Since the F-16, American fighter jets have been inherently unstable, deliberately, and so are some of the more recent foreign fighters. While a plane is flying straight forward without any maneuvering adjustments, small momentary changes in air pressure at one part of the plane's surface or another will always develop, which momentarily tug at the plane in one direction or another. With a stable plane, the resulting tiny lateral and/or vertical movement of the plane (if it happens at all) automaticly cancels out the pressure variation, so the plane keeps moving straight forward without having to do anything to compensate. With an unstable plane, the plane's tiny movement as a result of the same tug from the air would magnify the tug instead, jerking the plane out of alignment and sending it into a spin or tumble. An unstable plane can only maintain straight forward flight by making lots of really quick little adjustments in its steering to compensate. That's done by "fly by wire" electronics instead of by the pilot; the maneuvers the pilot intends to make are input to that computer system, which then calculates exactly how to do the maneuver while also constantly adjusting against turbulence to keep itself in the air and in control. Instability is good for maneuverability, as long as it can be kept under enough control that it's not too detrimental to safety, speed, or fuel efficiency.