AMC to Appendix N, N25.1(b) Qualitative fuel tank flammability assessment

(a)A conventional unheated aluminium wing tank is a conventional aluminium structure, integral tank of a subsonic transport aeroplane wing, with minimal heating from aeroplane systems or other fuel tanks and cooled by ambient airflow during flight. Heat sources that have the potential for significantly increasing the flammability exposure of a fuel tank would preclude the tank from being considered “unheated.” Examples of such heat sources that may have this effect are heat exchangers, adjacent heated fuel tanks, transfer of fuel from a warmer tank, and adjacent air conditioning equipment. Thermal anti-ice systems and thermal anti-ice blankets typically do not significantly increase flammability of fuel tanks. For these tanks, a qualitative assessment showing equivalency to the unheated aluminium wing fuel tank may be acceptable when considered with the following: 1 A description of the aeroplane configuration, (including subsonic, wing construction, etc.), 2 A listing of any heat sources in or adjacent to the fuel tank, 3 The type of fuel approved for the aeroplane, 4 The tank operating pressure relative to ambient static pressure, 5 The tank is uninsulated and made of aluminium, and 6 The tank has a large aerodynamic surface area exposed to outside air to transfer heat from the tank.

(b)Fuel tanks with an aerodynamic surface area to volume ratio (surface area/volume) greater than 1.0 have been shown to meet these criteria. Fuel tanks with a ratio less than 1.0 are not considered conventional unheated aluminium wing tanks. The aerodynamic surface area includes the area of the integral aluminium wing fuel tank that is exposed to outside air. It does not include any portion of a fuel tank that is shielded from free stream airflow, such as the front and rear spar, or an area under a fairing or wing thermal blanket.

[Amdt 25/6]