Each time a thundering jetliner passes overhead these days, Stacie Huffman warily scans the sky for plunging debris.
Only a twist of fate saved the high school sophomore from aluminum part that a Delta Airlines 727 dropped on her tennis match in April at Mount Rainier High School near Seattle. Just a few seconds before her coach beckoned her, Stacie, 16, was leaning on a fence at ground zero.
"There was a loud bang," she recalled. "I turned around and this big piece of metal was crashing into the fence 10 feet away."
Like cars losing hub caps and mufflers on the highways, airplanes periodically shed objects such as aluminum skins, access panels, fuselage doors, red hot turbine blades, frozen sewage, and even whole engines.
Hurtling toward Earth at hundreds of miles per hour, such fall out has punched gaping holes in roofs, crushed cars into pancakes, plunged into crowded swimming pools, smashed school desks, showered people with human waste, sent glass shards shooting through living rooms and set houses on fire.
Federal officials and aviation executives usually characterize these incidents as acts of God so rare they aren't a menace. But they acknowledge that no one knows with any certainty how often they occur, nor has anyone seriously studied the problem.
No deaths or injuries have been reported in such incidents, but the Federal Aviation Administration is not even 100 percent certain of that. Some safety experts and those who have had close encounters with falling flotsam say it is a problem that needs attention.
"Nobody has been killed yet yet yet, but it will be a public relations disaster the day it finally happens," said Chuck Miller, a noted air safety proponent and former government accident investigator. "Relying on statistical probability is a very short-sighted approach."
Poor maintenance and bad design are major culprits, according to engineers. Increasing use of older aircraft, more prone to corrosion and metal fatigue, also may be contributing factors. Toilets can leak sewage, which freezes at high altitude, then falls off as ice.
While the skies have grown more crowded with flights, once-vacant zones near big city airports have filled up with industrial and residential development. As a result, falling debris is far more likely to be noticed.
Objects from all types of aircraft are being reported with what appears to be growing regularity; several serious incidents have occurred this year alone.
Data formerly kept by the military suggest that things fall off combat aircraft every day. The Navy recorded an average of 460 incidents per year during the 1980s.
In the commercial world, Boeing received 200 reports of structural parts falling off its planes between 1989 and 1994. McDonnell Douglas officials said only several incidents occur each year involving their planes.
Not to worry, say FAA officials who characterize falling debris as statistically insignificant.
"It is a rare, rare event that this happens, but it does happen," said Anthony J. Broderick, the FAA's associate administrator for regulation and aircraft certification. "When you measure them in terms of probability, given millions flights, it is very, very low."
The FAA is supposed to investigate every report of a falling object, but Mr. Broderick says tracking down the offending aircraft is "generally impossible."
The agency has no records of ever fining an airline or undertaking disciplinary action against a pilot for dropping something, even in incidents that resulted in serious property damage.
One of the most serious cases occurred in 1983, when an engine malfunction aboard a DC-9 spewed hot turban blades that set brush fires and burned 15 rooftops in Newport Beach, Calif.
Steve Link's home in the exclusive Dover Shores area was gutted. Mr. Link says he and his wife wonder whether the stress caused by the incident triggered a miscarriage.
"It was a very tough time," Mr. Link said. "Ultimately, my wife couldn't deal with it and we had to sell the house and move."
Mr. Link said the airline disputed the damages and he ultimately had to hire an attorney to reach a settlement with Republic Airlines, which was later bought out by another company.
Airlines often refuse to accept responsibility when they are accused and fight demands for property damage compensation.
Executives at major airlines and their unions declined to talk publicly about the problem. However, one executive at a major airline privately called the incidents "major embarrassments."
The biggest thing ever known to have fallen off a plane was caused by stress corrosion on a Boeing 737, operated by Aloha Airlines. The entire top of the forward fuselage plunged into the Pacific Ocean in 1988, killing a flight attendant and injuring 61 others before landing. The incident prompted the FAA to intensify inspections and improve maintenance of older aircraft.