"It's not to the point where you can say every airplane going up is going to crash," Goglia said. "But it does raise the risk, bit by bit."
FAA officials insist they do closely monitor Northwest's Detroit hub. "We are there, day and night," Cory said.
But she acknowledged that much of the agency's monitoring is from afar. Even on days when no inspector is at Metro, inspectors analyze mechanical data via computer in Minnesota, where Northwest is headquartered.
The agency has moved in recent years to what Cory called a "risk-management type of system that puts more emphasis on making sure air carriers have a good system and they're self-auditing."
Focus on problems
Inspectors rely on data to spot problems, then focus inspections where there is an emerging pattern of malfunctions or bad practices.
For example, if airline records show that left tires on 747s are wearing away faster than expected, an FAA inspector might identify the tires as a risk area. The inspector will check maintenance records and aircraft logbooks, interview the mechanics responsible for tire maintenance, and might watch how they change and check tires and keep records.
"We're watching them, yes, and we're monitoring them," Cory said. "But we're also saying to the airline, 'Are you doing what you need to do in terms of quality control and assurance, instead of us doing it for you?' "
For all the advances in aviation technology, though, mechanics and safety experts say there is no substitute for walking the tarmac, eyeballing the engine and quizzing the mechanics.
"There is some validity to looking at the statistical data, but I think there still might be a need for a little more oversight and local presence," said Professor Dawna Rhoades, who researches maintenance issues at Embry-Riddle Aeronautical University in Daytona Beach, Fla. "Numbers don't always tell you a great deal."
Goodrich, with the FAA inspectors union, said showing up is half the battle. "If we're there, people will act differently; it's human nature," she said.
The Northwest strike hit during a period of record travel at Metro Airport. More than 18 million people passed through Metro in the first six months this year.
"Every gate is jammed," said Bob Rose, a striking Northwest mechanic and president of the local unit of the Aircraft Mechanics Fraternal Association. "You're running from plane to plane to plane."
The June audit, conducted by the Office of Inspector General for the U.S. Department of Transportation, noted that "pilots and flight crews are flying more hours, and aircraft are being utilized for more hours a day."
Airlines are shortening gate turnaround times between flights -- a practice that has allowed more flights, but provides less time for aircraft brakes to cool and for mechanics to detect problems. Auditors alluded to "incidents ... that may be precursors to potentially more serious incidents."
In Detroit, where union officials say Northwest cut its mechanics staff from 535 to 460 in the two years before the strike, mechanics describe their days as often being a scramble from gate to gate as they fix problems ranging from broken reading lights to failed power generators.
One morning earlier this month, Northwest mechanics recalled rushing to get a plane from Minneapolis that landed with a failed main power generator on its way to Boston.
With 23 minutes before a 10:33 a.m. scheduled departure, the plane was still on the ground, the cabin dark. Pressed for time, and knowing that a giant 747-400 with a long squawk sheet -- or maintenance list -- was bearing down on them in an hour, the mechanics said, they decided to power the Boston flight with an auxiliary generator instead of repairing the primary one. They said the move was safe and was allowed under FAA regulations.
The aircraft made its trip safely.
On the day the FAA inspector was in Iowa, Pence, the mechanic, said he wanted to alert the inspector to what he believed was an unsafe maintenance procedure on a plane so the FAA would "put pressure on the company" to make changes.