Friday, May 16, 2008
Twenty-five years ago, the greatest disaster in airline history killed 538 people, in part because of a radio glitch that still hasn't been fixed.
Mar 28, 2002 | Air travel, always a source of stress for the traveler, has gone full scale in the headache department. From the terrorist hijackings and the crash of American Flight 587, to the hassles now confronting travelers at our terminals, our skies have entered a new realm of insufferability in the mind of a worried populace.
It's a given that any vestiges of aviation's glamour days were long ago devoured in post-deregulation chaos, but our ambivalence toward flying never quite evolved into outright fear. It is different now, and the industry cannot afford to miss a step. Should another plane go down, whether from a terrorist's act of sabotage or a proverbial act of God, or should word emerge of some safety-oriented negligence, unprecedented numbers of citizens may be stowing their seat trays for the last time and opting instead for highways, trains and buses.
There are any number of things the airlines could do to preclude this. And one of these cannot be implemented fast enough: the long-needed installation of an inexpensive piece of equipment into the communications radios of airliners. While we fuss over cockpit doors, bombs and preflight security, this simple enhancement could save hundreds of lives.
The culprit is something called a "heterodyne." No, it's not a prehistoric animal or newly discovered subatomic particle, but the technical term for the phenomenon of two simultaneous radio transmissions blocking each other out.
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.
Airspace in and around major terminals is often congested with planes, all receiving and acknowledging instructions in rapid-fire succession. Blocked transmissions are very common. Anyone with a scanner or receiver that picks up the air traffic bands will be treated to the piercing squeals of heterodynes, often followed by third-party pilots chirping in with "You were blocked," or "You were stepped on," so that the instruction or acknowledgment can be repeated.
When maneuvering through the skies and along taxiways, pilots listen not only for their own instructions, but for those of other pilots as well. By creating a mental picture of what other aircraft are doing, they can orient themselves in the vast choreography of a crowded sky or tarmac. Should anybody offer an incorrect readback, acknowledge the wrong clearance or otherwise screw up, other pilots often detect the mistake.
Even in the worst congestion, such errors are rare and dangerous ones even more so. But the potential is always there, and the stakes are much higher than a heterodyne-induced headache or having to repeat yourself.
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.
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