For the purpose of this speed analysis I have assumed that the UA175 aircraft was travelling at a height of approximately 1000 feet at the time of impact and that the air temperature was approximately 20 degrees Celsius at that altitude. Under these environmental conditions the speed of sound (Mach 1.0) would occur approximately at 767 mph or 666 knots. Here is a list of UA175 speeds issued from official bodies that were presumably calculated using video footage of the WTC2 strike:
Massachusetts Institute of Technology - 503 mph / 436 knots / M0.653
British Royal Air Force - 575 mph / 500 knots / M0.750
Federal Aviation Administration - 586 mph / 510 knots / M0.765
National Institute of Standards and Technology - 546 mph / 475 knots / M0.714
Federal Emergency Management Agency - 590 mph / 514 knots / M0.771
As you can see there is a difference in speeds from one UA175 video to another, which does not make sense because the UA175 aircraft could have only been travelling at one speed prior to its collision with WTC2.
Not entirely happy with these official speed figures from MIT, RAF, FAA, NIST and FEMA I decided to calculate the approximate speed of the UA175 aircraft myself using 3 different video sources in an attempt to clear up the speed inconsistencies demonstrated by the official calculations and hopefully to identify one universal speed.
To calculate the approximate speed of the UA175 aircraft from any given video I simply cloned together two images of the UA175 aircraft at a known time interval from the same video footage and then measured the distance travelled between the two UA175 aircraft clones using the WTC2 towers east wall as a horizontal reference for measuring distances. With the time frame known and the distance travelled calculated it is possible to obtain a reasonably accurate speed for the UA175 aircraft for each different video.
All media used for my calculations came directly from good quality NTSC DVD sources. The results were as follows:
Park Foreman Video - 540 mph / 470 knots / M0.705
Evan Fairbanks Video - 565 mph / 490 knots / M0.735
"Unknown Video" - 492 mph / 428 knots / M0.642
As with the official calculations there is quite a variation in speeds obtained from one video source to another according to my analysis, but we can comfortably group together the RAF, FEMA, FAA, Park Foreman Video and the Evan Fairbanks video figures to produce a speed range between 560 mph and 590 mph.
The variation in speed from the differing video sources could easily be due to small errors in the calculations by each respective party and as the calculated speeds in this group are quite close, this could be one possible explanation.
However, the calculation performed by Eduardo Kausel (below) at MIT and my own calculation using the Unknown Video are well outside this 560 mph to 590 mph speed bracket, yet Kausel claims to have used various publicly available video recordings and states that the velocities for the two WTC planes were in excellent agreement with the flight data based on RADAR information provided by the NTSC.
How can MIT's Eduardo Kausel be producing a UA175 speed that are at such odds with FEMA, RAF, FAA, NIST and my own set of calculations?
Moreover Eduardo Kausel draws our attention to another important point...
"...the above data indicates that the terrorists flew towards the WTC close to the ground at nearly the full cruising speed of the planes, which is about 900 km/h (560 mph) at a normal altitude of 10km (33,000 feet). It is surprising that the inexperienced pilots that the terrorist were could still steer the planes at those speeds and hit their target head on. Also, considering that the air at low altitudes is much denser than that at normal cruising height, the pilots greatly exceeded Vne ("Velocity Never Exceed") and thereby risked disintegration of the aircraft by air friction."
I spoke to a former Boeing 767-200 Captain about the aerodynamic limitations of the Boeing 767-200 aircraft and he stated that it would be unwise to exceed an indicated airspeed of 400 knots (460 mph at sea level) at any altitude.
As mentioned before, the airspeed of 400 knots at sea level is well outside the maximum operating speed of the Boeing 767-200 and therefore the pilots would run the risk of either total structural failure or localised structural failures, namely wing fairings breaking off, engine cowlings breaking off, control surfaces breaking off or becoming inoperative and handling difficulties. None of these structural failures and handling issues would be of any benefit to the crew, so why did the alleged hijackers chose to operate their usurped aircraft at such a dangerously high airspeed such that it could have prematurely terminated their mission through complete structural failure, or made it unnecessarily difficult or even impossible to execute through partial structural failure and aircraft handling difficulties?
The high speed approach to the WTC2 target as seen in the visual records of the event would have created a vast flight deck work load at a critical point in the mission, while making navigation to the WTC complex unnecessarily difficult for the hijackers who were not renowned for their skills and experience operating large passenger aircraft like the Boeing 767-200! When you consider the fact that the Boeing 767-200's airframe would have smashed to pieces on the towers façade during impact under any speed condition you have to wonder what the alleged hijackers had to gain by choosing this high speed approach and high speed collision.
I was also informed by the former Boeing 767-200 Captain that there would NOT be any adverse effects on aircraft handling or performance as a result of nearing or achieving transonic airspeeds.
The only conclusion that can be drawn from this UA175 airspeed analysis is that the video recordings are all giving us different speeds because the UA175 aircraft was actually travelling at a different speed in each video!
The fact that all the videos apparently show a structurally intact Boeing 767-200 in controlled flight prior to its collision with WTC2 travelling at such a ridiculously high airspeed is another indicator that whatever the
UA175 aircraft was, it was not a production model Boeing 767-200, or it was simply something that has been added to the video recording in post production either to conceal what the video recording originally showed, or to add something to the recording that should have been there, but was in fact absent from the original recording.
We should consider the possibility that there is more than one person or group responsible for the forgery and / or manipulation of any given UA175 strike video and that there is seemingly a lack of accordance between these groups or individuals which could account for the speed inconsistencies demonstrated by the videographic record.
www.911research.dsl.pipex.com/ggua175/speed/ 
