But there have been no fatal crashes in which maintenance error was a cause since January 2003, when a US Airways Express flight, a Beechcraft 1900, went out of control on takeoff because of an improperly rigged tail. Statistically, the era of outsourcing appears to be safer than when airlines did most of the work themselves, although that does not suggest a cause-and-effect relationship.

The decade-long push to reduce the accident rate began with a “safety summit” in 1996, after the T.W.A. Flight 800 disaster off Long Island and the ValuJet crash in the Everglades of Florida. The summit was convened by the secretary of transportation at the time, Federico F. Pena, who declared a goal of zero accidents.

In 1997, a national commission on aviation safety and security, led by Vice President Al Gore and known as the Gore Commission, concluded that a more realistic goal would be to cut the rate of fatal accidents by 80 percent. Because crashes are sporadic, the goal was stated as the average of the most recent three years.

Despite the safety improvements since then, not all the trends are positive. Airports have lately recorded a disturbing number of what they call “proximity events,” in which a plane lands on a runway already occupied by another because someone made a wrong turn or a controller made an error.

On July 11, for example, a United plane in Fort Lauderdale, Fla., took a wrong turn onto a runway where a Delta Air Lines plane was supposed to land; the two came within 100 feet, according to the F.A.A.

“Probably the biggest threat of all, today, many, many people agree, is not so much a midair collision as a runway incursion incident,” said Richard Healing, an aviation safety expert and former member of the National Transportation Safety Board.

The F.A.A. has a radar system at many airports to warn tower controllers of conflicts on the airport surface, but the system can be confused by puddles on the pavement, which the radar sometimes misinterprets as airplanes. And it warns only the controllers, not the pilots directly.