Transport aircraft manufacturers, including business jet OEMs, will be required to develp operational limitations for all future Part 25 designs, under a proposed rule intended to eliminate "widespread fatigue damage" (WFD) as aircraft age. However, for existing type certificates, this proposal would apply only to jets with mtows of more than 75,000 pounds and opperated under Part 121 U.S. and Part 129 foreign carriers. Specifically excludded are business jets with mtows of more than 75,000 pounds, such as models manufactured by Bombardier and Gulfstream used under Parts 91 and 135. Aircraft would be prohibited from operating beyond the limit established. However, the rule would provide a means for operators to extend the limit, after which the aircraft could no longer be operated. Comments are due by July 17.