Some TCAS Units Miss Traffic, FAA Says

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The traffic alert and collision avoidance systems used in thousands of aircraft need to be upgraded, the FAA said this week, to prevent them from missing targets in high-density areas. During a flight test, a TCAS unit built by Aviation Communications & Surveillance Systems (ACSS) dropped several aircraft tracks because of interference limiting, the FAA said. The dropped tracks could "compromise separation of air traffic and lead to subsequent mid-air collisions," according to the proposed airworthiness directive. The fix will cost about $3,000 per airplane, the FAA said. The units are installed on about 7,000 aircraft operated by U.S. airlines and more than 1,800 business aircraft, an FAA spokeswoman told The Wall Street Journal.

The FAA said that during a flight test, the TCAS unit interrogated aircraft in a high-density airport area and some of the targets disappeared from the cockpit display or were not recognized. One occurrence of dropped tracks occurred for 30 to 40 seconds of a 90-minute flight segment. Operators have 48 months after the effective date of the AD to install the software upgrade. A spokeswoman for L-3 Communications, the parent company of ACSS, told the Journal that the company informed the FAA of the problem in the summer of 2009, and has been working on a fix. Service bulletins already have been issued to deal with it, she said.