Memphis, Tenn. ? Ron Carpenter and his fellow air traffic controllers were busy keeping more than 200 airplanes on course over seven states when their communication system crashed. Suddenly they couldn’t talk to pilots or call for help.

“Somebody just pulled out a cell phone,” Carpenter said. “Then everybody else says, ‘Hey, that’s not a bad idea.”‘

So at a major Federal Aviation Administration center, controllers were reduced to using their personal cell phones to ask other centers to help keep planes on course and avert disaster.

They succeeded, but now members of Congress want to know whether the Memphis failure last month was an isolated breakdown or evidence of a design flaw in a $2.4 billion project to upgrade telecommunications at air-control centers and other FAA installations across the country.

The FAA blames the disruption on the failure of a major AT&T phone line, but critics say that the trouble is deeper – that the new communications network being installed lacks sufficient backups.

“It’s engineered this way, and it’s going to happen again,” said Dave Spero, a vice president of the union representing FAA technicians.

During the breakdown, 100,000 square miles of airspace were closed off for more than three hours and flights around the country were canceled, delayed or diverted, adding to the woes of a flying public already fed up with disruptions.

The upgrade is called the FAA Telecommunications Infrastructure project, or FTI. The prime contractor on the 15-year project is Florida-based Harris Corp., which said in September that nearly 90 percent of the FAA’s entire system of more than 4,000 installations had been switched over.

The FAA told a congressional subcommittee that the Memphis outage was an AT&T problem and that an investigation was under way.