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Reconstruction of a South African Airways Boeing 747 has failed to reveal what started an onboard fire, which led to the loss of 19 crew members and 140 passengers. (Courtesy BEELD)

Cause Unknown

What brought down these five airplanes?

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Like the Royal Canadian Mounted Police, who always get their man, aircraft accident investigators are expected to solve every case. But Mounties don’t nab every crook, and occasionally, an airplane crash can’t be explained. Mystery compounds an accident’s tragedy, leaving families to wonder about the fate of those they lost. And from the perspective of those in the airline industry, understanding the cause of an accident is the only way sufficient measures can be taken to prevent a recurrence.

The tools and techniques of accident investigation continue to advance. In the past, flight data recorders could capture only five aspects of aircraft performance; some current digital devices can record more than 1,000. With new software, investigators can also make better use of that data in documenting an aircraft’s final moments. Not only has accident investigation grown more sophisticated, air travel has become safer. With these strides, unsolved—or partially unsolved—cases have become rarer. Still, one such rarity may be developing in the South Atlantic. As this article goes to press, the inquiry into the 2009 crash of Air France Flight 447 continues.

Investigators try to learn not only what happened in an accident but why. The “why” is usually the tougher question. In 1972, for example, as a British European Airways Hawker Siddeley Trident departed London’s Heathrow Airport, the leading-edge slats were prematurely retracted, and the airliner fell to the ground. Did a medical emergency incapacitate the captain? There is evidence suggesting this, but no proof.

Sometimes an accident stumps investigators until similar subsequent disasters establish a pattern and point the way to solution and prevention. That was the case with the infamous 1954 de Havilland Comet crashes, which stemmed from a flaw in the design of the airliner’s windows. In other cases, solutions are found but disputed. National Transportation Safety Board (NTSB) investigators concluded that the 1999 loss of an EgyptAir Boeing 767 and the 1997 loss of a 737 from Indonesia’s SilkAir were both caused by the pilots intentionally crashing the aircraft, but in each case, the airline’s government does not accept the verdict.

In some cases, investigators may need decades to close the books. In 1947, a British Lancastrian airliner crashed high in the Andes Mountains. It took 53 years for a glacier that had encased part of the wreckage to melt sufficiently for investigators to find the remains and examine them for clues. A team organized by novelist Clive Cussler has been searching the bottom of Lake Michigan for a Northwest Airlines Douglas DC-4 lost in 1950.

Here are five of the most stubbornly unyielding mysteries in aircraft accident investigation.

Northwest Airlines Flight 293

In the history of unsolved aviation accidents, the shroud of mystery has most often been the water that covers nearly three-quarters of Earth’s surface. That was the case for a Northwest Airlines DC-7C carrying six crew members and 95 passengers on June 3, 1963. Flight 293 was a charter, transporting members of the military and their families, as well as Department of Defense employees, from McChord Air Force Base, near Tacoma, Washington, to Elmendorf Air Force Base, near Anchorage, Alaska.

During the first half of the trip, radio communications indicated an uneventful flight. About two and a half hours after departure, though, the pilots requested clearance to climb from 14,000 to 18,000 feet. Controllers told them there was traffic at the requested altitude. No one replied. In that interval, something catastrophic occurred, but what? The answer lies under more than 8,000 feet of water in the Gulf of Alaska.

The report on the crash of Flight 293 is a slim document. The investigation chronicled the seemingly innocuous prelude to the flight: Aircraft mechanical condition, crew qualifications, and the like all seemed to be in order. In the aftermath of the accident, a recovery operation yielded only about 1,500 pounds of wreckage. Investigators conducted as much analysis as they could—determining, for example, that there was no indication of an inflight fire or explosion. The degree of fragmentation suggested that the aircraft hit the water at high speed. And the deformed shape of the seat backs indicated that the fuselage came down nearly inverted. The pattern of floating wreckage showed that the airframe probably remained intact until impact. The report considered the possible reasons the pilots requested altitude change, such as to avoid icing or turbulence. In the end, the Civil Aeronautics Board concluded its inquiry without a finding of probable cause.

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