind the panels.

Cox notes that if there were a lithium battery fire in a passenger's laptop computer, the crew are issued with containment boxes and gloves to handle hot objects, but no protection for the arms, body or face. And there is no well-rehearsed drill for handling lithium battery fires, which can generate huge heat through self-sustaining chemical reactions. Cox observes that ideally there needs to be a system for intervening in the chemical process - which is the principle on which halon extinguishant works - followed by the application of a coolant, like water. But at present the crew training, the drills, and the equipment are all either non-existent or inadequate.

Fatal fires on passenger aircraft may indeed be rare, but IATA calculated in 2002 that in-flight smoke events occur once in 5,000 flights, and diversions resulting from these about once every 15,000 flights. More recently the FAA said there are 900 reported smoke events per year in the USA alone, and these "frequently lead to diversion". One of the effects of the lessons learned from Swissair 111 is that pilots are much quicker than they used to be to make a diversion decision when smoke is detected. Obviously with Swissair 111 in mind, the FAA said that in the event of an in-flight fire, "delaying the aircraft's descent by only two minutes is likely to make the difference between a successful landing and evacuation, and a complete loss of the aircraft and its occupants".

Indeed, the SAFITA report concludes that rapid diversion is one of the primary mitigation techniques for reducing the risk of harm from onboard fire. The Swissair 111 report concluded that the crew should have diverted without any delay, although it cannot be asserted with any confidence that the aircraft could definitely have landed safely even if the pilots had acted with all possible speed. Lessons from both the recent Asiana and UPS freighter accidents reinforce this advice.

 

 Asiana

 Fire brought down an an Asiana 747F in just 18min

The FAA, having recently reworked its predictive model for freighter fire accidents, now forecasts that the average number of US-registered freighter fire-related accidents likely to occur during the 2012-21 decade - if no mitigation action is taken - is between two and 12, with six the median probability. The agency explains: "Approximately four of those are likely to be initiated by primary or secondary lithium batteries on the aircraft." The definition of "primary", in this case, is batteries as airfreight; "secondary" is airfreighted equipment fitted with lithium ion batteries, or lithium-powered equipment brought on board by crew and passengers. Almost all personal electronic devices are powered by lithium-ion rechargeable batteries.

Between March 1991 and October 2012, the FAA office of Security and Hazardous Materials Safety recorded 132 cases of aviation incidents involving smoke, fire, extreme heat or explosion involving batteries or battery-powered devices The found that lithium batteries were involved in the majority of the battery-caused incidents.

The SAFITA report describes the extent of the problem posed by personal equipment carried by crew and passengers: "On a typical flight, a single aisle jet carrying 100 passengers could have over 500 lithium batteries on board. These devices are not tested or certificated nor are they necessarily maintained to manufacture's recommendations. Replacement batteries from questionable sources can be contained within devices. 'Grey' market batteries may not be manufactured in accordance with international standards. It is possible that they have a greater probability than original equipment to overheat and cause a fire."

In April 2012, a passenger's personal electronic device burst into flames on a Pinnacle Airlines flight from Toronto to Minneapolis-St Paul. The SAFITA says: "During the in-flight service, the flight attendant noted that the device was on fire on the floor; its battery was burning several feet from the device. Using water from the service cart, the flight attendant put out the fire using wet paper towels. She then submerged the battery in a cup of water because it was still smouldering." The captain smelled the fire and diverted the aircraft.

Meanwhile those crews who carry electronic flight bags in the flightdeck should be aware that they are all powered by lithium-ion batteries.

Concern about lithium-ion batteries in aviation was considerably heightened by two high-profile battery overheat incidents on Boeing 787s within a week of each other in January. The first involved a Japan Air Lines aircraft on the ground in Boston, Massachussets, where the auxiliary power unit starter battery caught fire, and the other an airborne All Nippon Airways 787 where the main battery overheated. The two events grounded the type for more than three months while Boeing and its suppliers, watched by the FAA, redesigned the multi-cell batteries and their containment units. The intention was to reduce the likelihood of a thermal runaway, and to contain it effectively if one occurred. The 787 is the first aircraft to incorporate lithium-ion main batteries as part of its originally certificated design, although in other types they had been used to power ancillary units, like the emergency lighting in the Airbus A380.

So assessment of present and future onboard risks has to take account of recent technology developments, and also those in the pipeline. The widespread use of lithium-ion batteries is the obvious factor, but there are other changes too, including the growing use of carbonfibre reinforced plastic (CFRP) materials in aircraft primary structures like wings and fuselages. This is not to say that carbonfibre is reckoned to be a particular fire risk, but its behaviour when exposed to heat is different from that of aluminium, and while the behaviour of the latter is a known quantity, there is general agreement that the industry has more to learn about the results of CFRP's exposure to fire.

Meanwhile a more insidious fire-risk multiplier is the ever-increasing length of electrical cabling in wiring bundles in modern aircraft - an estimated 150km of insulated wire per aircraft. As aircraft and their systems become increasingly digitally controlled, as hydraulics are increasingly replaced by electrics, as systems redundancy is reinforced to boost dispatch reliability figures, and as a result of the exploding demand for in-flight entertainment systems, the length of cabling carrying electric energy in fat wiring bundles is growing fast. The SAFITA report says: "The increasing complexity of electrical installations, especially on larger aeroplanes with premium class cabins, will result in further issues. Such installations include, but are not limited to, IFE systems, electrically operated seats and charging systems for computers or other electronic devices. Each system installed in an operator's aeroplane can require unique procedures to deal with a failure or a problem that might result in an in-flight fire. Another issue is the addition of new systems to aeroplanes using existing circuit-breakers to power the new equipment." On top of this concern is the industry's acceptance, not formalised until early this century, that electrical wiring insulation has a shorter safe life than most airframes do.

Bigger wiring bundles all add to aircraft weight, so naturally manufacturers look for ways of making the cable core and its insulating layer lighter. The FAA has been worried by this side-effect of progress, and in 2008 observed: "Wire specifications should be revised to incorporate resistance to cut-through, abrasion, hydrolysis, and longer-term heat ageing." It was factors like these that led to the Swissair 111 tragedy. The fire began with short-circuiting in a wiring bundle, the insulating material itself began to smoulder, and then the fire moved into the fibrous thermal-acoustic hull lining blanket, which was contaminated with dust and the products of maintenance activity, like metal shavings, grease and even hydraulic fluid. The UK CAA's synopsis of the just-launched FAA/CAA fire-risk awareness campaign particularly addresses the serious risk of wiring-bundle damage combined with contamination like dust and moisture.

The purpose of SAFITA Part 1 was to identify the sources of fire risk, prioritise them and highlight mitigation strategies. The verdict was this: "The aviation industry and its regulators acknowledge that there will be ignition sources and fuel sources for fires within aeroplanes. Only through multiple layers of mitigation can the risk be kept to an acceptable level. To be effective these multiple layers will need to be re-evaluated regularly and available technology used wisely."

Meanwhile the studies for SAFITA Part 2 are continuing, seeking strategic approaches to better design, engineering, materials, and tools for fire detection and suppression. It is a tacit admission that the situation right now is simply unacceptable.