New, Smaller Planes Crowding Skies Once Left to Big Jets By MATTHEW L. WALD WASHINGTON, March 6 — For years the skies have been crowded with airplanes, but the planes' capabilities have kept them at different altitudes, with small, piston-driven models a few thousand feet above ground, commuter turboprops in the 20,000-foot range and jet airliners at 30,000 feet or higher. Now, to the dismay of aviation experts, an increasing number of planes may begin competing for space at the same higher altitudes. The turboprops are disappearing and being replaced by "regional jets," which fly at big-jet altitudes. Some...