By John S. McClenahen The current design for Boeing Co.'s Mach 0.98 passenger aircraft dubbed the Sonic Cruiser assumes that composite materials will make up about 60% of the plane. But maybe not. "As soon as we started talking about this being a mostly composite airplane, we began to get very interesting proposals from the aluminum manufacturers regarding new alloys with better properties and economics," Walt Gillette, vice president and program manager for the advanced-design aircraft disclosed July 24 at the Farnborough (England) Air Show. "We are looking at those proposals and evaluating what will be best for the airplane. At this phase of development, we aren't ready to make a decision about materials and that lets us keep an open mind and evaluate all possibilities to find the best answer." Chicago-based Boeing announced the Sonic Cruiser concept 16 months ago. The plane, if it gets a production go ahead, is expected to begin passenger service in 2008.