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Hot topic: Clean Sky's 'Environmentally friendly fire protection' system

For the past four decades, the cargo holds of airplanes have been protected with fire-suppression and extinguishing systems that use Halon 1301. It's an effective agent and poses no risk to passengers. However there are two threats on the horizon that must be urgently addressed — and Clean Sky is turning these threats into a unique opportunity for European aviation, thanks to the EFFICIENT (Environmentally Friendly FIre suppression for Cargo using Innovative greEN Technology) project.

"Halon 1301 is a very effective agent. You don't need much of it to achieve your fire protection goals, therefore it doesn't add much weight to the aircraft. It's safe for use in the concentrations we need, so there's no danger to people. So from fuel consumption, safety, and from an economics perspective Halon is perfect and that's why it's been used over the last 40 years on board commercial aircraft to protect the lower deck cargo hold from fire" says Rainer Beuermann, Fire Protection CoC Cabin & Cargo, Airbus Operations GmbH.

But despite the suitability of Halon 1301 for fire suppression in aviation, Beuermann also points out that there's an environmental downside: "Halon depletes the ozone layer, and this regulation needs to be seen in the context of our product evolution policy, so if Airbus decides in the future that we need to come up with a new aircraft programme because of competitor pressure, then this new aircraft would have to be equipped with a Halon free system".

Efforts to protect the ozone layer were adopted in 2008 under the Montreal Protocol, and ongoing initiatives by the EU to mitigate the ubiquity of Halon include the Commission's launch, last year, of an evaluation of the Regulation (EC) No 1005/2009 (the Ozone Regulation). It's a serious issue that will affect Europe's air transport, though the regulation only applies to new type certificates, not to current production or derivative aeroplanes.

There's another looming issue in the background — an obsolescence risk. A sub group within the UN environmental programme has assessed the availability of Halon on a worldwide basis, and has determined that global stocks of Halon are diminishing and could run out by the end of the decade. Therefore, in addition to the environment there's a business risk to aviation operations.

"Clean Sky's EFFICIENT project, promoted by Airbus, seeks to design, develop, manufacture and test an environmentally friendly and economically viable Halon-free cargo hold fire suppression system which will reduce environmental impact by reducing weight and lowering aircraft CO2 and NOx footprint" says Paolo Trinchieri, Project Officer at Clean Sky. "The project is a collaboration led by Cranfield University with SP Technical Research Institute of Sweden AB, London South Bank University, all striving to come up with an entirely new Halon-free solution which provides at least an equivalent level of safety compared to Halon 1301". 

If Clean Sky develops a replacement for Halon, in addition to the environmental benefits there will be a unique business opportunity for European aeronautical companies and for the associated supply chain. After all, an alternative for Halon will be needed worldwide, not just for European aviation.

But an artificially created substitute with similar chemical characteristics to Halon 1301 would take more than 10 years to develop, would be prohibitively expensive and, being a niche product would be too commercially risky to develop. There are Halon 1301 alternatives that are suitable for land applications but these are not compliant with the stringent fire suppression requirements in aviation. The EFFICIENT team has been examining the possibilities from a variety of angles:

"The first question when we started this project was: Are there any potential alternative agents? But there weren't any on our radar" says Airbus's Beuermann. "The SP Technical Research Institute of Sweden did some market research regarding alternative agents and developments on the market, looking at what is available and what might be available in due course. Based on this research they down-selected potential agents and came up with a final recommendation for further investigation, taking into account the stringent safety and performance requirements to be met by the agent. The current work now focuses on the use of inert gases as halon replacement agents based on the outcome of the agent-down selection".

Zodiac Aerotechnics is working on an On-Board Inert Gas Generating System that shall provide Nitrogen in such a quantity and quality allowing the safe suppressing of a cargo hold fire under all operational conditions. The manufacturing of the demonstrator unit is progressing and completion of the demonstrator is expected by mid 2019. 

"But the replacement of halon with an inert gas does not enable the design of some kind of simple drop-in-replacement solution" says Beuermann. "New system philosophies and combined multi-functional systems architectures need to be considered for the next generation of halon-free cargo hold fire suppression systems".

A partner in the EFFICIENT consortium, The London South Bank University, has already carried out some large-scale fire suppression tests, because the first consideration that will drive the design of the system architecture is how much agent is needed in order to achieve the fire suppression goals and what concentration is required. The University also did some lab scale fire suppression tests in order to specify the required extinguishing concentration, as well as some lab scale explosion tests to determine how much concentration is needed to contain a propane enriched atmosphere, related to the very specific fire test scenarios inside the cargo hold. 

Now that the required concentration is known, it is possible to use this preliminary data to design the architecture which is the backbone of the system, and it becomes possible to explore which technical options exist to either generate the agent on-board or to store it in high pressure cylinders.

"There are two ways to demonstrate this" explains Beuermann. "First you have to perform full scale fire suppression tests and then the second stream is we need to show that the agent distribution inside the cargo hold meets the specified design concentration. To suppress fire in the cargo hold you need to flood the entire cargo with the required design concentration and you need to maintain these same concentrations, bearing in mind that the cargo hold is not 100% airtight — we call it cargo hold leakage. As we lose some of the cargo hold atmosphere you have to compensate for any leakages. That means that under all operational conditions the system is able to build up and maintain the specified design concentration".

The pressure inside a cargo hold is significantly affected as an aircraft changes altitude during climb and descent phases of the flight, which in turn changes the agent distribution pattern. An important aspect of the project has been how to simulate realistic flight conditions, temperature and pressure changes — without the expense of using a real aircraft in flight, which would have been prohibitively expensive. Representative demonstrators are essential to reach the project's objective of reaching Technology Readiness Level (TRL) 6.

"One way we can work to achieve this is by using Computational Fluid Dynamics (CFD) to predict agent distribution inside the cargo hold" says Beuermann. "CFD results are of a very high quality but it takes a long time to get those results. Fraunhofer is working on a new simulation tool that allows faster assessment regarding novel or redesigned fire suppression architecture. It runs on a standard laptop to provide fast first design tests for a first risk assessment. This tool builds on the results obtained from the Clean Sky 1 project and this could help us regarding the development of new Halon-free systems".

The findings of these CFD tests are complementary to a series of representative ground-based tests which include a full scale fire suppression test campaign scheduled for the end of March 2019 at Cranfield University. Additionally, the first agent discharge and agent distribution tests under representative flight conditions are planned for Q4 of 2019 at the Fraunhofer Flight test facility in Holzkirchen, Germany. 

Fraunhofer's facility consists of a huge tubular pressure vessel containing part of the forward fuselage of an Airbus A310 complete with lower cargo hold, where it is possible to cool down the entire facility and change the pressure to create realistic in-flight conditions. The facility can even simulate descent profiles to represent changes in flight altitude to determine agent distribution contours over time. The results and information obtained from this testing are crucial in order to assess whether the fire protection objectives have been met or not.

"This is one of the beauties of Clean Sky 2, as we were not aware that this kind of flight test facility even existed in Germany, at least not here in Bremen" says Beuermann. "Clean Sky 2 has established the link to the flight test facility at Fraunhofer and has closed the gap in our validation and verification process because the conditions you have in a real flight are very difficult to simulate on the ground. By collaborating through Clean Sky we are learning where we can find new capabilities and skills within Europe that strengthen and reinforce the European research and technology network".