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 if 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.

FAA spokesman Paul Takemoto said the network has backup phone lines for emergencies and each center is served by more than one communications carrier.

“The failure was in a network we don’t own or operate,” Takemoto said. “It was a massive failure, according to what BellSouth has told us, that has never been experienced before and affected all of the customers in that region, not just the FAA.”

Aviation consultant Michael Goldfarb said backup at the Memphis center was obviously insufficient.

“Wal-Mart losing its power is one thing, but for the FAA to lose power when planes are in the air is another thing,” said Goldfarb, a former FAA chief of staff. “And to only have one line take down an air-control center, something is very wrong with that picture.”

AT&T; refused to talk about the breakdown except to say its cause was under investigation. Harris, which landed the contract in 2002, also had no comment.

The Memphis breakdown was the latest in a string of similar but less serious failures of the upgraded system at other air-control centers, said Spero of the technicians union. “This is the first time a whole center has gone down,” he said.

His union, Professional Airways Systems Specialists, has long been at odds with the FAA over the upgrade and other projects.

The Transportation Department’s inspector general has criticized the project for falling behind schedule and running over the original cost of $1.9 billion.

The Air Route Traffic Control Center at Memphis is one of 20 such FAA centers around the country.

When communications failed, controllers at other centers — summoned by cell phone — directed planes out of the Memphis airspace, which covers parts of Alabama, Mississippi, Arkansas, Missouri, Indiana, Kentucky and Tennessee.

All airline traffic within a 250-mile radius of Memphis was shut down and flights heading into the region were rerouted. Flight disruptions were reported in Nashville, St. Louis, Pittsburgh, Miami and other cities.

The FAA is under pressure from Congress and President Bush to reduce flight delays. In the first seven months of 2007, the industry turned in its worst on-time performance since the government began collecting comparable data in 1995.

“The delays are a major concern, but my first thought is about planes being up there without controls and that could jeopardize lives,” said Rep. Steve Cohen, D-Tenn., a member of the House aviation subcommittee.

Rep. James Oberstar, D-Minn., chairman of the House Transportation Committee, said his panel plans to look into the project.

“What makes our air-traffic control system so safe is it’s heavily reliant on backups, overlaps and redundancies,” Oberstar said. “If one system fails, there’s another to pick up for it. Something did happen and this backup was not available.”