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")
[bunk]Vd is a hard limit, meaning there is no additional margin of safety above Vd[/bunk]
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:
http://stanton.ice.put.poznan.pl/flutterlaboratory/
What is Vd (and Vne)?
All planes have a set of "Vspeeds" specified by the manufacturer, and certified by extensive testing and analysis. The Vspeeds are either specific speeds that are optimal for a particular purpose (such as Vglide, the best speed for a poweroff 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 767200. Vd for a 767200 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.
http://www.nasausa.de/centers/dryden/pdf/88390main_H2077.pdf
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:
http://lessonslearned.faa.gov/ElectraWings/4b.308.pdf
And this was codified a FAR 25.629 in 1965:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/9DD00A6A3FBA4A088525667200503326?OpenDocument
And essentially the same applies in the regulations that applied from 1978 to 1992:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/1AF764F921FAD01C862568E900643105?OpenDocument
How does this apply to UA175?
Vd for a 767200 is 420 KCAS/483mph, so 1.2 Vd is 504 KCAS/580 mph.
http://en.wikipedia.org/wiki/United_Airlines_Flight_175
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/BOEING767AirplaneFlightManual
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 lowspeed 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.
http://en.wikipedia.org/wiki/Knot_(unit)
http://en.wikipedia.org/wiki/Airspeed_indicator
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")
[bunk]Vd is a hard limit, meaning there is no additional margin of safety above Vd[/bunk]
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:
http://stanton.ice.put.poznan.pl/flutterlaboratory/
What is Vd (and Vne)?
All planes have a set of "Vspeeds" specified by the manufacturer, and certified by extensive testing and analysis. The Vspeeds are either specific speeds that are optimal for a particular purpose (such as Vglide, the best speed for a poweroff 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 767200. Vd for a 767200 is 420 KCAS below 17,854 feet, according to FAA certification data. (420 Knots Calibrated Air Speed, or 420*1.15 = 483 mph).
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.
http://www.nasausa.de/centers/dryden/pdf/88390main_H2077.pdf
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:
http://lessonslearned.faa.gov/ElectraWings/4b.308.pdf
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/9DD00A6A3FBA4A088525667200503326?OpenDocument
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/1AF764F921FAD01C862568E900643105?OpenDocument
Vd for a 767200 is 420 KCAS/483mph, so 1.2 Vd is 504 KCAS/580 mph.
http://en.wikipedia.org/wiki/United_Airlines_Flight_175
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:
Source: http://www.scribd.com/doc/210896551/BOEING767AirplaneFlightManual
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 lowspeed 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.
http://en.wikipedia.org/wiki/Knot_(unit)
http://en.wikipedia.org/wiki/Airspeed_indicator
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