A company that keeps tabs on the aircraft industry reports jet aircraft orders are booming as commercial airlines worldwide modernize their fleets with more fuel-efficient aircraft. The company, Forecast International, says more than 3,400 aircraft worth $114 billion are either on order or on option to order, most to be delivered over the next six years. In 15 years of tracking aircraft developments, ``I've never, never seen a replacement cycle like this,'' said company President Edward M. Nebinger. He said Tuesday airlines are ordering new planes because they can save 20 to 25 percent on fuel costs with newer models, with fuel being an airline company's largest operational expense. Nebinger also said the industry had weathered an era of cutthroat competition, prompted partly by the deregulation of the industry a decade ago, that has seen less-efficient carriers fold amid a series of mergers and consolidations. ``The survivors are economically stronger,'' he said. Newer models also reduce the risk of metal fatigue, which literally can cause a plane to fall apart. Nebinger recalled this year's Aloha Airlines incident, when the entire front section of the cabin roof flew off in flight, due to fatigue cracks. That plane had made 90,000 takeoffs and landings, he said. The Aloha Airlines incident and the recent crash of a 1973-vintage Boeing 727 jet in Dallas renewed questions about the reliability of aging aircraft, Nebinger said. His figures indicate the average age of a Boeing 707 is 20.3 years, while the average age of a Boeing 727 is 15.4 years. The average age of a McDonnell Douglas DC-8 is 21.4 years and the average age of a DC-10 is 12 years. Forecast International maintains computer data on aircraft inventories worldwide. Nebinger said jet planes are the last major items that the United States exports and his data provide encouraging news as the United States attempts to trim its trade deficit. U.S. manufacturers are expected to deliver 1,738 new aircraft, worth about $72.5 billion, through 1994. Of that, $25.4 billion is destined for American airline companies, while the remaining $47.1 billion will go to foreign customers, Nebinger said. But he also said European companies, specifically Airbus Industrie, are taking an ever-increasing bite of the market away from U.S. giants like Boeing and McDonnell Douglas, which 15 years ago claimed virtually the entire world market. He said Airbus Industrie was expected to deliver 852 aircraft through 1995, while other European producers will deliver about 575. Much of those will go to countries other than the United States, but about $12.8 billion will go to U.S. operators, he said. On balance, Nebinger said, ``the U.S. still emerges as a very strong winner in the import-export balance, with exports to Europe exceeding imports by $13.2 billion.'' The impact on the U.S. trade deficit, he said, ``cannot be overemphasized.'' Boeing has 1,275 aircraft either on order or option to order. McDonnell Douglas has 349. Airbus Industrie has 852, while Fokker, based in the Netherlands, has 257, and Embraer of Brazil has 208, according to Nebinger's numbers. To support the orders his company has tracked, new engines needed through 1994 will be worth $24.7 billion, Nebinger said, of which U.S. manufacturers will produce $15.6 billion, or 63 percent.