By Andy Pasztor Federal aviation regulators are proposing fixes to midair-collision warning devices installed on nearly 9,000 U.S airliners and business aircraft, after uncovering a safety problem during a test flight. The Federal Aviation Administration's proposed directive, made public on Monday, seeks to mandate software upgrades to widely used devices manufactured by a unit of L-3 Communications Holdings Inc. (LLL) During a flight test over a busy airport's airspace, according to the FAA, airborne collision warning systems manufactured by the unit, Aviation Communication & Surveillance Systems LLC, failed to properly keep track of all nearby planes. The agency said one aircraft disappeared for at least 40 seconds from cockpit displays, which "could lead to possible loss of separation of air traffic and possible midair collisions." Despite the proposal's broad sweep, regulators apparently concluded the problem doesn't pose an imminent safety threat because they want to give airlines and operators of business aircraft up to four years to complete the upgrades. Officials of New York-based L-3 Communications, which previously issued service bulletins dealing with the issue, couldn't immediately be reached for comment early Tuesday. An FAA spokeswoman said the company's so-called TCAS devices are installed on more than 7,000 U.S. airliners and more than 1,800 business aircraft registered in this country. Less than 100 U.S. military aircraft also use the affected TCAS devices, which provide pilots with computer-generated alerts and emergency instructions to avoid nearby aircraft. TCAS stands for Traffic Alert and Collision Avoidance Systems. Other U.S. and European companies manufacture similar systems, but those aren't affected by the FAA's proposed rule. The agency's move comes amid heightened scrutiny of airborne near-misses around the U.S. Prompted by a spate of dangerous midair incidents in the past year, The FAA months ago began a nationwide review of air-traffic control procedures and safeguards. More recently, there has been a focus on controller errors leading to a surge in midair incidents in the skies over Washington, D.C. And earlier this year, the FAA, the air-traffic controllers union and United Airlines pilots agreed to set up the first U.S. program to jointly collect and analyze voluntary reports about midair near-collisions. By shielding participants from punishment, controllers feel that the current drive to expand voluntary reporting programs and data-sharing initiatives has contributed to the recent spike in reports of hazardous midair incidents. One goal of voluntary data-sharing is to identify crowded sections of U.S. airspace where operational mistakes--ranging from altitude deviations by pilots to improper instructions issued by controllers--occur most frequently. Participants can then devise various prevention strategies to reduce risks. The FAA's proposed rule also coincides with ongoing agency and industry analyses of a different set of reports--previously supplied by European carriers--highlighting spikes in midair collision-avoidance warnings in recent years around major U.S. airports. The sites include Los Angeles, Denver, Dallas and Chicago. After analyzing data from more than four million flights, industry and FAA experts found that such cockpit warnings occurred many times more frequently over Southern California than any other busy air-traffic sector in the U.S. Some experts suspect large numbers of general-aviation aircraft around Los Angeles and San Diego, combined with FAA redesign of some approach and departure routes, may be to blame. Other experts have focused on slight differences between collision-avoidance technology used by U.S. carriers, versus the onboard devices generally used in Europe. But the FAA has said it is too early to draw definitive conclusions. The National Transportation Safety Board, which investigates airliner incidents and accidents, earlier this year started collecting its own reports of cockpit collision-avoidance warnings. The board wants to make sure that such incidents are promptly and fully reported, and that relevant radar data and detailed flight information gets passed on to investigators.