By MATTHEW L. WALD
WASHINGTON, - Now that it appears obvious that the Grumman Mallard seaplane that crashed Monday off Miami had metal fatigue, the airline and the Federal Aviation Administration are devising a way to inspect the four remaining Mallards for the same problem.
But the difficulty they face may illustrate why the problem was not discovered in the plane before it crashed.
Mark V. Rosenker, acting chairman of the National Transportation Safety Board, said Thursday that getting to the wing beam that came apart in the crash would require peeling back the metal skin of the plane.
Though many planes are carefully inspected for metal fatigue, it is much harder to find on models that are not extensively used, said John Goglia, an airplane maintenance expert and former member of the safety board.
Only four Mallards - all built 50 years ago or more - are operated commercially, all owned by the operator of the plane in Monday's crash, Chalk's Ocean Airways, so that airline never has the benefit of the experience of other carriers, Mr. Goglia said.
He noted that aircraft produced by companies that still existed usually had the manufacturer behind them.
"In this case, the manufacturer is gone, and there's probably nobody providing the support, so it increases responsibility on the operator," said Mr. Goglia, who is now a professor of aviation science at St. Louis University.
The board made the problem obvious on Thursday when it posted photographs on its Web site of the broken metal part, which shows the "beach marks" characteristic of metal fatigue. These are curvy lines, like those left in the sand by waves, showing flaws created by different stresses.
Fatigue can be found by a variety of inspection techniques, experts say, and the spot where it was found in the wreckage, on the spar, an internal beam near the fuselage, is the first place to look, said George H. Kizner, chairman of the mechanical engineering department at Vaughn College of Aeronautics and Technology, in Queens.
"That's where the stresses are the maximum," Mr. Kizner said.
Mr. Rosenker said his investigators were cutting the same part out of the opposite wing, to send it to a laboratory for analysis and to see whether it also showed fatigue.
One method for finding fatigue before an accident is to spread a dye on the metal, wait for it to seep into cracks and then wipe the surface clean. Then the technician puts a "developer" powder on the surface and waits for it to suck dye back out of the cracks, making the cracks more obvious.
Another is to put iron filings on the part being tested and run a current through the metal; the filings will gather at the cracks, because those are magnetic poles.
Another is to run electricity through the part and measure the magnetic field; cracks will produce a disrupted field.
Technicians can also use ultrasound; they spread petroleum jelly on the object to be tested and bounce sound waves off the target. Just as in medical diagnosis, the ultrasound will reveal internal structures, including cracks.
The safety board is probably months away from concluding that metal fatigue was the immediate cause of the Miami crash, said people with experience in its investigations. Mr. Rosenker went so far as to say that fatigue was "certainly one of the elements that is extremely interesting to us."
But the board only reaches conclusions when its members vote on a final report.
Investigators will also look into other questions, experts said. Mr. Goglia, who was on the board from 1995 to 2004, said this would include whether the inspection program, developed by the airline and approved by the F.A.A., was adequate, and whether the plan was followed.
Another issue, Mr. Goglia said, is the replacement of the original piston-driven engines with the more powerful turbines, which generate high-frequency vibrations.
"The engine mod is certainly suspect," he said. "This airplane has been flying since the 40's, and wings haven't fallen off the ones that haven't been re-engined."
Source: New York Times
