Some aviation experts want to wreck a few airplanes to see if they can determine when a plane becomes too old to fly.

Apparently nobody knows the answer, as underscored by the fact that 350 international aviation experts discussed the problem for three days last week, yet couldn't agree."There were people on both sides of that saying yes you should (etire old planes), and people on the other side saying that there is no life limit to an airplane, that you can fly it indefinitely as long as it is properly inspected and maintained," said Fred Farrar, a spokesman for the Federal Aviation Administration.

The conference was called after April's Aloha Airlines accident, in which the skin of a 19-year-old Boeing 737 peeled away in midair, sweeping a flight attendant to her death.

But while that debate continues, there was considerable support for having the federal government purchase old airplanes and test them to death to see how long they will last.

The plane makers and airlines liked the idea, as do some members of Congress.

"Research in government, whether it's the FAA or NASA, over the past several years has tended to focus on the new, not the old," Rep. Dan Glickman, D-Kan., told the conference. "We're looking for new avionics, new materials, new structures, new way of doing things - but some of the problems are basically old problems."

While the concept of testing airplanes goes back to the Wright brothers, virtually all tests done today are performed on new aircraft.

Asked how often The Boeing Co. tests older aircraft, company spokesman Jack Gamble said, "There really isn't that much done, and that would apply to all manufacturers." The Seattle-based firm is the leading commercial aircraft manufacturer.

Commercial plane makers such as Boeing and McDonnell Douglas expect their planes to last 20 years, based partly on a design philosophy sometimes called "two lifetimes." Here's how FAA administrator Allan McArtor recently explained it:

"There are several ways to build an airplane," McArtor said. "One is to take whatever the object would be and to test it. And when it fails, you declare the useful life to be half of whatever that test was. Then people who use that device feel comfortable, since it's been tested to twice as much as what's considered to be useful life."

Under that philosophy, planes would be retired when they reach age 20 - but often they aren't. More than 600 planes over 20 are still being flown by the major airlines.

A contrasting philosophy is that if you treat an old airplane like a classic car, babying it and catching any trouble early, it will last indefinitely.

But age is only one consideration. The numbers of flights, the level of maintenance and how the plane has been used are also factors.

In the Aloha Airlines accident, for instance, the Boeing 737 jet was 19 - still within the designed lifetime - but the plane's heavy Hawaiian island-hopping schedule in a humid and salty atmosphere strained the aircraft. Corrosion may also have been a factor in the accident.

If older airplanes are tested, the planes would eventually be destroyed. The process is called fatigue testing, and the plane never leaves the ground.

In a fatigue test, the body of the plane is pressurized and depressurized every five minutes, around the clock. In just days, a body of an airplane can be put through the equivalent of years of wear-and-tear.

The FAA has not decided if it should spend money to test old airplanes, spokesman Farrar said. "A lot of people point out it can be awfully expensive, Gamble said.