Towers of Terror

They were not the tallest towers in the world, just the most loved. They were not quite as famous as the Taj Mahal, just more visited. They were not just a tourist attraction but also the provider of livelihood of tens of thousands. They were not at the center of the universe, but they definitely seemed as if they were. They were not just yet another tall and pretty building but a symbol of a country’s financial might. One of the most photographed, the most visited, and the most symbolic shrine of architecture, the “twin towers” of the World Trade center in New York City, are no more.

The World Trade Center (WTC), located at the southern tip of the island of Manhattan was a large office and shopping complex with six buildings, interconnected by a huge basement or mezzanine. Two of the buildings were majestic, slender, square towers. Completed in 1973, the towers rose about half a kilometer into the sky. Vertical parallel lines zoomed skywards making it a breathtaking sight from any angle. Going up the towers were an astounding 205 separate elevators (lifts). With 418,600 square meters of space, the WTC is home to over 500 corporations, which employs about 40,000 people. The mezzanine consisted of several levels and housed a huge shopping complex, and three subway stations with six separate lines (or routes), including an underwater line to New Jersey. The 360-foot mast atop the north towered had 10 television antennas and numerous auxiliary antennas. It was the first office complex to have its own electronic telephone exchange, fiber optic cabling, and international data access.

Visitors flock to the twin towers, at a rate of 10,000 a day, to climb up and gawk at spectacular views of the city, the harbor and the green lady with a torch. A set of super fast, non-stop elevators whisk them non-stop to the upper deck, 110 floors above the city in 58 seconds. The observation deck was glass all around and on pleasant days the rooftop provides a beautiful open-air perch above the world’s most spectacular concrete jungle–New York City.

At 7:59 a.m. on Tuesday September 11^th 2001, American Airlines flight 11, a Boeing 767-200 jetliner with 91 passengers (capacity 168) and about 10,000 gallons of jet fuel (also called aviation kerosene) aboard, took off from Boston for the 2590-mile trip for Los Angeles. The expected flight time was over 5 hours. At about the same time (7:58 a.m.), same location United Airlines flight 93, another 767-200 with 38 passengers headed to San Francisco.

The controllers at Boston Logan airport guided American flight 11 from takeoff to the edge of the city and handed off to controllers at the FAA flight service center in Nashua, New Hampshire. Along with tens of other aircrafWar and Progresst on the same airspace, a particular controller at the Nashua Center closely controlled AA 11, commanding its speed, turns, climb rates and direction. About 15 minutes later, when the aircraft was at 29,000 feet, the controller instructed it to rise to the cruise altitude of 31,000 ft. The pilot did not respond and the plane did not ascend. Frantically, he repeated, “American one-one, do you hear?” He switched frequencies and tried again, but the result was silence. Then, suddenly the aircraft’s transponder stopped working.

Every aircraft, huge or tiny, flying in the US airspace is required to have an “altitude reporting transponder”. This is a small device that receives radar signals from the ground, and responds with an identity code (called squawk code) and the aircraft’s altitude. The transponder makes the aircraft more visible to radar and provides controllers with altitude information, as radar is not designed to detect altitude. In an emergency the squawk code is set to 7700 for mechanical trouble, 7600 for radio failure and 7500 for hijack. These codes cause displays on radar screens to flash and are used to send messages in hijack situations when the pilot is unable to tell the truth.

The aircraft was taken over by five knife-wielding people who had stabbed the flight attendants and scared the passengers and entered the cockpit. Obviously, the well-versed hijackers instructed the pilot to switch off the transponder and not communicate with the controllers. The pilot however occasionally and surreptitiously pressed the “press to talk” button on the yoke (a steering like control) making cockpit voices get transmitted, and hence the controllers heard fragments of the conversations. The controllers suspected a hijack was in progress, but were not sure. Radar images showed that the westbound plane made a southbound turn at 8:28 a.m. and flew over the Hudson River towards New York. The controllers presumed the plane would land on one of the NY area airports. The transmissions from the cockpit ended about 10 minutes after the turn, and hence someone other than the pilot took control. Due to the lack of the transponder the descent of the plane went undetected.

Seventeen minutes after the turn, at 8:48 a.m. the plane was flying past the island of Manhattan over the water. It made another, sharper turn to the left, and collided with the north tower of the WTC. The collision was made possible by descending the plane from 29,000 feet to about 1,000 feet, reducing speed from about 450 knots to about 350 knots and finally, seconds before the impact, accurately turning the plane 90º. The wingspan of the 767 is 150 feet across, and the WTC tower was just 200 feet across. Maneuvering a heavy, fuel laden, jetliner with that level of accuracy at a low altitude with a high airspeed, can only be done by a trained pilot, and possibly only by a pilot trained to fly that particular kind of airplane.

The collision sprayed kerosene into the building and the result was an explosion and a fireball. The New York TV stations had helicopters in the air to look for morning traffic and aimed their airborne cameras at the burning north tower and live pictures were on TV, at 8:50 a.m. Within minutes the national cable channels (CNN, MSNBC) picked up the live feed. Since the New York TV stations broadcast from atop the antenna on the north tower, the signal did not really get broadcast, but the satellite feeds into the cable TV system worked. Apart from a few people in New York City (about 35% of household in New York do not have cable TV), the world watched the event live.

As the audience watched the picture of a smoldering tower, another plane flew into view and within seconds crashed into the second tower. Suddenly realization dawned that this was not a freaky accident, but a purposeful, planned atrocity. This plane was United 93 from Boston, which had flown further west before turning back towards New York and arrived at 9:03 a.m., 15 minutes after the first collision. By then the military had been informed and F-16 jet fighters were patrolling aimlessly looking for hijacked aircraft. Two other passenger airplanes were reported missing.

The other two were AA 77 and UA 93, both Boeing 757’s the former from Dulles airport in Washington and the latter from Newark Airport in New Jersey. Both had taken off about 8 a.m. AA 77 flew to Ohio, turned back and circled over the US Capitol in Washington DC and crashed into the Pentagon at 9:43 a.m. UA 93 flew to Cleveland turned back and then there was a sequence of passenger and hijacker fights culminating in a crash at a remote part of Pennsylvania at 10:10 a.m. Recurrent rumors say this plane was shot down by military fighters, the Defense department vehemently denies, the FBI thinks that theory is not to be ruled out yet.

The World Trade towers burnt fiercely and then collapsed. The South tower, which was hit later, collapsed first at 10:05 a.m., about an hour after impact. The North tower collapsed 10:28 a.m. about one-an a half hours after impact. The towers “imploded”, that is, they fell on their own footprints in a cloud of smoke and dust. The buildings had a “tube structure” that is, they are built like huge tin cans – strong on the outside and relative weak on the inside. The vertical pillars connected by horizontal beams keep them rigid and resistant to sway. However the explosion in the center and the resulting fire melted the beams, and the pillars fell into the building causing the implosion. The spectacular implosion also buried thousands of people—fathers, mothers, sons and daughters. Mostly breadwinners, they all disappeared into a rather large cloud.

For the next few days the entire county was close to paralyzed. Productivity zoomed down, TV watching zoomed up. One of the major mainstays of the American economy—air traffic—was completely shut off.  About 100,000 departures from 15,000 airports did not happen each day, every day. Millions of people were stranded. Restarting this monstrous service will take days if not weeks. Business came to a virtual standstill. The stock market remained closed. The blow to a faltering economy was massive, and the effects will linger for years.

Is a lack of security and complacency to blame for the tragedy? Not really. How does one ensure that 1.6 million passengers per day are searched closely? Not possible, total security cannot be implemented. Security can be (and will be) increased, but with a toll of efficiency. The detriment is the reduction in capacity of the travel system and an increase in price of tickets. It may seem to be a small price to pay, but the effects on the economy may be significant. Loss of civil liberties will be a bigger toll.

About 16 people, with no guns, in less than two hours took down the Mecca of the financial economy and the Mecca of the US Defense Department. In the process they snuffed out about 10,000 lives, affected the lives of several millions of people, and created the possibility of a worldwide recession. This is a completely new form of global warfare. The technique uses the infrastructure of a country to attack itself. We know how to fight wars with tanks, ships and aircraft. We know how to engage people in a war zone. We do not know how to fight a war where the enemy is us.

Over the last ten, maybe fifteen years, the US military has been embarking on a mass scale modernization program with the vision of a changing landscape of war. War would not be fought, they say, with just tanks and aircraft. In years to come, it will be an increasingly targeted, information driven warfare, say the experts. Communication automated control, soldiers sporting wearable computers and automated warfare machinery with computerized sensors and actuators would be the cutting edge of the technological warfare. Winning wars would depend on who has the latest technological wizardry. The tragedy at the southern tip of Manhattan throws all such visions into limbo.

The Twin Towers of the World Trade Center are no more. The painful lessons learnt will resound all over the world in forms unpredictable. Those who are celebrating the coordination, the planning and the uncanny success of the mission, will eventually learn a sad lesson. People living in Manhattan will eventually come to grips with the gaping space in the skyline. For all those of us who are ex-residents of Manhattan, or avid visitors of the city, the towers are still there, just invisible.

Note: This article was compiled from information available as of September 13^th.

Partha Dasgupta is on the faculty of the Computer Science and Engineering Department at Arizona State University in Tempe. His specializations are in the areas of Operating Systems, Cryptography and Networking. His homepage is at http://cactus.eas.asu.edu/partha