Boeing will be required to demonstrate that the 787’s composite wing and fuel-tank structure can withstand a post-crash fire long enough for passengers to evacuate safely, under a special certification condition proposed by the US FAA.
Published on Monday (9 April), 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”.
Existing rules are based on the service history and extensive full-scale fire testing of conventional aluminum structures. And while it has previously approved composite fuel tanks in horizontal stabilizers, the FAA says the 787’s carbonfiber composite wing “will introduce a new fuel tank construction into service”.
Aluminum is highly thermally conductive, says the FAA, and readily transmits the heat of a ground fire to fuel still in the tank. This spreads the heat over the wing surface and prevents localized hot spots, delaying structural collapse or burn-through beyond the time needed for evacuation.
In addition, as the fuel in an aluminum tank is heated, fuel vapor rapidly accumulates in the ullage, or empty space, pushing the fuel-air mix beyond its flammability limit before the wing burns through and reducing the risk of explosion.
The heat capacity of aluminum and fuel will delay burn-through or wing collapse beyond the time required for evacuation time, says the FAA, adding that studies show preventing fuselage burn-through for 5min can significantly enhance survivability.
Because carbonfiber has low thermal conductivity, the FAA says “additional substantiation by test and analysis will be required to show that the 787 provides an acceptable level of safety” during an external fuel-fed fire.
To be consistent with the capability of aluminum structures, “the 787 fuel tanks must be capable of resisting a post-crash fire for at least 5min”, says the FAA. Under the proposed special condition, Boeing will be required to demonstrate compliance at a range of fuel loads from minimum to maximum.