Investigators probing the breakup of a Russian airliner above Egypt is probably to spend a lot of time examining just a few milliseconds of sound. They'll be trying to tease as much as possible from the Metrojet Airbus A321's two black boxes, recovered from the Sinai Peninsula, where the plane's pieces came to rest Oct. 31. But if previous crashes involving explosions are a guide, the sound and data the devices recorded won't provide much to definitively answer investigators' biggest question: Was it a bomb? While sound-wave spikes and sonic signatures captured by the recorders have been a valuable tool to determining what happened in crashes such as last year's missile strike against Malaysia Airlines Flight 17, they have taken a back seat to evidence drawn from shards of metal and other physical remains, according to a review of crash reports. "It was a nice thing to corroborate the other physical information that we had," said Bernard Loeb, the former head of the U.S. National Transportation Safety Board's aviation division. "But it was, in and of itself, of little value." Evidence from intelligence sources suggests that a bomb placed aboard the plane by Islamic terrorists brought it down as it was flying from Sharm el-Sheikh for St. Petersburg on Oct. 31, according to U.K. and U.S. officials. All 224 people aboard died. The pair of black boxes -- painted bright orange so they're easier to locate in a crash -- have been recovered and are being reviewed, according to a statement by Egypt's Civil Aviation Ministry. The recorders capture sounds from the cockpit and chronicle a plane's mechanical systems. They are built to withstand the rigors of a crash, from massive impacts to jet-fuel fires. Power Needed Despite those protections, they can't function without electric power. They typically fail almost immediately after an explosion, limiting their value, according to the crash reports. Dutch investigators probing the Malaysia plane shot down above Ukraine on July 17, 2014, heard only 20 milliseconds of sound after the explosion on the cockpit recorder aboard the Boeing Co. 777, according to their final report. A spike in noise lasting 2.1 milliseconds was identified as "the sound of a pressure wave associated with an explosion," the report said. Investigators found that the sound pulse reached the recorder's four microphones -- one for each pilot station and another capturing ambient noise in the cockpit -- at slightly different times. That allowed them to calculate that the explosion occurred outside the cockpit just above and to the left of the plane. While that estimate wasn't conclusive, it matched the pocked fuselage where shrapnel from the missile hit the plane, according to the report. The cockpit recording on TWA Flight 800, which went down off the coast of New York in 1996 in a ball of fire after its fuel tank exploded, captured a "very loud sound" that lasted only 0.117 seconds before the recorder lost power and failed, according to the NTSB's final report. In the TWA investigation, conducted amid claims that a bomb or missile was involved, the NTSB gathered recordings from previous aircraft breakups such as Pan Am Flight 103, a Boeing 747 downed by a bomb over Scotland in 1988. Investigators also recorded the sounds of various types of bombs, simulated structural failures and other explosions in abandoned aircraft. Their analysis found that sound waves from bombs left a distinct signature different from other sudden failures, helping them rule out a bomb. Critical Clues Loeb, who oversaw the TWA investigation and participated in other cases in which explosions brought down planes, said the recorders have another purpose: They can help rule out other possible causes. In the Malaysia accident, for example, the recorders showed that the plane was operating normally until it was hit by the missile. Still, the most critical clues in the TWA investigation were the twisted metal fragments dredged up from the Atlantic Ocean, Loeb said. Indentations in steel beams and the way skin on the fuselage peeled off the plane provided investigators a road map for where the explosion occurred and how it began. Investigators combing the desert in Egypt will have to conduct the same meticulous review of the Metrojet aircraft's wreckage, he said. "They are going to have to have the physical forensic evidence or we're not going to know," Loeb said. Source: Bloomberg
