A remarkable thing happened on American airplanes last year: For the first time since the beginning of commercial flight, there were no fatalities on any US airline anywhere in the world. This preliminary statistic from the Federal Aviation Administration is a powerful antidote to the public concern that escalated after the ValuJet and TWA Flight 800 crashes of 1996. Moreover, it highlights the slow, steady progress toward safer skies - progress fueled by innovations as basic as better fire-resistant materials in passenger cabins, and as sophisticated as ultrasonic scanners that peer deep into metal layers of fuel tanks looking for hints of cracks and corrosion. Thousands of new ideas like these are the pinions of safety in America's skies. Yet reasons abound for even greater vigilance. Air traffic continues to boom, making accidents more likely. Concerns are unabated, too, about things like airport safety and pilot error. So the industry is counting on these innovations - and new efforts to eliminate human mistakes - to help keep planes safely aloft. "We're long past the point where there's a single thing that will dramatically boost airline safety," says Clint Oster, a professor at the University of Indiana's School of Public and Environmental Affairs in Bloomington. "Today, safety is much more evolutionary than revolutionary." Numbers tell the story of the extraordinary progress in airplane safety over the past four decades. In 1959, there were more than 26 accidents for every 1 million takeoffs. Today there are fewer than two. Yet today's rate has not shown much improvement for about 25 years. So as air traffic grows, experts worry the number of crashes will rise if the accident rate doesn't drop. As a result, last year President Clinton called for an 80 percent cut in this accident rate over the next decade. Part of getting there will be tighter regulation. In fact, 1998's zero-fatality rate is partly due to tougher safety standards for smaller "puddle-jumper" commuter planes. (The zero-fatality number doesn't include the Sept. 2, 1998, crash of Swiss Air Flight 111 off Nova Scotia, because it was not a US carrier.) The industry is also counting on a continued string of innovations, such as the ground-proximity warning system that screams, "Whoop, whoop! Pull up, pull up! Terrain, terrain!" to pilots flying dangerously low. Or there's the collision-avoidance system, which similarly warns pilots if they fly too close to another airplane. The computer even tells each plane's pilots which direction to fly to avoid a midair crash. More useful every day is today's better weather forecasting - and the delivery of those data to pilots. One new system allows pilots to get weather predictions for flight paths from their home computers over the Internet. Mechanics are also seeing many innovations. The fuel-tank scanner, a microwave-oven-size device that scouts out corrosion, uses the same technology that looks into the wombs of pregnant women. Before its arrival, mechanics had to drain fuel tanks and climb inside to eyeball any cracks. That process took two days. Today's takes two hours. OTHER maintenance goes on even while a plane is aloft. Sensors monitor potential trouble spots and beam reports to the ground. There, computers analyze the trouble and can print out job tickets for the ground crew. When the plane arrives at the gate, the right mechanic is there to work on it. Better design plays into safety, too. Today's engines often last 35,000 or more hours - at least 10 years - without being taken off the wing for major repairs. All this adds up to better safety - and a better bottom line. "Airlines have learned that good safety is good business," says Aaron Gellman, director of Northwestern University's Transportation Center in Evanston, Ill. If commercial airlines needed any reminder, there was the May 1996 ValuJet crash in Florida, which left 110 dead and the low-cost carrier's image in tatters. There were 340 passenger deaths that year and 54 in 1997. To be sure, there are disputes between regulators and airlines over safety issues, with regulators goading airlines into adopting new measures and airlines complaining they cost too much. One current area of concern is airport security. Newly released FAA documents, requested by The New York Times, show gaping holes in airport security. Federal agents posing as passengers smuggled guns and other weapons aboard planes with ease, the Times reported Monday. But perhaps the biggest concern is human error - by everyone from security personnel to pilots. About 75 percent of all accidents are human-error related, says David Stempler, president of the Air Travelers Association in Washington. Three-quarters of those errors occur because people don't follow procedures. "We try to make shortcuts, or we forget to do one little thing," he says. Sometimes pilots rush through checklists, for instance. But new systems are being developed to ensure that pilots check each item on the list. Even here, technology can help - sometimes.