FAA, leveraging similar requirements issued more than a decade ago for the Boeing 787, has issued special conditions that Boeing must follow to demonstrate the 777-9’s composite fuel tanks withstand tire debris impact.
The requirements dictate that "tire-debris impact to any fuel tank or fuel-system component, located within 30 deg. to either side of wheel rotational planes, may not result in penetration or otherwise induce fuel-tank deformation, rupture (e.g., through propagation of pressure waves), or cracking sufficient to allow a hazardous fuel leak,” FAA said.
Testing must be done using a tire debris fragment size that is 1% of the tire mass, and the fragment load must be "distributed over an area on the fuel tank surface equal to 1.5% of the total tire tread area,” FAA continued.
The results must also demonstrate that fuel leaks within the defined debris-impact area triggered by larger debris pieces will not lead to “hazardous quantities of fuel” entering the engine inlet, auxiliary power unit inlet or cabin-air inlet.
“Fuel-tank surfaces of typical transport airplanes have thick aluminum construction in the tire-debris impact areas that is tolerant to tire debris larger than that defined in...these special conditions,” FAA said. "Consideration of leaks caused by larger tire fragments is needed to ensure that an adequate level of safety is provided where composite material is used.”
The requirements are identical to conditions FAA issued in 2007 for the 787—the first large commercial air transport aircraft with composite wings. In addition to focus on debris impact, the requirements take into account pressure waves that could damage a fuel tank. A pressure wave-induced fuel tank rupture played a role in the July 2000 crash of an Air France Concorde.
Boeing is preparing the first 777-9 for its inaugural flight. By later this year, four 777-9 aircraft will be involved in the flight test and certification program, with first deliveries to Emirates expected to get underway around May 2020.