Millimeters of Aluminum Separate Airplane Passengers From Open Sky

April 5, 2011— -- In the middle of the California Mojave desert, engineers cut through the aluminum roof an old plane in an airplane graveyard.

Many of the planes here are used in safety tests as engineers search for ways to make planes safer and more durable.

On top of a Boeing 737, similar to the one that was ripped open in mid-flight last week, a worker tore through metal to show what stands between the passengers and the open sky: merely millimeters of metal.

Last week, a fuselage rupture in the plane's roof sent a Southwest flight into a terrifying but controlled descent of 36,000 feet. Investigators have attributed that incident to a crack in the plane's aluminum skin.

Stuart Witt, a pilot and general manager of the Mojave Air and Space Port, said even though the aluminum shell doesn't look impressive, it's safe and has been doing its job for more than 70 years.

"Millions of people will fly today, and they will arrive safely in aluminum planes," Witt said.

But the aluminum skin can "fatigue"-- meaning it can crack, stretch and break apart over time. Those cracks can be hard to see, however, so inspectors rely on devices that use electricity to check for microscopic cracks.

John Nance, aviation expert and ABC News contributor, said inspectors were forced to rethink how they conducted those inspections after the roof of Aloha Airlines Flight 243 was torn off in 1988, killing one person. He said inspectors may have to do the same now.

"Nobody thought cracks were going to be a problem in the middle of the fuselage," Nance said on "Good Morning America" today.

Since Friday's incident, the federal government has issued emergency inspections on 175 Boeing 737s. Southwest owns 80 of those planes.

Nance pointed out that this was not a problem specific to one airline.

"What we're looking at is not a Southwest problem. What we're looking at is a problem for aging aircraft," said Nance.

There are alternatives to the aluminum skin. Aviation pioneer Dick Rutan said a material called composite, made up of mostly carbon fiber, is the future of flight. Some larger, commercial airlines use it today.