One by one, the oldest Boeing 737s returned to the sky Monday, but not until completing emergency inspections that had some eye-catching results: In 50 percent of the planes examined, half had extensive wear in power lines running through their wing fuel tanks.

As of Monday morning, 47 aircraft had been inspected, and mechanics found some signs of abrasion in half of the wiring bundles examined, Federal Aviation Administrator Jane Garvey said."I think any time we see abrasion in wiring we are concerned," she said on NBC's "Today" show.

Garvey, however, tried to allay the concerns of passengers scheduled to travel aboard a 737.

"Certainly if it's a 737 that's flying today, then it's not part of the fleet that we are the most concerned about," Garvey said. "The ones we are concerned about will not fly until they've been inspected, until we're sure, until the air carriers are sure that it is safe."

United Airlines, which canceled 54 flights Sunday to make the emergency inspections, said it expected to have all of its 18 affected aircraft operating Monday. Southwest Airlines, which has 35 such airplanes in its all-737 fleet, had mechanics work in teams of three over the weekend to finish the inspections.

"Basically, from a customer standpoint, the inspections have been seamless," Southwest Airlines spokeswoman Kristie Kerr said.

Doug Clowers found his United flight from Ronald Reagan Washington National Airport to Chicago canceled, "but they made sure they didn't mess up my plans too much. They made sure I was on the next available flight," he said.

The emergency inspection, which affected 15 percent of the 737s operating domestically, was the broadest order by the FAA affecting commercial aircraft since McDonnell-Douglas DC-10s were grounded in 1979.

It came three days after the FAA gave airlines up to a week to inspect their oldest 737s for signs of wear in the wiring or the pipes that carry the wires through the fuel tanks.

A recent inspection of a Continental Airlines 737 found both exposed wires and holes in the piping believed to have been caused by electrical sparks. Officials feared the mixture of fuel, air and electricity could spark an explosion such as the one that downed TWA Flight 800.

On Saturday, mechanics inspecting a United Airlines 737 found one wire bundle that showed signs of sparking in one spot and a second area where the wires had been worn bare, most likely from vibration over the plane's 50,000 hours in flight.

That prompted the FAA to revise its original order, ordering all Boeing 737-100 and -200 series aircraft immediately out of passenger service until they could be inspected and repaired. The affected planes have cigar-shaped engines mounted directly under their wings. U.S. carriers operate 179 of the aircraft.