Reading The Wreckage
Air crash investigators train students to fit little pieces into the big picture.
- By Eric Adams
- Air & Space magazine, July 2001
(Page 3 of 5)
All training stresses that accident investigation is a team effort. After a crash is reported, the investigators, led by the Investigator In Charge, break into groups, each of which gathers evidence about the pilot, the powerplant, the aircraft structure, air traffic control, the weather, and other factors. They interview witnesses and begin sorting through debris. The students are taught that they must account for every component of an aircraft, if for no other reason than to rule it out as a contributing factor.
One of the biggest challenges in that respect is determining whether components were damaged before the accident or during it. Among the most thoroughly addressed subjects is fire. Wall passes around a small charred canister. It’s an oxygen container identical to the one that started the fire that brought down ValuJet Flight 592 in the Florida Everglades in May 1996, killing 110 people. This one was used to test whether such a fire could have occurred, and its presence silences the room. “Always suspect the possibility of in-flight fire,” Wall says. “When a fire is in the slipstream and fueled by lots of oxygen, such as in an engine, temperatures can exceed 3,000 degrees, whereas on the ground they will usually stay below 2,000 degrees.”
Materials leave distinctive signatures when subjected to certain temperatures, he continues, thus possibly revealing whether the fire occurred in the air or after the crash and where it originated. Rubber hoses, for example, will melt at 400 degrees, aluminum alloys at 1,000 degrees, and stainless steel at 3,100 degrees. Furthermore, when aluminum melts on the ground, it will puddle due to gravity, but when it comes apart in an in-flight fire, it produces the “broomstraw effect,” in which the points where the metal has come apart will look stringy.
Other damage might require closer inspection. Andy McMinn, a TSI staff instructor, briefs us on metallurgy and how to use it to determine what caused a part to fail. Throughout his talk, he refers to crystal structure—how atoms are arranged—and discusses the various ways parts can change, depending on whether they have been subjected to fatigue, violence, or some other stress. Parts usually fail by overload, the material can be ductile or brittle, and the failure will leave important clues. “Fatigue manifests itself in ‘beach marks,’ like tide marks in the sand,” McMinn says. “If a rectangular plate is pulled and compressed thousands of times, beach marks will indicate the direction of crack propagation. When the cracks reach a critical size, you’ll see progressive failure and then instantaneous failure.”
The course’s aggressive pace is relieved by breaks and lunches—but even then there’s something to see. Instructors show videos of airplanes crashing and documentaries about the accident investigation process. The classroom itself contains exhibits on investigation processes and equipment, examples of component failure, an enormous cut-away of a Teledyne piston engine, and framed, illustrated case studies showing the effects of inexperience, bad luck, aircraft neglect, and poor judgment. One shows an old Cessna sitting crushed, nose down, on the side of a mountain. The pilot, who had been killed, had only a student license that had expired five years before, and a list found in the cockpit showed about 30 repair items that needed attention, but there was no evidence that any of the repairs had been completed. The airplane had had its last annual inspection five years before the crash.
Mike Grimes, a Teledyne engineer who lectures on engines and propellers, knows that in accidents such as that any number of factors could have contributed. So he tells us to start at the front of the airplane and decide whether the engine was running when the airplane crashed—and then determine what might have caused the engine to stop. “Look for propeller damage that requires a lot of energy,” he says. “Are there massive chunks of aluminum torn out of the leading edge of the blade? Is the hub broken?”
But what if a propeller is missing? Why did it drop off?
“How do you find a missing propeller?” he asks. “You can’t use [FAA tracking] radar data because the propeller is too small, but you can use it to see where the airplane started coming down. That’ll give you a general idea. Then the insurance adjuster becomes your best friend. Why? He’s got the checkbook. Get him to place an ad and offer a reward. Everyone with an SUV is going to be out looking for that prop.”