1. Mick West

    Mick West Administrator Staff Member

    A point sometimes raised in 9/11 discussion is that the the planes, and in particular UA175, were flying above their maximum "safe" speed, with the claim being made that they would immediately experience flutter, and hence be uncontrollable, and quickly break apart. The implication being that the planes that impacted the World Trade Center, the Pentagon, and the field at Shanksville, were not stock planes, but were planes that have been modified in some way, or were entirely different types of plane in disguise - possibly even not the same planes that took off that morning, but some other planes that were craftily swapped in at some point in the flight.

    While this seems like an incredible, even ridiculous theory, it's raised often enough that it's worth looking in detail into how flutter occurs, and at what speed, so we can see if this is in any way relevant to the events of 9/11.

    What is the Claim?

    The specific claim made by Pilots for 9/11 Truth is:
    (From "SkyGate 9/11")

    As we will see, this is incorrect, and there is a FAA certification margin of safety of 20% above the Vd limit.

    What is Flutter?

    Flutter is a resonance effect where, at certain speeds and conditions, oscillations of parts of the plane are amplified by the passing air, and grow in amplitude. This can lead to loss of control and damage to the plane. Severe flutter can rip the wings or stabilizers off a plane. Here's what it looks like in simulation:

    What is Vd (and Vne)?

    All planes have a set of "V-speeds" specified by the manufacturer, and certified by extensive testing and analysis. The V-speeds are either specific speeds that are optimal for a particular purpose (such as Vglide, the best speed for a power-off gliding descent) or are high or low limits beyond which a particular operation should not be attempted (such as Vlo - the maximum speed you can fly with landing gear lowered).

    For this discussion the most relevant V speeds are Vne (V Never Exceed) and Vd (V Dive). Vne is the speed that that pilot should never take the plane beyond. Vd is the speed beyond which structural problems are predicted to start to occur (or have been observed to start to occur, in which case it is denoted Vdf). Vne is less than Vd (usually around 90% of Vd), so for purposes of this discussion, Vd is the maximum "safe" structural speed of the aircraft.

    What is Vd and the observed impact speed for UA175?

    The fastest 9/11 plane, UA175, was a 767-200. Vd for a 767-200 is 420 KCAS below 17,854 feet, according to FAA certification data. (420 Knots Calibrated Air Speed, or 420*1.15 = 483 mph).
    The estimates of speed for UA175 range from 503 mph (visual analysis) to 590 mph (radar analysis).

    At what speed would flutter become a problem?

    Flutter is not something that simply happens at one particular speed. The onset and amplification of flutter depend upon a variety of factors, including the loading of the plane (in particular the fuel loading in the wings), and the turbulence of the atmosphere. Even then the flutter requires some initial oscillation (excitation) of the affected structure in order for it to start.

    NASA has a very detailed document on the history of flutter testing, in which they discuss the problems of getting sufficient excitation to induce flutter. Solutions to this problem included flying around looking for areas of high turbulence, attaching small rockets to the wings, or having rotating weights inside the wings.
    So the onset of flutter is determined by a combination of speed and excitation, not simply speed alone. However as speed increases, the ability of the plane to dampen any excitation decreases, so the amount of excitation required to induce flutter will decrease. However we can't say for sure that flutter will occur at a certain speed. Just that it is increasingly likely to occur as speed increases.

    At what speeds is Flutter NOT a problem?

    If we can't tell at what exact speed flutter will occur, then perhaps we can find a speed below which it will not occur. We know it should not occur below Vd, but the CAR 4b.308, proposed in 1964, says:
    And this was codified a FAR 25.629 in 1965:

    And essentially the same applies in the regulations that applied from 1978 to 1992:
    How does this apply to UA175?

    Vd for a 767-200 is 420 KCAS/483mph, so 1.2 Vd is 504 KCAS/580 mph.

    So, based on the above, we can say that a 767 should not experience flutter (or any loss of control) below 580 mph. And given that flutter requires additional excitation, and some initiation time, and that the very highest estimate for speed for US175 was 590mph, then it seems unlikely that flutter would have caused significant problems for the pilots of UA175, who were intent solely on crashing the plane into a building.

    Some additional clarifications follow:

    Does Vd vary with altitude?

    Yes, however the Vd limit of 420 Knots is the Calibrated Airspeed (essentially the same as the indicated airspeed) , which is the same as the true airspeed at sea level. The FAA certification document says:

    Note that Vd is a certification speed, not an actual operating limit, so it does not show up in the 767 flight manual. However the lower speed Vmo does:

    Source: http://www.scribd.com/doc/210896551/BOEING-767-Airplane-Flight-Manual


    closer in for clarity:


    Notice the Vmo is a constant indicated airspeed below 26,000 feet. This means that the Vmo of a 767 is 360 knots at sea level. The same applies to Vd, meaning Vd of a 767 is 420 knots at sea level.

    MPH vs. Knots, KTAS, KIAS, KCAS, and KEAS

    The multiple ways of indicating the speed of a plane area source of confusion here. Firstly there MPH vs. knots. We are all familiar with MPH, and knots is simply nautical miles per hours. A nautical mile is 1.15 standard miles. So 1 knot is 1.15 mph. So 420 Knots i 420*1.15 = 483 mph.

    Now with all the K...AS units, these are speeds measured in knots, however they are AIR SPEEDS, i.e the speed of the plane relative to the air it is moving though. If you imagine a plane flying through calm air at 300 knots, then it's moving over the ground at 300 knots. The ground speed is the same as the air speed. However if the plane is flying at the same airspeed into a 50 knot headwind, then while the airspeed is still 300 knots, the ground speed will only be 250 knots.

    The airspeed mentioned above is the "true" airspeed, KTAS (Knots True Air Speed). Unfortunately there is no simple way of measuring this from on the plane.

    KIAS is the Indicated Airspeed. This is the speed that the airspeed indicator will show. This is roughly accurate at sea level, but at higher altitudes will show a lower number due to reduced air pressure.

    KCAS is Calibrated Airspeed. This can be a little confusing, KIAS varies by plane, KCAS is essentially the KIAS for a hypothetical perfect airspeed indicator. So to convert KIAS to KCAS we are correcting for instrument error and variations in temperature. Here's an example conversion chart showing the corrections are fairly small.
    For most practical purposes here KIAS and KCAS can be considered to be the same.

    KEAS is the Equivalent Airspeed. Wikipedia describes it as "Equivalent airspeed (EAS) is the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed (TAS) and altitude at which the aircraft is flying.[1][2] In low-speed flight, it is the speed which would be shown by an airspeed indicator with zero error.[3] It is useful for predicting aircraft handling, aerodynamic loads, stalling etc."

    The important thing to remember in the context of this discussion (planes flying into buildings near sea level) is that all these values are essentially the same thing.


    Attached Files:

    Last edited: Aug 19, 2018
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  2. Pete Tar

    Pete Tar Moderator Staff Member

    If VD is dive speed , VNO never exceed speed, what's VMO and what's VG?

    Also the constant mixing of airspeed in miles per hour with knots tends to make threads on this subject a blur of incomprehensible specialised nitpicking. (I'm referring to the other thread, this one is quite clear.)
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  3. Mick West

    Mick West Administrator Staff Member

    i.e. the speed you should normally never go faster than in regular operations.

    VG is the glide speed. Engines off. Low speed. designed to maximize time in air and distance. Not relevant here.
  4. Pete Tar

    Pete Tar Moderator Staff Member

    So in the context of the title
    EasyJet 737 incident debunks Pilot for 9/11 truth VG diagram video
    it's a glide diagram?
  5. MikeC

    MikeC Closed Account

  6. Mick West

    Mick West Administrator Staff Member

    No, that's a V-G diagram. Velocity (speed) against G-force. Also sometimes called a V-n diagram. It shows the range of speeds and g-forces that are considered safe.

    I'm going to edit that tread title for clarity.
  7. jaydeehess

    jaydeehess Senior Member

    Ahhh, acronyms, is there anything they cannot do?
    No, IMHO.:D
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  8. jaydeehess

    jaydeehess Senior Member

    In a diagram/graph, the name would refer to each axis, whereas in the speed designation the second letter is a subscript and is an indication of the specific flight condition.
    Vd. As opposed to Vd
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  9. jaydeehess

    jaydeehess Senior Member

    Nice gif BTW in the OP
  10. BlindIdiots

    BlindIdiots Member

    False, the U.S does not use aircraft regulations created in 1964 in 2014

    The parts of CAR 4b.308 were incorporated into 14 CFR 25.629 (which is the same as FAR 25.629) in November 1964 http://lessonslearned.faa.gov/ll_main.cfm?TabID=4&LLID=7&LLTypeID=10

    What happened to CFR 25.629 then?

    It was reformed, and the parts that say


    Were removed, so that regulation does not apply anymore.

    You can view the current CFR 25.629 here : http://www.ecfr.gov/cgi-bin/text-idx?rgn=div5;node=14:

    As you can see, there is no longer any mention of the above.

    I would remove the "debunked" label from the title now if I were you, Mick.
    Last edited: Mar 27, 2014
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  11. WeedWhacker

    WeedWhacker Senior Member

    14 CFR 25.571(c) that was posted above. Excerpts:




    (iii) -- At speed VD the airframe must withstand wind gusts of up to 15 fps when below 20,000 feet. What kind of wind gusts and or turbulence was there on 9/11 affecting UAL 175?
  12. BlindIdiots

    BlindIdiots Member

    I was referring to the outdated regulation Mick posted.

    Wind gusts or Turbulence have nothing to do with flying at 500 knots at sea level.

    But I will debate 9/11 aircraft with you on pilotsfor911truth.org, stop being so afraid of that site and come here.
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  13. Mick West

    Mick West Administrator Staff Member

    So it's just 1.15 Vd, not 1.20?

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  14. WeedWhacker

    WeedWhacker Senior Member

    This thread is already specific (tangentially related) to the 9/11 events.

    And, the point that I made and that was ignored was exactly the fact that on 9/11 the air was particularly smooth. Therefore the claims made at the site "pilotsfor911truth" are moot. There is nothing to "debate".
    Last edited: Mar 27, 2014
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  15. Mick West

    Mick West Administrator Staff Member

    This thread is specific to flutter - with perhaps a general extension to aeroelastic stability. Please keep it to that.
  16. WeedWhacker

    WeedWhacker Senior Member

    The concept of flutter involves outside forces (such as wind gusts, etc) acting on an airframe in order to induce. Is this not covered in the OP?

    Am I mistaken here?

    (Color text altered by me)
  17. Mick West

    Mick West Administrator Staff Member

    Sorry, I was referring to the discussion of PF9 as being off topic.

    Any argument against the requirements of being flutter free up to 1.15 Vd? If not, then I'll update the OP. It changes the numbers from:

    Vd for a 767-200 is 420 KCAS/483mph, so 1.2 Vd is 504 KCAS/580 mph.


    Vd for a 767-200 is 420 KCAS/483mph, so 1.15 Vd is 483 KCAS/555 mph.
  18. jaydeehess

    jaydeehess Senior Member

    So the new regs would have applied to aircraft being designed/tested after the revision.
  19. Mick West

    Mick West Administrator Staff Member

    I think the 1964 regs that I quoted were proposed, not actual. They were in response to a crash:

    The 767 was first produced in 1981, according to Wikipedia's sources.
  20. jaydeehess

    jaydeehess Senior Member

    What I was getting at was that it seemed odd to retroactively apply new regulations to aircraft already in service, and that I have expected that to not be the case.
  21. WeedWhacker

    WeedWhacker Senior Member

    No. That seems to have been one of the pertinent changes in the CFR, relaxing the "1.2 VD" requirement. I'd venture to say that modern designs (i.e. the B757, B767) far exceed the "minimums" in regards to flutter onset probability. Personal experience tells me that they both have very robust airframes.
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  22. Mick West

    Mick West Administrator Staff Member

    Not too relevant in this case though, as it was either 1.2Vd, or 1.15Vd

    Seems like it was 1.2Vd in 1967 at least.
    http://books.google.com/books?id=eI...=0CC8Q6AEwAQ#v=onepage&q=25.629 1.2vd&f=false

    I'm not clear when it changed. But that would be good to know, for completeness.
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  23. Pete Tar

    Pete Tar Moderator Staff Member

    Why would you?
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  24. Mick West

    Mick West Administrator Staff Member

  25. WeedWhacker

    WeedWhacker Senior Member

    To clarify, an airplane's "VD" speed plus 20% (same as 1.2 X VD). Through 1990 means that when the B757/767 family of airplanes were in the design stages (circa late 1970s, as they were introduced in the early 1980s), there were sufficient reasons for the airframe manufacturer (Boeing) to indeed build a very robust product, and one that had a tendency to aerodynamic flutter designed out of it.

    Again, this disputes the various claims seen in myriad "9/11 conspiracy" sites.

    AND, to add....even if we only use a figure of 15% (as cited in recent CFR regulations)...this is minimally different. The same facts remain (the very strong and above "minimum" standards that are built into modern, circa 1980 and forward, Boeing designs).

    Coupled with the other evidences we've seen of earlier Boeing airframes that endured stresses far outside the flight test regime, and still survived?

    The evidence is conclusive to call the claim that the airplanes seen on 9/11 were "not" as already proven? (i.e. regularly scheduled airliners). Debunked.
    Last edited: Mar 27, 2014
  26. deirdre

    deirdre Moderator Staff Member

    plus everything ive read says AT LEAST 15%. Don't know if that matters.
    bold mine.
  27. WeedWhacker

    WeedWhacker Senior Member

    You are correct.
  28. Pete Tar

    Pete Tar Moderator Staff Member

    This is my favourite bit.

    From the link in
    which I was scanning in the vain hope that a discussion of the change to the relevant VD figures would magically leap out from the dense text.
  29. deirdre

    deirdre Moderator Staff Member

    ah. o_O
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  30. jaydeehess

    jaydeehess Senior Member

    Post #7 redux
  31. Mick West

    Mick West Administrator Staff Member

    The history of 25.629:
    http://rgl.faa.gov/Regulatory_and_G...enView&Start=1&Count=200&Expand=11.138#11.138 (http://archive.today/0H5Eo)
    http://rgl.faa.gov/Regulatory_and_G...9DD00A6A3FBA4A088525667200503326?OpenDocument (http://archive.today/B7qfJ)
    http://rgl.faa.gov/Regulatory_and_G...1AF764F921FAD01C862568E900643105?OpenDocument (http://archive.today/dwKQ4)
    http://rgl.faa.gov/Regulatory_and_G...12E1EB546266FC538525667200503A50?OpenDocument (http://archive.today/jQfRq)
    1992 (current regulations)
    http://rgl.faa.gov/Regulatory_and_G...91070D90B083322F8525667200503C26?OpenDocument (http://archive.today/qlp3K)
    Since the 767 began development in 1972 (production started in 1978, and the first 767 entered service in 1981), the initial design would have been done with the 1965 regulations in mind. And the regulations were only relaxed from 1.2Vd to 1.15Vd in 1992. Hence I feel the OP is still accurate.
    Last edited: Aug 19, 2018
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  32. WeedWhacker

    WeedWhacker Senior Member

    Great sourcing, and quite conclusive.
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  33. jaydeehess

    jaydeehess Senior Member

    Certainly lives up to the thread title.

    I would imagine that Rob Balsamo will soon be retracting his group's claims.:rolleyes:
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  34. deirdre

    deirdre Moderator Staff Member

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  35. Keith Beachy

    Keith Beachy Active Member

    Wind gusts and turbulence would have a lot to do with flying at 500 knots at sea level - a big reason why low level flights for large USAF aircraft were studied and limited because of airframe fatigue; a big reason Vmo is 350 for large aircraft low, yet we fly at 500 high.
    The Vg diagram pilot for truth have is fake, made up using graphs from other planes. A real Vg diagram is good at one weight, one altitude, and pilots for truth Vg diagram was made up by Balsamo to fool people who don't understand a Vg diagram.
    I flew over Vmo, the aircraft did fine. The only damage I have seen on a Boeing jet flying over Vmo, was torn skin on some Al honeycomb under the leading edge.
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  36. SpaceCowboy

    SpaceCowboy Rob Balsamo of P4T Banned


    Don't you find it curious that "MikeC" and "TWCobra" have not endorsed or agreed to what you have posted in this thread? And yet people like "weedwhacker" and "Keith Beachy" have?

    I'll give you a hint....

    "in equivalent airspeed at both constant Mach number and constant altitude"

    Anytime you wish to actually learn from real and verified pilots on the matter, instead of interpreting the FAR's based on your bias... feel free to make a post at the P4T forum and become educated.

    Hmmm... will you approve this post?
  37. Mick West

    Mick West Administrator Staff Member

    Perhaps you could just explain your objection, with numbers?
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  38. WeedWhacker

    WeedWhacker Senior Member

  39. Keith Beachy

    Keith Beachy Active Member

    You posted enough to debunk the pilot for truth nonsense. Balsamo posts the fake Vg diagram he photo-shopped. It has the wrong curves, no altitude, no gross weight, only BS. Boeing does not publish a speed of structural failure. You posted real definitions, the right stuff which aircraft are designed to. What Balsamo posts is hearsay, what some journalists makes up about Vd, and other quote mined nonsense to drum up business for the "offer no theory" wild claims to fools gullible kids and conspiracy theorists. How many more years will Balsamo use his fake Vg diagram.
    Your did research the pilot for truth audience is unable to comprehend. Funny seeing Balsamo's buddy make up nonsense about it.
    EA990 was 50 knots over Vd of 420 KCAS; Balsamo switches between KEAS and KCAS to make the numbers look better. It involves math, and usually 911 truth is not very good at math.
    Your research gets to the heart of high performance jets and airliner, whereas Balsamo has his fake Vg diagram. His generic Vg diagram hoax works with his audience pilots for truth, where Balsamo changes users' avatars and names to make weak personal attacks.
    You know you have the right stuff when Balsamo goes on the personal attack mode, and claims to have verified pilots at pilots for truth; Lear comes to mind, but he sees aliens, and knows we have based on the dark side of the moon - a pilot for truth expert, good for many a quote-mining bonanza of woo on 911 issues. Yes, we can all go to FAA, plug in the names and we see our type ratings, and find I have an ATP, and Balsamo does not. https://amsrvs.registry.faa.gov/airmeninquiry/
    Pilots for truth have less than 0.1 percent of all pilots supporting the fake Vg diagram claims of structural failure. I find it ironic the pilot for truth followers and leaders make note that some planes have failed inside the flight envelope, thinking full scale rudder reversal and other mishap problems are really in the envelope of normal flight. The Vg diagram is not about misuse of the controls or malfunctions, it is about aerodynamics, and what a plane can do.
    What does Boeing say about flight over Vmo?
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  40. Mick West

    Mick West Administrator Staff Member

    Arn't KCAS and KEAS essentially the same at sea level?