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The radio glitch that killed more than 500 people

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Anyone who has used a traditional analog communications radio knows that 2 sets of communications can not go on at the same time.  When two people “key up” at the same time with about the same power on the same frequency a “heterodyne” is produced and neither transmission is heard.

This has been a challenge of VHF communications in air travel

From Salon ( http://www.salon.com/technology/feature/2002/03/28/heterodyne/index.html?x)

Normally, flights communicate with air traffic control (ATC) via two-way VHF radios. While tuned to a particular frequency (the spectrum used by air traffic rests between 118.0 and 136.97 MHz), a pilot or controller clicks the microphone, speaks and waits for an acknowledgment or “readback.” It differs from talking on the telephone, for example, as only one party can speak at a time.

The trouble arises when two — or more — microphones are clicked at the same instant. The transmissions are effectively canceled out, rendered unintelligible in a noisy hail of static or a high-pitched squeal. Speaking simultaneously, the transmitting parties do not realize the block has occurred.

And sadly enough, the lesson here is not so much of what could happen, but what has already happened. For this month marks the 25th anniversary of the world’s worst air disaster, a crash between two airplanes that never left the ground, caused in part by a blocked transmission, a heterodyne. Most people have never heard of Tenerife, a small, frying pan-shaped speck in the Atlantic. Tenerife is one of the Canary Islands, a rocky chain off the coast of Morocco, governed by the Spanish. The big town on Tenerife is called Santa Cruz, and its airport, at the base of a cascading mountain, is called Los Rodeos. On March 27, 1977, Los Rodeos was the scene of the worst airplane crash in history.

Long delayed and overly complex solutions to simple challenges?

In the past, the FAA has eventually gotten around to legislating a host of important regulations after various accidents. After two high-profile midair collisions, one in 1978 and a second in 1986, an airborne traffic collision avoidance system, known as TCAS, is now found in the cockpit of every airliner. Following the crash of ValuJet in the Everglades in 1996, fire suppression was mandated for cargo holds. And after a long pathology of something euphemistically called CFIT, or, “controlled flight into terrain,” ground proximity warning systems (GPWS) became standard equipment.

Generally, not only were these fixes mandated long after they should have been, but they came in the form of expensive, overly complex warning systems. Things like GPWS and TCAS probably thrilled the engineers who designed them, but their color-coded depictions, variable-pitch aural warnings and multistage alarms often use up more gray matter than a pilot may have to spare in the heat of battle.

This time what’s needed is not another acronymic “system” of high-technology prowess, but a back-to-basics, low-tech solution to an old and very high-stakes problem. The fix is so low-tech, in fact, the airlines and regulators should be ashamed and embarrassed even to debate the matter.

In some instances, serious problems do not require cumbersome or costly solutions. It’s too late for those killed on Tenerife, but 25 years later, another clipped transmission could find us back on a foggy runway asking, “Why?”

So, what is the easy low-tech solution?

Read about CONTRAN – http://www.aatl.net/publications/contran.htm

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Written by frrl

July 20, 2010 at 7:39 am

Posted in Uncategorized

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