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Wednesday, December 23, 2015

Observed Deceleration of UA 175 during the Impact at the South Tower

A Rendez-vous with Physics

Written by 9/11 Truth Outreach Staff

UA 175 figure 1Figure 1: UA 175 enters visual range of Evan Fairbanks’ camera south of the South Tower
UA 175 figure 2Figure 2: UA 175 traveling unobstructed prior to impact
UA 175 figure 3Figure 3: UA 175 at about point of impact

One of the more distracting discussions about the destruction of the Twin Towers is that, despite the large number of eyewitnesses and video recordings of the planes hitting the Twin Towers, some people insist that no aircraft actually impacted the structures, and that all the videos of the event – from television networks to private individuals – were faked. While speculation may be part of the process in formulating a hypothesis, speculation cannot answer the question, “what physically happened.” To answer that question, analysis of the available evidence is needed.

The South Tower was hit by United Airlines Flight 175 or (as some speculate) another plane swapped for Flight 175. The current analysis does not distinguish between these two possibilities, so for the purposes of this article we will simply call the impacting aircraft “UA 175.” What we will demonstrate here is that the tail of UA 175 decelerated upon impact with the South Tower.

Observing the deceleration of the aircraft during impact is what one would expect of a real plane, but this observation would be unlikely if the footage were faked. This simple observation supports the authenticity of the video and confirms the presence of an actual aircraft with substantial mass.

Newton's third law of motion states: interacting objects exert equal but opposite forces on each other. Upon impact, equal forces will be exerted on the plane and the tower at the point of impact. With the initial momentum of the plane in the northward direction, we would expect the South Tower to accelerate and sway northwards. Conversely, the South Tower will tend to decelerate the aircraft. Because the aircraft takes time to move through its own length as it enters the building – and energy is expended destroying both the structure of the building and the airframe – the tail section would be expected to decelerate some amount before it reaches the face of the building. This slowing is due to the force of impact transmitted through the structure of the plane.

Deceleration of an aircraft’s tail due to internally transmitted forces is observed every time we see an aircraft land at an airport – the plane brakes to a stop as a single structural entity without the momentum of the tail continuing to push itself through the rest of the plane into the cockpit. The Internal air-frame structure transmits decelerating forces from the braking forces of the wheels and the braking forces of the engines to decelerate the tail – of course much more gently than upon impact with a structure like the Twin Towers.

While aircraft are not designed to withstand a substantial jolt upon impact, the inherent structure is designed to connect all parts of the aircraft and will not cease to perform this function in the event of a catastrophic event. The many survivors of plane crashes are a testament to the structural integrity of the airframe during a catastrophic landing.

UA 175 figure 4Figure 4: Tail of UA 175 begins to diverge from constant-speed red-outline enveloper
UA 175 figure 5Figure 5: Tail of UA 175 continues to diverge from constant-speed red-outline envelope
UA 175 figure 6Figure 6: Just before disappearing, the tail of UA 175 has slowed considerably from constant-speed red-outline enveloper

In a video analysis with sufficient clarity, the impact can be analyzed and the deceleration can be observed. Fortunately there is one video with sufficient clarity to allow analysis of the UA 175 impact that provides the results discussed here. This analysis is based on the Evan Fairbanks video which has the best resolution for analysis. The margin of error for measurements in other videos, such as the one by Scott Myers, does not allow the deceleration to be measured with confidence (that is, the measurements are within the margin of error). The positions of the cameras are sufficiently distant that the parallax effect (effect of different lines of sight) can be safely disregarded.

Visual Analysis by Achimspok

The visual analysis presented here was performed by Achimspok and is available on his youtube channel in its entirety. His video shows the analysis at various speeds – but with little explanation. His analysis superimposes a red-outline envelope around the aircraft which is moved at a constant speed as determined by speed of the aircraft as it approaches World Trade Center Building 2 (the South Tower) before impacting the building. During and after the impact, the red-outline envelope, which is overlaid on top of the video, continues to be moved at this constant speed. If the tail of the aircraft decelerates, it will not keep pace with the constant speed red-outline envelope superimposed on the video.

Viewing the sequence from Figure 1 through Figure 6, the motion of the tail is seen to diverge from the constant-speed red-outline envelope as it decelerates.

  • Figures 1 and 2 shows that the tail is within the constant-speed red-outline envelope.
  • Figure 3 shows the very beginning of the impact and the tail is contained within the red-outline envelope.
  • Figure 4 shows the tail has traveled a shorter distance than the red-outline envelope indicating that it has decelerated compared to the constant-speed red-outline envelope.
  • Figure 5 shows the tail is clearly outside the constant-speed red-outline envelope, thus illustrating a significant deceleration compared to the red-outline envelope. The word ‘significant,’ as used here, means a deceleration greater than the margin of error.
  • Figure 6 shows the maximum observable displacement. The tail is shifted outside the red-outline envelope by approximately the length of the top of the tail.

Achimspok did not provide a numerical estimate of the deceleration.

Numerical Analysis by Eric Salter

UA 175 figure 7Figure 7: Salter's final frame showing a constant-speed white-outline envelope ahead of the decelerated tail section.

Another analysis performed to measure the deceleration was done by Eric Salter a professional video editor. Salter’s analysis was performed to quantify the loss in kinetic energy as the plane encountered the South Tower. In his analysis, he initially calculated that the tail decelerated by 18 percent from the beginning of the visual observation through the end of the available observation periods.

Using his measurement technique, the tail was shown to decelerate from 612 pixels per second to 502 pixels per second. After receiving feedback about the alignment in the last frame, Salter re-estimated the final speed to be 553 pixels per second, which implied a 10 percent deceleration.Salter’s analysis clearly indicates a deceleration greater than the margin of error and in fact, shows a significant deceleration.

Figure 7 shows an overlaid image that Salter developed that is similar to Achimspok’s figure 6. Similarly, figure 7 shows the tail section lagging behind the constant-speed white-outline envelope.


This analysis does not address whether the plane was UA 175. It does, however, show that there was an observable deceleration of the aircraft, which supports the hypothesis that the aircraft impact is authentic. Furthermore, the authenticity of the Evan Fairbanks video will be supported in future articles that describe the video evidence of the acceleration and sway of the South Tower in a northward direction and a review of survivor testimonies confirming the sway. These analyses should satisfy critics that there was an actual aircraft impact with substantial mass.

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