LAST month's crash of Swissair Flight 111 off Halifax, Nova Scotia, has revealed a disturbing flaw in the rules governing black box flight recorders. Because of a loss of power, the final minutes of voice and flight data are missing. Safety experts are calling for changes to ensure that recorders are backed up by battery power.
The crash left 229 people dead, yet without the missing data the tragedy may never be fully explained. Investigators with the Transportation Safety Board of Canada (TSBC) have found that the aircraft's cockpit voice recorder and flight data recorder stopped working six minutes before the aircraft disappeared from radar screens.
TSBC spokesman Dana Doiron says both recorders were powered by the third of the aircraft's three main power lines. The MD-11 airliner that crashed has three engines, and each line gets power from a different engine. Prior to the crash, the aircrew had reported smoke in the cabin. If they followed standard safety procedures to identify the source of an electrical fire, they may have switched off the number three line first in an attempt to isolate the problem.
Investigators are still piecing together what really happened. If the recorders had been backed up by battery power, their job might have been easier. What is the point of making black boxes virtually indestructible, ask some industry sources, if they can be switched off by pilots or knocked out if one of an airliner's power lines fails?
"You need to gather data right to the end, says Denis Chagnon, spokesman for the industry's global safety regulator, the Montreal-based International Civil Aviation Organization. Chagnon expects the final report on the crash to recommend the introduction of mechanisms to keep black boxes working even if power fails.
An airliner's voice recorder tapes all cockpit noise, including radio conversations, pilots' voices and engine noise. It records on a 30-minute loop, so that in the event of a crash, investigators can listen to the aircraft's final minutes. The flight data recorder, taping on a 25-hour loop, records key parameters such as altitude, airspeed and flap position. Currently, the only part of the recorders that have battery power are their underwater locator beacons, which emit an acoustic signal for 35 days.
John Thom, a spokesman for Boeing in Seattle, which makes the MD-11, says that providing battery power to existing black box recorders may be counterproductive-because the tape is a loop, the data would be overwritten if the devices weren't quickly recovered. But Albert Reitan, a specialist in voice recorder analysis at the US National Transportation Safety Board in Washington DC, says this problem could be solved by adding a "g-switch" between the battery and the recorder that would sense the force of the crash and cut out.
So why haven't such devices already been made mandatory? Les Dorr, a spokesman for the US Federal Aviation Administration in Washington DC, says that one reason his organisation hasn't acted is that, in the event of a total power failure, the systems that feed information into the flight data recorder would also shut down. "The data recorder would function, but there would be nothing on it," he says.
But Reitan notes that five key instruments in modern aircraft, including the airspeed indicator and the altimeter, are low-powered liquid crystal display devices that themselves have battery back-up. So there would still be some key data to record. "In any investigation, the more information you have the better off you are," he concludes.
Author: Paul Marks
New Scientist magazine 3 October 1998, page 4
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