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Metal fatigue cited in crash

NTSB investigators find a flaw in a wing that fell off a Chalk's plane. The FAA will look at the company's other planes.

By BILL ADAIR and TAMARA LUSH
Published December 22, 2005

WASHINGTON - It didn't take long for experts at the National Transportation Safety Board to spot the problem in the severed wing of the Chalk's Ocean Airways seaplane that crashed in Miami Beach: a crack in a metal beam that holds the wing to the fuselage.

By simply looking at photographs of the crack, NTSB metallurgists in Washington made a quick determination that the beam suffered from metal fatigue. The problem can grow undetected and make a plane break apart, as the Grumman G-73T Mallard did shortly after takeoff Monday afternoon from a shipping channel near Miami Beach. The two pilots and all 18 passengers were killed.

Mark Rosenker, the NTSB's acting chairman, said in an interview Wednesday night that the crack "was an amazing find. It was quite obvious."

Chalk's announced it was voluntarily grounding its other four G-73Ts and was cooperating with the investigation.

Records from a 1992 inspection of the plane indicate corrosion was found in the same area of the beam as the fatigue crack. Mechanics sanded away the corrosion, polished the area and made sure there was no significant damage to the structure, according to the records.

Rosenker said that he was unfamiliar with that incident but that investigators were examining all records.

The FAA plans to examine the other Chalk's planes to see if they have corrosion or other problems in that area of the wings.

"We are pulling the skin back on one wing and looking at other aircraft to see if there are any other similar issues," said FAA spokeswoman Laura Brown.

Rosenker said investigators were unable to get any information from the plane's cockpit voice recorder, but said they had found a key piece of wreckage - the matching piece of the beam.

It will allow them to put the pieces together and try to determine why the beam broke.

"To have both sides is a very interesting opportunity for us," he said. "It allows us to match everything up."

NTSB officials emphasized that the investigation had just begun and that it will take a year for the agency to make a formal decision on the probable cause. The NTSB usually determines a crash is caused by several factors. But based on Wednesday's developments, it seems likely metal fatigue played a role in making the plane's right wing break off in a fireball.

Fatigue cracks can start because of corrosion, a manufacturing flaw or because that portion of the plane was struck by an object. The cracks spread because of repeated stress, like a paper clip bent backward and forward until it breaks.

The wing of the Grumman Mallard is supported by two spars, metal I-beams that endure the stresses of flight. The NTSB found the rear spar on the right wing had a crack with the tell-tale signs of fatigue: wavy lines known as "beach marks" because they resemble the rippling left in the sand after repeated waves.

Michael Marx, the former chief of the NTSB's metals lab, said fatigue cracks are quite apparent in the wreckage of a crash.

"It becomes pretty easy for somebody to spot," he said.

John Cox, an accident investigator in St. Petersburg, said wing spars are designed to be so strong that they won't break in flight.

"It's very rare," Cox said. "They are so robust and carefully designed that they are not an item you expect to fail."

The FAA requires that planes be inspected for problems such as cracks. The inspections are done visually and using electrical currents, X-rays or ultrasound.

Like many airlines, Chalk's relied on mechanics from other companies to do some of its inspections, including those for corrosion. But those inspections can be difficult because the spars are sealed inside the wing, which must be disassembled for mechanics to see them. Because it is a 58-year-old plane, it lacks modern panels that would make inspections easier.

The NTSB will try to determine why the cracks were not found in previous examinations.

The NTSB has investigated similar accidents involving forest service planes that crashed in 2002 because of broken wings. The agency said the accidents were caused by fatigue cracks that were not detected because of inadequate inspections.

Perry Christie, the prime minister of the Bahamas, said his government is following the crash investigation closely.

The airline's decision to ground its four remaining planes leaves the residents of Bimini - and the many tourists who want to visit - with few options. Although the tiny island is only 50 miles from Miami, no commercial carrier flies there, unlike Freeport or Nassau.

With the loss of Chalk's planes, only boats and expensive charter services make trips there.

"Chalk's represents a lifeline to this economy," Christie said.

[Last modified December 22, 2005, 01:12:39]