The U.S. Air Forces new F-35 (Joint Strike Fighter) is moving from prototype to production and serious weight gain problems have been  encountered. The aircraft is supposed to have a maximum take off weight of 25 tons. But as designers checked that all portions of the aircraft were strong enough to deal with every maneuver the aircraft is meant to handle, it was found that many structural elements would have to be stronger. The prototype aircraft are not built as strongly as the final production model, and some weight gain is expected in the final version. But in this case, the F-35 might end up being five tons heavier. Even if this is avoided, this will put more pressure on the air force to switch more quickly to using a larger proportion of unmanned aircraft.