Accident investigators tough task; how they'll determine why Germanwings Flight 9525 crashed

Cockpit voice recordings and, hopefully, flight data will provide the main clues to investigators trying to understand what caused Germanwings Flight 9525 to crash.

But that is just the beginning. Drawing on decades of experience with crashes around the globe, French officials will analyze thousands of pieces of wreckage, maintenance log books and other clues to determine what led the Airbus A320 to crash into a mountain, killing all 150 passengers and crew.

"Mapping the accident site is a very detailed process that can take days and sometimes weeks," warns Jim Hall, former chairman of the National Transportation Safety Board.

Investigators lead by the France's BEA, an agency that investigates air accidents, will photograph, chart and document each piece of wreckage as well as the human remains. While there is no indication of terrorist activity, the crash site is treated like a crime scene until proven otherwise.

Other members of the team will start to listen to the cockpit voice recorder for any clues. The recorder takes audio feeds from four microphones within the cockpit, documenting all the conversations between the pilots, air traffic controllers as well as any noises in the cockpit. In this case, the entire flight, starting with taxi and takeoff, should be recorded. Investigators will look for clues in the pilots' voice tones, words and listen for sounds of explosion or alarms in the background.

Psychologists will also study the recording to listen for voice inflexions to see if the pilots were drunk or impaired, tired or angry with each other.

"The cockpit voice recorder will be very interesting, perhaps by its silence," says Steve Hull, an accident-investigation consultant with RTI Forensics. If that is the case, it could indicate that the pilots were unconscious.

The flight data recorder captures 25 hours' worth of information on the position and condition of almost every major part in a plane. Investigators will initially focus on the engines, flaps, rudders and cockpit instruments.

Even if the two data recorders — often called black boxes, despite being bright orange — don't offer clues, experts are confident that authorities will still piece together the chain of events.

"The wreckage itself will provide a lot of information," Hall says.

If there is usable data on the recorders, that will initially lead the direction of the investigations, says Alan E. Diehl, a former air safety investigator with the NTSB and a former scientist for human performance at the Federal Aviation Administration.

"Both will point you in directions of what is critical," Diehl says. "Based on what you learn from the recorders, you might focus on key pieces of wreckage."

The four possible causes of any crash are human error, mechanical problems, weather, criminal activity or a combination of two or more. Diehl says investigators will work backward, starting by eliminating what didn't happen.

"You're usually dealing with a jigsaw puzzle with many of the pieces missing," he says. "You start eliminating things that didn't happen."

As the debris field is mapped and photographed, Diehl says, a key question there will be: What's missing? If engine parts or bits of the tail, for instance, are missing from the main crash site then investigators can presume they broke off earlier in the flight and might have been part of the problem.

Fragments of metal will also be brought back to laboratories and studied under microscopes.

The metal can often show what happened to a plane. For instance, if the plane was pushed beyond its structural limits and pulled apart, there would be evidence of it. Microscopic looks at the wreckage would also tell if there was fetal fatigue or corrosion. If there was an explosion, the metal would show such an outward force and there would be chemical residue and pitting.

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Scott Mayerowitz can be reached at http://twitter.com/GlobeTrotScott .