Fuselage airframe

A fuselage airframe of a helicopter comprising a subfloor compartment for a fuel tank, encompassed by a concave cover shell, standard frames and a floor panel covering substantially entirely said subfloor compartment above the concave cover shell, wherein at least one differential frame is mounted to the concave cover shell, said at least one differential frame being perpendicular and symmetric with regard to a longitudinal axis and having an upper section above the floor panel and a lower section below the floor panel and respectively lateral frame root regions merging the upper and lower sections, said at least one differential frame partitioning the subfloor compartment between the standard frames, characterized in that the lower section of the at least one differential frame has a web with variable-heights with respect to the cover shell between a minimum height at the longitudinal axis and a maximum height at the lateral frame root regions, and in that at least one transversal beam is provided, said at least one transversal beam bridging the entire web portion of the lower section with the variable-height of the at least one differential frame and being hingedly attached at its both lateral ends to the lower section of the at least one differential frame by means of at least one bolt.