| 11 September 2001>News Stories>First Tower to Fall Was Hit at Higher Speed, Study Finds
First Tower to Fall Was Hit at Higher Speed, Study Finds
esearchers trying to explain why the World Trade Center's south tower fell first, though struck second, are focusing on new calculations showing that the passenger jet that hit the south tower had been flying as fast as 586 miles an hour, about 100 miles an hour faster than the other hijacked plane.
The speed of the two planes at impact has been painstakingly estimated using a mix of video, radar and even the recorded sounds of the planes passing overhead.
Two sets of estimates, by government and private scientists, have surfaced, but both show that the plane that hit the south tower at 9:02 a.m., United Airlines Flight 175, approached the trade center at extremely high speed, much faster than American Airlines Flight 11, which hit the north tower at 8:46 a.m.
In fact, the United plane was moving so fast that it was at risk of breaking up in midair as it made a final turn toward the south tower, traveling at a speed far exceeding the 767-200 design limit for that altitude, a Boeing official said.
"These guys exceeded even the emergency dive speed," said Liz Verdier, a Boeing spokeswoman. "It's off the chart."
The speed of the planes is far from the only factor that will be important in explaining why the south tower, which was struck between the 78th and 84th floors, fell within 56 minutes and the north tower, which was hit between the 94th and 99th floors, stood for 102 minutes.
Ultimately, it was the combination of structural damage and the fires, fueled by thousands of gallons of jet fuel, that brought the buildings down. The south tower was also hit at a lower point, meaning there was more weight bearing down on the damaged floors.
But the difference in the towers' survival times, which translated into a difference in the amount of time tenants and rescue personnel had to get out, could be related in part to the planes' speeds, researchers said.
"Clearly one plane came in faster and had more energy," Dr. Jerome Connor, a professor of civil engineering at the Massachusetts Institute of Technology who is studying the collapses, said of the new calculations, in which he was not directly involved.
"If one building had more damage, it would take less for the heat to build up enough for it to come down," Dr. Connor said. "That would help explain why the building that was hit second, fell first."
The high speed of United Flight 175 may also have complicated the hijackers' mission, because it would have been more difficult to make accurate adjustments in the plane's direction, several pilots said. Loud and repeated alarms would also have been sounding in the cockpit.
"The faster you go, the less time and room you have for error," said Tim O'Toole, a former 767 pilot and staff engineer in safety department of the Air Line Pilots Association.
The flight data recorders from the two planes have not been found; Boeing officials said these so-called black boxes are not designed to survive the forces they encountered in the collapse.
But a researcher at Massachusetts Institute of Technology, by closely studying videos of the attack, has estimated the planes' speeds. The Federal Aviation Administration, in consultation with the National Transportation Safety Board, has come up with its own estimates, based on radar and video.
The M.I.T. analysis, by Eduardo Kausel, a professor of civil and environmental engineering, found that the United plane was traveling an estimated 537 m.p.h., while the American plane, the first to hit, was traveling 429 m.p.h.
The Federal Bureau of Investigation said the government's analysis put the speeds at 586 m.p.h. for the United flight and 494 m.p.h. for the American one.
In both cases, the planes were flying much faster than they should have been at that altitude: the aviation agency's limit below 10,000 feet is 287 m.p.h.
Investigators could not say for sure why one plane was traveling faster than the other; it may have been accidental choices of novice pilots, or perhaps the second group of hijackers feared being shot down. But what is clear is that at impact, this difference was important.
The energy of motion carried by any object, called the kinetic energy, varies as the square of its velocity, so even modest differences in speed can translate into large variations in what the building had to absorb.
That means that while the United jet was traveling only about a quarter faster than the American jet, it would have released about 50 percent more energy on impact.
"The difference is enormous," Dr. Kausel said of the energy created by the impact of the planes.
Even at a speed of only about 500 m.p.h., a partly loaded Boeing 767 weighing 132 tons would have created about three billion joules of energy at impact, the equivalent of three- quarters of a ton of T.N.T., according to another team of researchers at M.I.T.
Only about 6 percent of that energy would be used up in cutting more than 30 exterior steel columns, said Dr. Tomasz Wierzbicki, a professor of applied mechanics at M.I.T., who did his research with a student, Liang Xue. But some 25 percent would go into ripping up floor structures and 56 percent in damaging structural columns in the core.
The energy poured into the core at this speed would probably be enough to damage or break about 23 of the 47 columns in the core. At a higher speed, more may have been damaged.
Aviation experts have disagreed over just how difficult the mission was for the hijackers, who had limited flying experience and had probably never operated a real commercial jet. The high speeds added to the complexity of their task.
The typical cruise speed of a Boeing 767-200 at 35,000 feet is 530 m.p.h. The lower the plane goes, however, the thicker the air becomes, so the slower the plane must travel to avoid excessive stress.
Flying a Boeing 767 straight ahead at 1,000 to 1,500 feet would not be too difficult, even at more than 580 m.p.h., and it would most likely not threaten the structural integrity of the plane, a half a dozen pilots and a Boeing spokeswoman said.
But accurately turning the plane at that speed and maintaining the proper pitch, or up and down movement, is difficult, the pilots said, particularly for a novice pilot, and turning at that speed would have put excessive stress on the plane.
An automatic pilot device could have directed the hijacked planes to Manhattan, if the hijackers knew how to enter certain coordinates into the computerized flight management system. But as they approached the city, the hijackers almost certainly had to take manual control of the aircraft, because the automatic pilot in navigation mode is not accurate enough to target the center of building, pilots said.
Video of the approach of United Flight 175 to the south tower shows that it banked westward in the final moments, its right wing going up, its left wing down. That maneuver may have been intended to maximize damage to the building. But it has been interpreted by some pilots as a sign that the hijacker nearly missed the tower.
"It was unfortunate luck," said Richard Fariello, a retired T.W.A. captain who works as a consultant to NASA. "The way he was headed, he could have just clipped it perhaps with one wing. There is a good chance that would have been the case."
Structural engineers cannot yet say how important a role the planes' speed played in how quickly the towers collapsed. Aside from the fact that the second plane hit a lower floor, it also struck more to one side of the tower's face, presumably causing asymmetric damage that could have made it more difficult for the tower to reapportion its loads among surviving structural columns.
But determining the force and energy of impact is the starting point for any effort to understand what failures within the buildings eventually caused collapse, said Dr. Shyam Sunder, chief of the structures division at the building and fire research lab of the National Institute of Standards and Technology.
"It's important to have the speed of the plane and the direction that it hit for any analysis that we do relating to aircraft impact on the structure," Dr. Sunder said.
If the plane that hit the south tower had been traveling slower, and the tower perhaps had stood longer, it is still unclear how many more people would have survived. Even though the south tower fell in only 56 minutes, fewer tenants died in it than in the north tower. In large part, that is because many of the people who worked in the upper floors had evacuated during the 16 minutes between the two attacks. But extra time might have meant that those trapped above the impact zone at the south tower would have found the one emergency exit stairwell that was still passable.