Boeing may have to prove new twinjet's wing and fuel tank structure capable of withstanding fuel-fed blaze
Boeing is confident it can comply with a proposed US Federal Aviation Administration certification requirement to demonstrate that the 787's carbonfibre-composite wing and fuel-tank structure can withstand a post-crash fire long enough for passengers to evacuate safely.
The FAA has proposed a special certification condition requiring "additional substantiation by test and analysis" that the 787 wing tanks are capable of resisting an external fuel-fed fire for at least 5min. The notice proposing the special condition says "applicable airworthiness standards do not contain adequate or appropriate safety standards for [composite] wing and fuel tank structure with respect to post-crash fire safety".
"We have done some of the testing to demonstrate burn-through resistance and the outcome has been very favourable. We see no surprises," says Jeff Hawk, 787 director for certification, government and environment. He says Boeing will be able to comply with the FAA's special condition through materials testing and analysis, without having to stage a large-scale demonstration.
Existing certification rules are based on the service history and extensive full-scale testing of conventional aluminium structures. Aluminium is thermally conductive and transmits the heat of a ground fire to fuel still in the tank. This dissipates the heat and prevents localised hot spots that can cause auto-ignition and delays structural collapse or burn-through beyond the time needed for evacuation.
Carbonfibre is a better insulator than aluminium, Hawk says, so the fuel will not heat up as quickly, reducing the possibility of auto-ignition, while the composite structure is less prone to melting. "The physics are different and the figure of merit is 5min, so we are not talking about an extended period," he says.
The difference in thermal conductivity has another effect on fire resistance. As fuel in an aluminium tank heats up, vapour rapidly accumulates in the ullage, pushing the fuel-air mix beyond its flammability limit and reducing the risk of explosion. There will be less heat in a composite tank, and it may take longer to reach the upper explosive limit, but Hawk says the 787's nitrogen-enrichment system will ensure the empty fuel-tank spaces are inerted in a crash.