Get Informed

Tuesday, November 20, 2018

Plane Impact and Tower Sway at the WTC on 9/11

Examining Momentum Transfer on the Towers

Written by

During the seventeenth anniversary of the events of 9/11, a number of the unsupportable explanations about the mechanics of "what happened" on 9/11 resurfaced. The 9/11 TAP contact-us form received numerous comments requesting that 9/11TAP become a proponent of positions, including, faked aircraft impacts at the Twin Towers on 9/11.  In particular, the comments expressed disagreement with an earlier article entitled Observed Deceleration of UA 175 during the Impact at the South Tower.  The resurgence of this proposition was amplified by a number of strident individuals commandeering e-mail threads and “shouting down” arguments based on an analysis of the evidence. In this article, we will look at the issue of a large plane impacting the Twin Towers from several perspectives.

From this analysis, it is not possible to state exactly what type of a plane impacted the Twin Towers, but it is certain that the Towers were subjected to momentum-based impacting forces that caused them to sway after the impact.

While the 9/11 Truth Action Project focuses, largely, on outreach and education about the events of 9/11, it is imperative for people doing outreach:

  1. To understand basic physics,
  2. To understand how evidence is gathered,
  3. To understand what conclusions can be drawn from the collected data, and
  4. To understand the limits and caveats associated with the conclusions.

It is especially important that activists who do outreach to the public, who reach out to professionals, and who prepare public documents avoid making statements that can not be supported – and which can be shown to be erroneous.

9/11 TAP provides a comprehensive collection of materials that can be used by people doing outreach so that they can speak authoritatively about the events of 9/11 and the questions that are unanswered. The destruction of the World Trade Center and especially the destruction of World Trade Center Building 7 is the major focus for 9/11 TAP.

Basics Physics

There are many aspects of 9/11 where simple physics validates the observations reported by many witnesses and substantially disproves alternative hypotheses.   While the alternative hypotheses may not be completely disproved, it does raise the "raises the bar" for any alternative hypotheses – because there are factual observations, supported by basic physics, that must be explained away.  This process of hypothesis, alternative hypothesis and the use of other factors – physics in this case – is at the heart of the process known as scientific inquiry. 

A key issue for those concerned about the events of 9/11, where basic physics has "raised the bar" so high that the official explanation cannot be true, is the collapse of World Trade Center Building 7 under "free-fall" acceleration for approximately eight stories.

South Tower Impact – Transfer of Momentum

The key physics principle that supports the presence of a large plane impacting the Twin Towers is the transfer of momentum. The observation supporting this transfer of momentum the sway of the towers captured in video footage taken during and immediately after the plane impact.  This video record of building sway is supported by accounts from the occupants of the towers.  In the Scott Meyers’ video (entire video here), the top of the South Tower is seen to sway to the right and then to begin its return to center before the sway is obscured by smoke. In this video, the impact of a plane into the South Tower is recorded, and the sway of the South Tower can be observed. A shortened version focuses on the sway and uses stationary red and green lines to highlight the motion resulting from the momentum transfer.  (Note: multiple cycles of sway in the mid-tower region can be seen in the entire Scott Meyers’ video.)


WTC Sway Zoom Composite withLineandBoxes 275px 865a2Figure 1: Sway of the South Tower for first 6 seconds shows ~20 inch movement (Click to Enlarge)Figure 1 shows a sequence of the tower’s sway with the images of the top of the South Tower spaced at approximately 0.75 second intervals. While the video provides a better way to observe the swaying motion, relative to the red and green reference lines, this sequence of still images provides a framework for analyzing that movement.

The images in Figure 1 have a stationary, straight vertical orange line (at a constant location relative to the stationary red and green reference lines). This orange line is shown to the left of a slightly curved yellow line. The curved yellow line follows the movement of a window opening with the width of the line approximately the width of a window opening. The window glass is approximately 22 inches wide and fills most of the 26 inches between the perimeter columns (perimeter columns are located every 40 inches. Because the columns are 14 inches wide, this leaves a 26 inch opening for the window and frame.)

The difference between the straight orange line and the curved yellow line shows the amount of sway over time of the sequence of frames. From this analysis, it takes approximately three seconds for the building to travel from its pre-impact, resting location to the point of maximum sway. This is consistent with the analysis that was performed and documented by NIST. Figure 2 is a graph provided by NIST that analyzes the sway that they documented at the 70th floor (NOTE: the NIST image was modified from the published version to focus on the first 40 seconds). Figure 2 shows that an entire period (cycle) of the swaying motion would take approximately 11 seconds. This is consistent with the three second period of sway (approximately) from the initial resting position to the maximum sway of the tower to the north shown in Figure 1.

NIST WTC Figure 2 15 PerimeterColumnSway v2 275px 1f74bFigure 2: Period (resonant frequency) of South Tower sway is approximately 11 seconds (Click to Enlarge).The amount of sway observed in Figure 1 is approximately the width of the line – or approximately 22 inches. This is consistent with the NIST estimate of movement at the top of the tower. According to NIST, the amount of deflection at the top of the tower was approximately 2 feet. According to the NIST NCSTAR:

The maximum deflection at the top of the tower was estimated to be more than 1/3 of the drift resulting from the original design wind loads (about 65 inches in the N-S direction) [NIST NCSTAR1-2, WTC Investigation, p 29]

The observed sway – most of the width of a window – in Figure 1 is consistent with the 1/3 of the wind induced drift (e.g., 22 inches is approximately 1/3 of 65 inches).  Because the period of sway is approximately 11 seconds this means that the maximum displacement of the Tower would occur at approximately 3 seconds after impact.  Due to the similarity of the North and South Towers, the period of sway would be the same for both towers.

North Tower Impact – Transfer of Momentum

Staehle Image 01 275px 72c5eFigure 3A: Pre-impact image by Wolfgang Staehle (Click to enlarge).There are only two video recordings of the North Tower impact. The Naudet Brothers video is the most well known and provides the most detailed view of the impact.  The second video is a time-lapse study of lower Manhattan with one frame taken every four seconds. This time lapse study was intended as a work of art by Wolfgang Staehle.

A few months after 9/11, the publication Vanity Fair obtained rights to publish three of the images. None of the others have been released into the public domain and this image sequence is largely unknown. The artist has stated his disappointment that, of his entire project, people are most interested in the moments covering the events of 9/11.  The 9/11 Memorial Museum is the only place where additional frames are available for public display.  However, these are not available for close-up public inspection.

 Staehle Image 02 275px ea876Figure 3B: Post-impact image by Wolfgang Staehle (Click to enlarge).

Recently, in September 2018, the video was put on display at the Brooklyn Historical Society.

Staehle installed a pair of cameras on top of a building in Williamsburg in early September of 2001, pointed them towards Manhattan, and set them to take a picture every four seconds for the next several weeks, along with other cameras clicking away in Berlin and in the bucolic German countryside. Staehle thought that his art piece would show the banality of daily life at the beginning of the 21st century, said one of the organizers of the event.

Staehle Image 03 275px 8fcaaFigure 3C: Last available post-impact image by Wolfgang Staehle (Click to enlarge).

Because the three frames available from Vanity Fair have a sufficient amount of detail of the Twin Towers – and they were taken from a solid stationary mount directly to the east of the WTC complex – they provide a unique side-view perspective on the plane impact. From these three frames, the sway of the North Tower due to the transfer of momentum can be observed.

Because the broadcast antenna on the top of the North Tower is approximately 8 feet wide at its base, and the antenna stands out clearly in the images, any significant motion can be observed.

Staehle Image 01 Antenna f4de5Figure 4A: Pre-impact antenna location

Figure 3A shows the pre-impact position of the tower as the plane approaches. The plane is observed probably(rough estimate) 1200 feet from the North Tower and would take approximately 1.5 seconds to travel to the impact point.  Figure 4A shows a close-up of the antenna.  Most of the antenna is seen to the right of the reference line and only a slight band of white is visible to the left.

Staehle Image 02 Antenna 75c65Figure 4B: Post-impact antenna location

Figure 3B shows the post impact image documenting the location of the North Tower four seconds later as a fireball begins to evolve. Assuming the value of 1.5 seconds before impact, the tower would be approximately 2.5 seconds into the sway. This would suggest that the Tower has not swayed to its maximum southward extent when the image was captured. The full southward sway would not be reached for another 0.5 to 1.0 second when the tower would start to sway back toward the center.  Figure 4B shows a close-up of the antenna at this point.  In this image, the antenna is seen with substantially more white to the left of the reference line – and less to the right of the reference line. While the difference between these two images is not dramatic, it is sufficient to substantiate the sway hypothesis. 

 Staehle Image 03 Antenna c5ae8Figure 4C: Last antenna location

Figure 3C shows the location of the North Tower another four seconds later. This third (and last) image captures the location of the Tower approximately 6.5 seconds after impact, which based on the observed periodicity of the South Tower’s sway, the North Tower would have swayed to the maximum southward extent and returned to approximately the location of the pre-impact position as it continued swaying toward the north. Because of the limitations posed by only having these three images to work with, the full sway cannot be observed and analyzed.  Figure 4C shows a close-up of the antenna in this last available position.  Again, a sliver of the antenna is seen to the left of the reference line and most of the antenna is seen to the right of the reference line.

Staehle Image Antenna 6b445Figure 5: Comparison of antenna locations


From the composite image shown in Figure 5, what is clear, is that the antenna has swayed to the south a quantifiable amount across the face of the antenna.  While the movement captured in the images is small, and the location in the oscillation cycle is not known with precision, there is no other explanation for this movement except the momentum transfer from the impacting plane to the tower.  

The following video reviews the three images from a variety of perspectives.

NIST Comment on Sway

According to NIST:

Both WTC1 and WTC2 were stable after the aircraft impact, standing for 102 minutes and 56 minutes, respectively. The global analyses with the structural impact damage showed that both towers has considerable reserve capacity. This was confirmed by analysis of the post-impact vibration of WTC2, the more severely damaged building, where the damaged tower oscillated with a peak amplitude that was between 30 and 40 percent of the sway under hurricane force winds for which the towers were designed and at a period nearly equal to the first two translation and torsion mode periods calculated for the undamaged structure. [NIST NCSTAR 1, p 148]

Explosives Could Not Create These Swaying Motions

Explosives would not be able to create this sway or, at a minimum, would have provided other tell-tale signs of very large explosive ejections of mass.

The primary reason explosives could not be responsible for the sway is: for an explosive forces to propel the South Tower northward, it would have been necessary for the explosions to forcefully propel a large amount of material in the southerly direction.  To be equivalent to the impacting plane – assuming the impacting plane that initiated the sway was 120 tons and collided at approximately 550 mph –  the mass of approximately 60 passenger cars would need to be "exploded out" at 550 mph to create the recoil necessary to sway the tower an equivalent amount (give-or-take a dozen or more cars).  No such projectiles were recorded in any of the videos that would be consistent with massive internal explosion(s) of sufficient magnitude to create the observed sway.

Secondly, for an explosion to have caused this movement, there would need to be a significant internal structure for the forces to have pushed against without destroying the resisting structure (resist the "recoil" effect). This is because the internal structure would need to be strong enough to resist the explosive forces in order to transfer the explosive forces to the rest of the structure—without destroying the structure in the process (e.g., bending, breaking or severing the internal columns).

Finally, explosions would have created simultaneous forces up through the floors above the impact area and down through the floors below the impact area. There are no videos that record damage in the form of exploded out windows above and below the impact area (excepting those that were in the path of the momentum based projectiles).

 Occupant Accounts of Tower Sway

Professionals who deal with human factors of building design related to emergency evacuation were interested in the reaction of the occupants to event initiation and their behavior during their exit from the building. Because of the difficulty in locating a statistically significant number of WTC survivors for first-person interviews and/or surveys responses an alternative approach was employed.  The selected method for investigating the evacuation relied on published accounts available in the public domain.

Through various means, published stories of World Trade Center (WTC) occupants were collected to document the evacuation process and provide the background material for a more detailed the National Institute of Standards and Technology (NIST) report on occupant behavior during the evacuation. These first-person accounts came from newspapers, radio and television programs, e-mail exchanges, and a variety of web sites. The accounts analyzed included survivors from several locations in each tower, providing a distribution of floors from the upper, middle, and lower floors of the two towers. In total, 745 accounts were analyzed representing 435 survivors from WTC 1 and WTC 2.  The results were documented in the NIST report "Analysis of Published Accounts of the World Trade Center Evacuation. Federal Building and Fire Safety Investigation of the World Trade Center Disaster (NIST NCSTAR 1-7A)"

This accounts were coded and then analyzed to obtain a statistical model of the evacuation experiences from survivors located on the different floors of the two towers.  Section 5.4 of the report was entitled First Cue Reported and stated that:

The first cues of the event that were mentioned in the accounts were found to differ depending on which tower the persons were located in. For WTC 1, the first building hit, the most common first cue of the event reported by 146 people (69% of people in that tower) was 'building movement,' such as feeling the building sway and tremble – many thought the building was going to tip over. WTC 2 occupants most commonly reported first becoming aware of the event from 'visual' cues (96 people) such as fire, debris and smoke, most likely coming from WTC 1. ...

 Nist 1 7A Table5 2 3ff11


A large number of "first cues" were related to the observation of office contents moving or noticing a jolt/sway motion.  These accounts corroborate the motion captured in video.


From this analysis, it is clear that a large mass impacted both the North and South Towers at the World Trade Center on 9/11. This building sway is conclusive proof that a large mass impacted the towers. For this movement to have occurred there would have needed to be a large sudden push to the north to make the South Tower sway – and a large sudden push to the south to make the North Tower sway.  No mechanism, other than plane impacts, is consistent with the observations.  Those who claim otherwise have no basis for their alternative assertions. 

One of the frequently stated reasons for disbelieving large planes impacted the Twin Towers is the belief that a plane could not have penetrated the exterior perimeter columns.  A high-level supplemental explanation of the damage to the exterior perimeter columns is presented as an appendix to this article.  The salient points of the explanation is that the perimeter columns were "bolted together" with only four 7/8" bolts.  It is at the location of these bolts that the perimeter column splices failed due to shear forces, breakage or pull-out.  With these modes of bolt failures, the perimeter columns are seen separated at the ends of the trident sections.  Significant bending of the steel perimeter columns can occur once a splice has failed.  The perimeter columns, themselves, do not appear to be severed.


This section provides supplementary material that augments the previous discussion.

Selected Statements by WTC Occupants About Sway

David Frank – ...The Quantum|ATL suite is located on the 78th floor of WTC #1 ... Our lives changed forever at 8:45 ... The explosion rocked the entire structure. Instantly, the windows just above us blew out and debris on fire tumbled past along with tens of thousands of 8 =BD" x 11" paper. I thought, "confetti". The "explosion" noise was bright, metallic and deafening. Simultaneously, the building groaned and leaned south dramatically in slow motion. Would the structure hold? I braced myself with my feet to keep from sliding. Then back we went in reverse. This time I braced with my hands. At this point, I don't remember if we went back and forth again, but I do remember that the swaying stopped…

Adam Mayblum – Adam Mayblum enjoyed the storms that rumbled off the Atlantic. As they lashed his windows and strafed the steel beams, Adam would scoff: You think that's power? I'm on the 87th floor of the World Trade Center. That's power. During the worst storms, the cords on his window shades would appear to sway a few inches, but it was an illusion. They actually hung straight, held steady by gravity. It was the tower that swayed, to absorb the weather.

When Adam felt the first rumble Tuesday morning, he glanced at the cords. They were oscillating like a pendulum, 3 feet in either direction [e.g., similar to Foucault's pendulum – as the building swayed they remained hanging straight down].

Kim White – Kim White, 32, an administrative assistant at Thebeast, a financial tech company on the 80th floor of 1 World Trade Center, was talking with an office temp when the first plane struck. All of a sudden the building shook, then it started to sway. We didn't know what was going on. I ran towards the reception area. It was completely collapsed ...

Richard Cruz – Richard Cruz was getting off the elevator on the 92nd floor of the south tower when the plane hit the north tower. "There was mass hysteria, people were screaming," said Cruz, 32. "I heard a lady's voice saying "Go back! Go back! There's been an explosion!' I smelled smoke and I saw a lot of paper flying like confetti." Cruz rushed to the stairwell along with other panicked co-workers from Aon Risk Services, an insurance brokerage company. At the 63rd floor, he decided to look out a window. [...] As Cruz rejoined the stream of people on the stairs, the second plane hit — his tower. "The whole building moved and it was swaying back and forth. I heard a muffled boom and I thought everything was just going to collapse. People were rushing and merging together and going crazy."

Newton's Third Law of Motion

The observed sway is an example of the principle known as the conservation of momentum –which is based on Newton's third law which states: In a collision between two objects, object A and object B, there is a force on A due to B (FAB). However, because of Newton's third law, there is an equal force in the opposite direction on B, due to A (FBA).

The sum of the forces is zero in a closed, lossless, system. This can be written mathematically as:

0 = FAB + FBA



Consequently, the impulse experienced by objects A and B must be equal in magnitude and opposite in direction. The forces act between the objects when they are in contact.

Additionally, for explosives to have moved one of the towers and caused it to sway, the conservation of momentum would have required a huge mass to be ejected in the direction opposite of the Tower sway.  

Aircraft Impact Damage to the Perimeter Columns

During the seventeenth anniversary many comments were made regarding the outline of the plane into the WTC Twin Towers suggesting that the plane could not have gone through the tower as observed.  Many commenters referred to the plane passing through the Twin Towers in such a manner as to resemble a "Wile E. Coyote" impact (from the Looney Tunes and Merrie Melodies Cartoon series).  These characterizations of the impact are incorrect. 

The observations, documented by NIST show that much of the observed damage to the exterior perimeter columns was in the regions of the center of the plane's mass (between, and including, the engines) impacted the building.  At the locations where the "light" ends of the wings (e.g., lower mass) impacted the Tower, it was observed that the aluminum cladding on the columns and windows suffered significant damage, however, the wings did not penetrate the exterior perimeter columns in these areas. 

The exterior perimeter columns were connected to each other, above and below, by four A325 7/8" bolts that passed through an end plate (at a "column splice"). Perimeter columns were only welded together at the mechanical floors. It is at the location of these bolts that the perimeter column splices failed due to shear forces, breakage or pull-out.  With these modes of bolt failures, the perimeter columns are seen mostly destroyed at the ends of the trident sections.  Significant bending of the steel perimeter columns can occur once a splice has failed.

Figure A1 shows the points in the South Tower where the perimeter columns separated as well as the less frequent points where the perimeter columns were destroyed mid-span. 

 NIST WTC Figure E 38a PerimeterColumnDamage 275px 927feFigure A1: Observed perimeter column damage to the South Tower showing breaks at location of bolted connections (Click to Enlarge).

Some of the columns shown in Figure A1 are damaged significantly and have been pushed into the building and, apparently, crushed the concrete floor slab (e.g., snow-plow effect).

A future article will discuss various points of failure in the exterior perimeter columns.

Related Events
No events

Additional Info


find your coordinator 8ac61

Subscriptions Banner

donation form banner

No events

BlueBar b5252