ByJohn S. McClenahen Third-quarter billings this year for the general aviation industry reached $2.1 billion, 1.6% better than the July-September quarter in 2000. But, also reports the Washington-based General Aviation Manufacturers Assn., even as the value of shipments was rising, the number of aircraft shipped was falling 13.4% to 593 from 685 the year before. Compared with 2000's third calendar quarter, shipments of piston-powered, turboprop, and turbofan jets all declined during this year's third quarter. For the first nine months of 2001, shipments of general aviation aircraft were valued at $6.5 billion, 2.4% better than the $6.3 billion recorded during the first nine months of 2000. However, the number of aircraft shipped is down 6.2% for the first nine months of 2001, with an 8.5% drop in shipments of piston-powered aircraft and a 6% decline in turboprop shipments more than offsetting a 0.7% increase in the number of turbofan jets shipped. Some 1,877 airplanes were shipped during this year's first nine months, compared with 2,002 during the first three calendar quarters of last year.