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“I can’t even recall a case of losing more than a single noncritical instrument, so the idea of all critical flight displays going out at once is pretty radical,” Mr. Smith said.

Also, electrical failures that cause communication blackouts are more dangerous nowadays, given the post-Sept. 11 fear of terrorists seizing the cockpit.

It isn’t known how many of the 633 A320-series jets operated by U.S. carriers are flying without the required modification because airlines do not have to notify the FAA about each one. United said it has completed work on about 90 percent of its fleet of 152 Airbuses covered by the FAA’s directive, and Delta said it has made the fix on 124 of its 126 planes. US Airways said it has modified “more than 60 percent” of its 189 affected Airbuses.

About 2,400 of the planes in service with non-U.S. carriers are required to make the modification, according to Airbus. A spokesman for the European Aviation Safety Agency said the organization doesn’t have figures on the number of planes fixed.

A pilot who recounted a 2009 incident on NASA’s Aviation Safety Reporting System said that 28 years and 20,000 hours of flying experience couldn’t help him explain why the cockpit was “like walking into a simulator with no power or batteries on … only light was the moon.” The website does not identify the airlines or airports involved.

On the Newark flight, Mr. Cochran told investigators, nearly all cockpit indicators and gauges were lost, including his standby attitude indicator, a display that enables pilots to keep a plane at the correct angle. His primary attitude indicator also failed but re-emerged shortly before landing.

“If they’d had bad weather, they could have lost the airplane, absolutely,” said Mr. Moss, who has conducted accident investigations and served as an expert witness in aviation cases. “It was just dumb luck that it was daytime and the visibility was good.”

In the Newark tower, a chilling thought occurred to controllers as Flight 731 circled back without warning: Was this another 9/11 about to unfold?

“You could see him making a hard right and then another turn; he’s deviating off his course and loaded with fuel,” a controller working that day recalled. The controller spoke to the AP on condition of anonymity because of rules against talking to the media. “He turned back east and was going right toward New York, and I thought, ‘Oh, here we go again.’”

A 2006 failure described by Britain’s Air Accidents Investigation Branch was similarly alarming. Ninety minutes into an EasyJet flight from Spain to England, electronic instrument displays and radio communications went dead. As the pilots struggled to fix the problem, the Airbus stopped sending radar signals for 10 minutes.

With “no means of knowing where the aircraft was or what had happened to it,” French air traffic controllers diverted another plane that would have passed through the same airspace less than 20 seconds apart, according to the British report.

The plane landed safely in England with the pilots trying unsuccessfully to reach the control tower with cellphones. They told investigators they worried they would be intercepted by military aircraft if they tried to land at another airport.

Bill Bozin, Airbus Americas vice president for safety, said the company took steps to address the problem before the Newark emergency, issuing two service bulletins in 2007 recommending electrical system modifications. Unlike a regulatory agency, an airplane manufacturer can’t require airlines to make safety upgrades.

Mr. Bozin said increased awareness of the problem has improved the situation “immensely,” even though many planes are still flying without the required modification — an automatic power switchover.

“With both Airbus, through its communication with its customers, and FAA, which has put out safety bulletins on this issue, we feel that the procedures have been sufficiently emphasized that we are safe right now, even before we get the ultimate solution, which is the automatic switchover,” he said.

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