Air travel's communications killer

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.


P. Smith
March 29, 2002 1:30AM (UTC)

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.

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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.

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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.

On that Sunday afternoon, just before 2 p.m., two Boeing 747s touched down at Los Rodeos. One was a KLM (Royal Dutch Airlines) flight from Amsterdam. The other a Pan American flight from New York. Both were charter flights.

And neither, as fate would have it, was supposed to be on Tenerife at all. Both were originally headed to nearby Las Palmas, where their passengers, nearly 700 in all, were meeting cruise ships bound for the Mediterranean. But earlier in the afternoon, a bomb exploded in a flower shop at Las Palmas airport, and all flights were diverted. Both KLM and Pan Am, along with many others, put down on Tenerife until Las Palmas opened up again.

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They taxied to the terminal building, parking right next to each other on the tarmac. It was a hectic scene at the normally lazy Los Rodeos. The ramp was crammed with airplanes -- most of which weren't scheduled to be there in the first place. Jet engines whined, horns blared, cars and trucks darted between aircraft and beneath wingtips.

At just about 5 o'clock, with Las Palmas accepting traffic and the airplanes refueled, the Pan Am and KLM crews received taxi clearance and began to move. Meanwhile, the weather had become terrible, with thick fog, light rain and very little visibility.

The Pan Am jet, in a note of historical irony worth mentioning, was identified on its blue-and-white hull as the Clipper Victor. The doomed Victor was already, in fact, no stranger to notoriety. Seven years earlier this very same airplane made history when it completed the first revenue flight of a Boeing 747, from New York to Heathrow on Jan. 21, 1970. Somewhere on its nose was the dent from a shattered champagne bottle.

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>From Clipper Victor, the view from the upper-deck cockpit, some 40 feet above the ground, was a commanding one, high enough for Capt. Victor Grubbs and his crew to see over the roof of the terminal, and above the tails of the other airplanes. Except for one: the KLM 747, which carried the name Rhine.

As the oldest operating airline in the world, KLM was and remains a proud airline with an excellent safety record. In the captain's seat of the Rhine sat Jacob van Zanten, one of the company's top instructor pilots. Frequent fliers may have recognized van Zanten's handsome visage as he walked through the concourse, or down the spiral staircase in the first-class cabin of the 747, for van Zanten himself stared out from some of KLM's advertisements -- in his captain's seat, silver-haired and square-jawed, assuring the world of the punctuality and competence of his country's national airline.

Because of the congestion, the normal route to Runway 30 at Los Rodeos was blocked. Departing planes would have to taxi on the runway itself, in a procedure called a "backtaxi." Reaching the end of the strip, they'd make a 180-degree turn and then take off in the opposite direction.

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Both KLM and Pan Am were given permission to backtaxi simultaneously. Van Zanten would go first. He would steer to the end, wheel his 747 around in a great U-turn, and then hold in position until granted permission for takeoff. Behind him was Capt. Grubbs in the Pan Am Clipper. His instructions were to eventually turn clear of the runway to allow van Zanten's departure.

Because of the fog, the airplanes could not see one another. And neither was visible from the control tower. The airport was not equipped with ground tracking radar.

Finally in position for departure, the KLM crew called for its ATC route clearance. This is not a takeoff clearance, but a procedure outlining turns, altitudes and frequencies for use en route. It is normally received well before an aircraft reaches the runway, but the KLM crew had been too preoccupied with checklists and taxi instructions to ask for it until now.

At 5:06, the KLM first officer, sitting just to the right of van Zanten, verified the route clearance with the control tower. He then uttered these mysterious words: "We are now, uh, at takeoff."

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For whatever reason, the KLM crew believed it had been cleared for takeoff. "We gaan," van Zanten told his crew. "Let's go." Releasing the brakes of his mammoth machine, the Rhine began barreling down the fog-shrouded runway, completely without permission.

"We are now at takeoff" is not standard phraseology among pilots, its intent perhaps vague. But it was explicit enough to get the attention of both the Pan Am crew and the control tower.

Almost immediately, the tower radioed back to KLM, saying, "OK, stand by for takeoff. I will call you."

At the same time, the Pan Am crew, still on the runway and quite concerned with KLM's final remark, made a call as well. "And we're still taxiing down the runway," announced the first officer.

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Either of these transmissions would have been, should have been, enough to stop van Zanten cold in his tracks. Realizing his mistake, there was still time to discontinue the roll. That is, if he'd heard either one.

But because both the controller's and Pan Am's calls were made at the same instant, the only audible sound in van Zanten's ears was the crackle and squeal of a five-second heterodyne.

Further confusion arose as the KLM 747 accelerated. The second officer leaned to van Zanten and asked, "Is he not clear, that Pan American?"

"Oh yes," van Zanten replied emphatically.

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Capt. Grubbs is heard saying nervously, "Let's get the fuck out of here." And with that, he and the crew saw the lights of the lumbering KLM jet emerging out of the fog. "Get off! Get off! Get off!" yelled the first officer.

Seconds before impact, van Zanten shouts and attempts to leapfrog his aircraft over the Pan Am, dragging its tail along the pavement for 70 feet. Its landing gear just lifting from the pavement, the Rhine slams into the side of the Clipper Victor, which had veered sharply to the left to avoid the collision.

The KLM aircraft settled back to the runway, skidded another thousand feet and was consumed by fire before a single one of its 248 passengers and crew members could escape. Gutted by fire and explosion, 335 people aboard the Pan Am plane also were killed. (There were 54 survivors from the Pan Am, including the entire cockpit crew.) The combined total of 583 victims represents the highest-ever death toll in an airplane crash of any kind.

Rarely is the heterodyne discussed as a central factor in what happened that day in 1977. Instead, the crux of the event is lost in a quarter-century-long legacy of human factors analysis: The pilots were stressed and tired. They used incorrect terminology. The tower controller was distracted by a soccer game playing in the background. And so forth.

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Sure, if van Zanten hadn't initiated his takeoff roll without permission, nobody would have died that day. But since when have we left our fate entirely in the hands of one person's judgment? If that were the case, there'd be fewer dials, alarms and flashing red lights in a cockpit. People make stupid mistakes, and technology is supposed to be there to back us up. There are products for sale than can greatly lessen -- even eliminate -- the danger from most occluded radio calls. These products are cheap, effective and readily available. Added to an aircraft's communication system, the devices monitor VHF channels and inhibit a pilot or controller from speaking onto an already busy frequency.

Worldwide, some airlines have voluntarily equipped their fleets with such units, the most common of which is made by British Aerospace and trademarked as CONTRAN. One airline, Britannia, a large U.K. operator of 757s and 767s, began outfitting its planes with CONTRAN in 1999. Virgin Atlantic also volunteered to test it on some of its 747s. A ground-based CONTRAN for ATC was put into operation at several British airports.

But despite their fiscally precarious positions, and even with their reputations on the line, it is doubtful the rest our biggest airlines will follow the likes of Britannia and Virgin. Sensible as it may seem, it rarely ever happens this way. Instead, the industry will need some coaching and persuasion from the FAA, its best friend and/or worst enemy, depending on the issue or whom you ask.

While the use of anti-blocking units like CONTRAN is recommended and encouraged by authorities both in the U.S. and abroad (the FAA, the U.K.'s CAA, etc.), it is not mandatory. Not until this technology is required aboard all commerical aircraft will frequencies remain clear of heterodynes and their potential dangers.

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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?"


P. Smith

MORE FROM P. Smith

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