Motivated by concerns over rising costs of Jet-A fuel and the current limitations of “drop-in” fuel substitutes, it is proposed that biomethane (or Bio-LNG) provides a promising sustainable aviation fuel. This paper discusses some technical considerations for converting a jet airliner to biomethane fuel. Following consideration of aircraft configuration alternatives and performance issues, a conceptual design is presented where cryogenic methane is stored in both an insulated wing-box and under-wing pods. It is concluded that the weight penalty of such a cryogenic fuel system would be relatively modest, hence the range and payload capability of existing Jet-A aircraft can be matched.