Making aging planes safer

The Philadelphia Inquirer's website, philly.com, has an interesting article about experimental work being done by scientists and engineers from Drexel University in order to better understand metal fatigue in aircraft.

Metal fatigue means what it sounds like. Repeated low levels of stress weaken a piece of metal. But unlike people, metal does not recover with rest.

Fatigued metal is deformed microscopically, in ways not fully understood. Tiny cracks develop, and eventually the material can no longer carry its weight.

You can demonstrate this with the humble paper clip. Flex it back and forth a few times, and eventually it snaps.

Similarly, an airplane flexes in various ways during flight, though the impact takes much longer to see.

One type of stress is due to changes in air pressure.

On the ground, pressure inside and outside the cabin is the same. But high in the sky, the pressure inside an airliner is typically about 8 pounds per square inch greater than the outside pressure.

The aluminum skin of the plane stretches slightly as a result, said Drexel's Tein-Min Tan, an associate professor of mechanical engineering.

The circular cross-section of a typical midsize airliner expands an inch or so in flight, he said. The circumference returns to its original size upon landing.

The article goes on to describe the methods used to artificially stress metal airplane panels in order to study the process. This work is being carried out at an FAA facility in Atlantic City.

In addition to studying metal fatigue, the researchers are also beginning to carry out similar studies on the new composite materials of the type that will be used to build the next generation of planes, such as the Boeing Dreamliner.

Composites, typically graphite fibers in an epoxy matrix, are even lighter and stronger than aluminum. But unlike aluminum, they can't be dented, so there may be no visible signs of underlying damage.

The Drexel team is also testing composites, using ultrasound and other nonvisual damage-detection methods to see what's best.

In April of this year, the FAA proposed a rule to limit how long commercial airplanes could fly. This kind of rule, which the FAA can enact after reviewing comments due in September, signals a departure from previous thinking in the aviation industry, which held that planes are safe to fly indefinitely, as long as they are properly maintained.

The FAA proposal would place different limits on each model of airplane, depending on data from the manufacturer.

It would affect commercial planes that weigh 75,000 pounds or more when loaded. About 1,600 smaller regional jets would be excluded.

If planes were retired at the end of their current "service goals," the rule would phase out 602 airplanes over 20 years - 15 percent of cargo planes and 10 percent of passenger craft.

But the rule would allow operators to apply for extensions, so long as tests demonstrated the crafts were safe.

Most airlines have not yet commented on the proposal, and have asked for more time to review it. The National Transportation Safety Board supports the measure, but urged the FAA to extend it to smaller planes.

Read the whole article here: Drexel works to make aging planes safer, philly.com

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