9/11: How hard is it to hit a building at 500mph?

[MikeC]

OK . . . so how does one find the breakup speed of a 767 or 757...

As has already been pointed out, Vne and Vmo are structural stress limits that aer designed to preserve teh airliner for decades of use.

they are not instant-disintigration-if-you-go-1-knot-over-the-limit speeds.


You know this already - why did you bring it up again?

 

[Rico]

Edit: Disregard

 

[George B]

As has already been pointed out, Vne and Vmo are structural stress limits that aer designed to preserve teh airliner for decades of use.

they are not instant-disintigration-if-you-go-1-knot-over-the-limit speeds.


You know this already - why did you bring it up again?

I placed the info there for reference . . . that is why I asked the question how would one find the speeds at which breakup would be expected?

OK . . . so how does one find the breakup speed of a 767 or 757...

 

[MikeC]

For reference - Silk Air 185 broke up over Indonesia in 1997.

the NTSB records its vertical speed in its report:



now if I'm reading that correctly that is a descent from roughly FL346 (34,600 ft) to FL 195 (19,500 ft) between 9:12:17 and 9:12:41 - 24 seconds to descend 15,000 feet - or about 37,500 feet per minute. Flight 175 dove about 24,000 feet in slightly more than 5 minutes - slightly less than 5000 feet per minute.

Later in the report the NTSB notes that teh speed of the a/c wouldhave reached greater than "1.2 Vd" - resulting in the empenage breaking off.

Vd is defined as "descent speed" and "analytical flutter clerance speed"

further analysis of the breakup and known "flutter" speeds gives 2 speed figures....hmm...it doesn't want to upload te picture - i'll see if I can get it in another post of via PM to mick for inclusion.

but the analysis says that ther are 2 flutter modes - 22 Hz at 5000 feet and 570 KEAS, and 12 Hz at 3000 ft and 600 KEAS - KEAS is "Knots equivalent airspeed", which you can find defined on Wiki:

KEAS is "knots equivalent airspeed", the calibrated airspeed corrected for adiabatic compressible flow for the particular altitude.

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And further on the Wiki page for Equivalent Airspeed we get:

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.

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So KEAS means the same aerodynamic load as the a/c would experience at that speed at sea level. Given that density is lower at higher altitude the actual airspeed - True Airspeed/TAS - is always higher than the EAS.

 

[MikeC]

Let's se if that picture will post:



good - so, my understanding is that the a/c must have been flying faster than those flutter speeds to break up at or below 18,000 feet - and given that those speeds are EAS, the True Airspeed must have been much higher.

 

[TWCobra]

George, the 250 knots limit below 10000 feet is a US ATC limit and nothing more. I have had a 767 at 350 knots below 5000 in less restrictive airspace with no problems.

 

[Rico]

I was going to say the same, but I think that red text transferred over when he quoted it from the document.

 

[George B]

Let's se if that picture will post:



good - so, my understanding is that the a/c must have been flying faster than those flutter speeds to break up at or below 18,000 feet - and given that those speeds are EAS, the True Airspeed must have been much higher.

570 and 600 Knots would be around 656 mph and 691 mph . . . and if those are EAS at 18,000 feet the speeds at sea level would they be lower not higher . . . .

 

[Jazzy]

270 knots is equal to 234.6237 miles per hour

No it isn't, George. And those decimal points are meaningless even when applied to the correct figure, which is 311 mph.

 

[George B]

You are assuming again - if ground effect had any influence on his dive that may well have been to make the dive shallower and give precisely the result that happened rather than hitting the ground earlier!

Stop inventing things.

I am not inventing anything . . . because one is thinking through a process is not an indictment . . . It is questioning the analysis . . . God forbid someone questions anything . . . If you don't like the question don't answer them!!!!!

 

[George B]

No it isn't, George. And those decimal points are meaningless even when applied to the correct figure, which is 311 mph.

Sorry the conversion equation on the net was incorrect or I put in the wrong numbers . . . what post was that in?..

 

[Jazzy]

570 and 600 Knots would be around 656 mph and 691 mph . . . and if those are EAS at 18,000 feet the speeds at sea level would be lower not higher . . . .

Manufacturers take such studies into account, which is why they design structures quite capable of dealing with loadings up to the onset of flutter, which ia around Mach 0.94 or 722 mph at sea level. They then instruct the operators to never exceed 70% of this.

This airplane survived passing through the sound barrier before encountering the ground. All the paper aboard survived the impact.

[video=youtube_share;nehx-FOCunU]http://youtu.be/nehx-FOCunU[/video]

 

[George B]

Manufacturers take such studies into account, which is why they design structures quite capable of dealing with loadings up to the onset of flutter, which ia around Mach 0.94 or 722 mph. They then instruct the operators to never exceed 70% of this.

This airplane survived passing through the sound barrier before encountering the ground. All the paper aboard survived the impact.]

So is that 722 mph at 20,000 feet or sea level? Speed of sound is 707 mph at 20,000 feet . . . http://www.fighter-planes.com/jetmach1.htm


British made aircraft. . . .you know they make a stronger aircraft. . . LoL!! The break up of an aircraft I am sure is a function of time, speed and altitude, etc. . . . straight down . . . Very short exposure to the speed of sound. . . . Not a good comparison Me thinks. . . .

 

[Rico]

Well during the last moments . . . would there not have been an altitude change cause by ground effect . . . a last moment rise in the trajectory . . . I am sure the pilot couldn't have reacted quick enough??

To answer your question, the answer is no.

Ground effect isn't a speed bump. In fact, one hardly even feels it in a course of flight. An airplane isn't just going to bounce back up because you take away some induced drag for a tad bit of lift, especially during the last couple seconds of flight in a dive. The effects of ground effect in this case is immaterial. Think about it, we are talking about an aircraft with six digits of weight in pounds doing a steep accelerated dive towards an object on the ground. At this point, it's flight path is mostly govered by gravity and engine power.

 

[MikeC]

570 and 600 Knots would be around 656 mph and 691 mph . . . and if those are EAS at 18,000 feet the speeds at sea level would they be lower not higher . . . .

If you do not understand the defintion you should ask about it, not make totally eroneous comments:

1/ EAS is how fast you would be going AT SEA LEVEL to generate the same aerodynamic load (ie drag).
2/ Air is less dense at higher altitude, and so imposes less load/drag at the same true airspeed (ie 600 kts at sea level gives higher drag than 600 kts at 18,000 feet)
3/ Therefore, the TAS required at altitude to generate a given EAS is ALWAYS HIGHER - because you have to go faster in the thinner air to get the same drag.

I hope that helps.

 

[George B]

If you do not understand the defintion you should ask about it, not make totally eroneous comments:

1/ EAS is how fast you would be going AT SEA LEVEL to generate the same aerodynamic load (ie drag).
2/ Air is less dense at higher altitude, and so imposes less load/drag at the same true airspeed (ie 600 kts at sea level gives higher drag than 600 kts at 18,000 feet)
3/ Therefore, the TAS required at altitude to generate a given EAS is ALWAYS HIGHER - because you have to go faster in the thinner air to get the same drag.

I hope that helps.

I think that is what I was trying to say . . . if someone quotes a TAS at 18,000 of 656 mph the equivalent air speed at sea level is less say 600 mph . . .

570 and 600 Knots would be around 656 mph and 691 mph . . . and if those are EAS (should have said TAS) at 18,000 feet the speeds at sea level would they be lower not higher . . . .

 

[MikeC]

I think that is what I was trying to say . . . if someone quotes a TAS at 18,000 of 656 mph the equivalent air speed at sea level is less say 600 mph . . .

Ah - OK - sorry for any confusion then.

Yes the equivalent airspeedat sea level wil be lower - the formulae is given on the wiki page for equivalent airspeed and isn't too difficult - you just need to know the air density at the 2 altitudes. you could probably find those figures for a standard day and do an indicative calculation of the difference.

 

[George B]

No it isn't, George. And those decimal points are meaningless even when applied to the correct figure, which is 311 mph.

235 knots is 270 mph . . . I simply reversed the numbers . . . I am sure you have done something like that before . . .

270 knots is equal to 234.6237 miles per hour

 

[Unregistered]

I am very skeptical of a 757 being able to achieve approximately 500 mph at 1,000 ft. In the Flight 1771 situation I would like to know how they determined it broke the sound barrier and even if it did it was in a dive-bomb. An 757 is designed to fly at around 500 mph max at approximately 30,000 ft. When you drop down to 1,000 ft you have 3x the amount of Drag because the air density is 3x greater at 1,000 ft than at 30,000 ft. Drag force is directly proportional to density. So now you have the engines having to push the aircraft through all that drag and generate 3X the amount of thrust as well as the control surfaces and structure holding-up. But then there is another problem with the engines... they are tuned to operate at max performance at certain altitudes and because you have 3X the mass flow rate coming in the engine then the bypass duct will have to dump the excess air, if it cannot then your compression ratios, air-to-fuel ratios etc. will be off. If this happens you are going to have some feedback issues between the compressor and the turbine. So, there are all sorts of issues with not only the Drag but also whether the engines could handle the excess air mass flow. The engine is likely to cut-off if you can't maintain the proper compression and expansion ratios.

I've only had one class on jet propulsion and haven't done the calculations on this example but we did do calculations on scenarios where altitudes were changed but nothing as drastic as trying to drop to 1,000 ft and maintain 500 mph+. I wanted to do this a few years back but never got around to it. As my focus is in aerodynamics I have a much bigger problem with the lack of the tip and bound vortex during impact.

P.S. I do not believe any 757 hit WTC. If anything I think they were probably something like the JASSM missile painted-up like planes. These claims of "millions of people saw the planes hit" are speculation and the amateur videos taken that day have been shown to be fakes. What I believe happened is that the CG images of a plane was over-lapped onto real (some suspect CG) footage of WTC and aired as LIVE broadcast. I do not accept holograms or remote control, etc. They simply hijacked the airwaves - all of the big news stations were airing the same footage and doing reports from the same location. So, all of the info stream was being centrally controlled.

 

[Rico]

On Flight 1771, they found a badly damaged Flight Data Recorder. The FDR used hardened steel, and they measured the compression on the remains of the FDR casing. Based on this, they measured that the impact of 1771 came close to 5,000 g's.

As for the 757 issue, it didn't start out at level flight. Flight 77 more or less dove towards the pentagon. Flight 175's (Boeing 767) flight path indicated a relatively steep descent too (as per radar returns from their transponder), and there is little reason to believe that Flight 11 was any different. Gravity plays a big role.

 

[SR1419]

the amateur videos taken that day have been shown to be fakes.

Can you please elaborate?

 

[MikeC]

I am very skeptical of a 757 being able to achieve approximately 500 mph at 1,000 ft. In the Flight 1771 situation I would like to know how they determined it broke the sound barrier and even if it did it was in a dive-bomb. An 757 is designed to fly at around 500 mph max at approximately 30,000 ft. When you drop down to 1,000 ft you have 3x the amount of Drag because the air density is 3x greater at 1,000 ft than at 30,000 ft. Drag force is directly proportional to density. So now you have the engines having to push the aircraft through all that drag and generate 3X the amount of thrust as well as the control surfaces and structure holding-up.

and engines generate a great deal more thrust from that denser air in order to accelerate teh aircraft and get it to climb too.

I recall as a youngster being told that 747-200 engines generated only about 8000lbs thrust at cruise altitude viz 30-35,000 at sea level which is where it is measured.

Aboe youhaveexamples of the accident reports for aircraft that did and did not disintigrate in the air - 4500mph at sea level is not enough to generate the flutter that is the ACTUAL destructive mechanism - not drag.

Her you go - Basic Gas Turbine engine principles:

The performance of the gas turbine engine is dependent on the mass of air entering the engine. At a constant speed, the compressor pumps a constant volume of air into
the engine with no regard for air mass or density. If the density of the air decreases, the same volume of air will contain less mass, so less power is produced. If air density increases, power output also increases as the air mass flow increases for the same volume of air.

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But then there is another problem with the engines... they are tuned to operate at max performance at certain altitudes and because you have 3X the mass flow rate coming in the engine then the bypass duct will have to dump the excess air, if it cannot then your compression ratios, air-to-fuel ratios etc. will be off. If this happens you are going to have some feedback issues between the compressor and the turbine. So, there are all sorts of issues with not only the Drag but also whether the engines could handle the excess air mass flow. The engine is likely to cut-off if you can't maintain the proper compression and expansion ratios. [/quoet]

Arrant nonsense. Turbine engines generate a constant compression ratio and it is volumetric - you compress a volume of air regardless of its density. Designers are perfectly aware of drag and the like, and "compression and expansion ratios" are determined by the physical size of the air path and the power being generated by the compressors.

whoever you copied this from has no idea how a jet engine works, and you shuoldlnot repeat "information" you do not understand.

I've only had one class on jet propulsion....

You could at least have paid attention in it then!

.....and haven't done the calculations on this example but we did do calculations on scenarios where altitudes were changed but nothing as drastic as trying to drop to 1,000 ft and maintain 500 mph+. I wanted to do this a few years back but never got around to it.

Did it ever occur to you in your scenario design that you could dive to 1000 feet to increase speed?

As my focus is in aerodynamics I have a much bigger problem with the lack of the tip and bound vortex during impact.

more meaningless drivel.

P.S. I do not believe any 757 hit WTC.

that would probably be because both aircraft were Boeing 767's -why am I not surprised by your error??!!

If anything I think they were probably something like the JASSM missile painted-up like planes. These claims of "millions of people saw the planes hit" are speculation and the amateur videos taken that day have been shown to be fakes.

ther are over 100 amateur videos - can you provide some actual evidence to support your claim that they are all fakes?

What I believe happened is that the CG images of a plane was over-lapped onto real (some suspect CG) footage of WTC and aired as LIVE broadcast. I do not accept holograms or remote control, etc. They simply hijacked the airwaves - all of the big news stations were airing the same footage and doing reports from the same location. So, all of the info stream was being centrally controlled.

and how about some actual evidene of "central control" of the media, and also of the eye witnesses on the ground, the pilots in the air that were nearly hit by the errant aircraft, the air traffic controlelrs who monitored its transpoonder, and the survivors of the dead passengers and crew?

 

[MikeC]

I am very skeptical of a 757 being able to achieve approximately 500 mph at 1,000 ft. In the Flight 1771 situation I would like to know how they determined it broke the sound barrier and even if it did it was in a dive-bomb.

Yes - exactly - the hijacker put it into a dive from 22,000 feet.

That is why it went fast enough to disintigrate. That is why 500 mph is NOT fast ernough to disintigrate!

An 757 is designed to fly at around 500 mph max at approximately 30,000 ft. When you drop down to 1,000 ft you have 3x the amount of Drag because the air density is 3x greater at 1,000 ft than at 30,000 ft. Drag force is directly proportional to density. So now you have the engines having to push the aircraft through all that drag and generate 3X the amount of thrust as well as the control surfaces and structure holding-up.

and engines generate a great deal more thrust from that denser air in order to accelerate teh aircraft and get it to climb too.

I recall as a youngster being told that 747-200 engines generated only about 8000lbs thrust at cruise altitude viz 30-35,000 at sea level which is where it is measured.

Above you have examples of the accident reports for aircraft that did and did not disintigrate in the air - 450 mph at sea level is not enough to generate the flutter that is the ACTUAL destructive mechanism - not drag.

Here you go - Basic Gas Turbine engine principles:

The performance of the gas turbine engine is dependent on the mass of air entering the engine. At a constant speed, the compressor pumps a constant volume of air into
the engine with no regard for air mass or density. If the density of the air decreases, the same volume of air will contain less mass, so less power is produced. If air density increases, power output also increases as the air mass flow increases for the same volume of air.

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But then there is another problem with the engines... they are tuned to operate at max performance at certain altitudes and because you have 3X the mass flow rate coming in the engine then the bypass duct will have to dump the excess air, if it cannot then your compression ratios, air-to-fuel ratios etc. will be off. If this happens you are going to have some feedback issues between the compressor and the turbine. So, there are all sorts of issues with not only the Drag but also whether the engines could handle the excess air mass flow. The engine is likely to cut-off if you can't maintain the proper compression and expansion ratios.

Arrant nonsense. Turbine engines generate a constant compression ratio and it is volumetric - you compress a volume of air regardless of its density. Designers are perfectly aware of drag and the like, and "compression and expansion ratios" are determined by the physical size of the air path and the power being generated by the compressors.

whoever you copied this from has no idea how a jet engine works, and you shuoldlnot repeat "information" you do not understand.

I've only had one class on jet propulsion....

You could at least have paid attention in it then!

.....and haven't done the calculations on this example but we did do calculations on scenarios where altitudes were changed but nothing as drastic as trying to drop to 1,000 ft and maintain 500 mph+. I wanted to do this a few years back but never got around to it.

Did it ever occur to you in your scenario design that you could dive to 1000 feet to increase speed?

As my focus is in aerodynamics I have a much bigger problem with the lack of the tip and bound vortex during impact.

more meaningless drivel.

P.S. I do not believe any 757 hit WTC.

that would probably be because both aircraft were Boeing 767's -why am I not surprised by your error??!!

If anything I think they were probably something like the JASSM missile painted-up like planes. These claims of "millions of people saw the planes hit" are speculation and the amateur videos taken that day have been shown to be fakes.

ther are over 100 amateur videos - can you provide some actual evidence to support your claim that they are all fakes?

What I believe happened is that the CG images of a plane was over-lapped onto real (some suspect CG) footage of WTC and aired as LIVE broadcast. I do not accept holograms or remote control, etc. They simply hijacked the airwaves - all of the big news stations were airing the same footage and doing reports from the same location. So, all of the info stream was being centrally controlled.

and how about some actual evidene of "central control" of the media, and also of the eye witnesses on the ground, the pilots in the air that were nearly hit by the errant aircraft, the air traffic controlelrs who monitored its transpoonder, and the survivors of the dead passengers and crew?

 

[George B]

So is anyone willing to introduce the survey to a pilot Forum as I said I don' t know the lingo?

I think we need some assistance from real pilots on this task . . . I don't talk the lingo and would be most suspect by the site managers. . . . I don't mind doing the grunt work but I need a proper introduction, etc. . . .

Flight 77. . . Pentagon source


http://en.m.wikipedia.org/wiki/American_Airlines_Flight_77


NTSB sources


http://www.documentingreality.com/f...t-175-video-data-impact-speed-study-ua175.pdf


http://pilotsfor911truth.org/p4t/Radar_Data_Impact_Speed_Study--AA11,_UA175.pdf


Flight experience source


http://www.911myths.com/index.php/Flight_School_Dropouts

Q) Could pilots with a commercial licencse have hit the the World Trade Center and the Pentagon with a 767 or 757 at the speeds observed on 9/11?(the pilots were assumed to have earned a commercial license within the previous year) (Tower #1 = 473 - 510 Knots Groundspeed, Tower#2 = 430 Knots Groundspeed, Pentagon = 460 Knots Groundspeed)

1) Yes, it would be fairly straightforward
2) Yes, but it would have needed a lot of concentration
3) Yes, but it would have been very difficult
4) Yes (other)
5) Probably, but they were pretty lucky
6) Probably not, it would be very hard to do
7) No, the plane would be impossible to control at that speed
8) No, the plane would have fallen apart at that speed
9) No, the plane could never even reach that speed.
10) No (other)
11) I don't know

 

[Met Watch]

Flight 77. . . Pentagon source


http://en.m.wikipedia.org/wiki/American_Airlines_Flight_77


NTSB sources


http://www.documentingreality.com/f...t-175-video-data-impact-speed-study-ua175.pdf


http://pilotsfor911truth.org/p4t/Radar_Data_Impact_Speed_Study--AA11,_UA175.pdf


Flight experience source


http://www.911myths.com/index.php/Flight_School_Dropouts

So is anyone willing to introduce the survey to a pilot Forum as I said I don' t know the lingo?

You were the one that came up with the survey idea and now you want one of us to post it to an airline forum? C'mon George, it's really not hard. Just go find a forum dedicated to professional pilots, post the poll, and just watch the results come in. There is no extra explaining needed or anything.

 

[George B]

POLL: Could pilots with a commercial licencse have hit the the World Trade Center and the Pentagon with a 767 or 757 at the sp

7) No 27.0% (53)

6) Probably not, it would be very hard to do 14.3% (28)

2) Yes, it would be fairly straightforward 12.2% (24)

8) No, the plane would be impossible to control at that speed 10.7% (21)

1) Yes 9.2% (18)

11) I don't know 8.7% (17)

9) No, the plane would have fallen apart at that speed 6.1% (12)

10) No, the plane could never even reach that speed. 4.6% (9)

4) Yes, but it would have been very difficult 4.1% (8)

3) Yes, but it would have needed a lot of concentration 2.0% (4)

5) Probably, but they were pretty lucky 1.0% (2)

Blank (View Results)(45)


Non-Blank Votes: 196

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[George B]

You were the one that came up with the survey idea and now you want one of us to post it to an airline forum? C'mon George, it's really not hard. Just go find a forum dedicated to professional pilots, post the poll, and just watch the results come in. There is no extra explaining needed or anything.

Which Forum do you recommend??

 

[Met Watch]

Which Forum do you recommend??

http://www.airliners.net/aviation-forums/

I'd recommend them.

 

[Unregistered]

Arrant nonsense. Turbine engines generate a constant compression ratio and it is volumetric - you compress a volume of air regardless of its density. Designers are perfectly aware of drag and the like, and "compression and expansion ratios" are determined by the physical size of the air path and the power being generated by the compressors.

whoever you copied this from has no idea how a jet engine works, and you should not repeat "information" you do not understand.

No, they don't compress regardless of density. When you are dealing with higher density air it becomes increasingly difficult to compress. You will have to dump the extra mass somehow. You've have the same Area, same Velocity and different Density so your mass-flow rate is higher - 3x higher. Everything is interdependent and they are designed to have maximum performance at their cruise altitude. You can't just take the engines down to 700 or 1000 ft and have them perform at 3x the thrust levels. However is claiming that doesn't understand how jet engines work.

I didn't copy and paste I have a Masters in Aerospace Engineering and I have taken jet propulsion courses, rocket propulsion, etc.

Did it ever occur to you in your scenario design that you could dive to 1000 feet to increase speed?

The aircraft could dive but it would only gain performance benefits while in the dive. What we have from videos do not show a steep dive. As the aircraft encounters the increased air density it will slow down and diminish any speed increase. But I was simply addressing the fact that the engines could not push the aircraft to those speeds and since we see a relatively shallow approach it is unlikely that much advantage was due to gravity.

that would probably be because both aircraft were Boeing 767's -why am I not surprised by your error??!!

767, my mistake.

 

[George B]

No, they don't compress regardless of density. When you are dealing with higher density air it becomes increasingly difficult to compress. You will have to dump the extra mass somehow. You've have the same Area, same Velocity and different Density so your mass-flow rate is higher - 3x higher. Everything is interdependent and they are designed to have maximum performance at their cruise altitude. You can't just take the engines down to 700 or 1000 ft and have them perform at 3x the thrust levels. However is claiming that doesn't understand how jet engines work.

I didn't copy and paste I have a Masters in Aerospace Engineering and I have taken jet propulsion courses, rocket propulsion, etc.


The aircraft could dive but it would only gain performance benefits while in the dive. What we have from videos do not show a steep dive. As the aircraft encounters the increased air density it will slow down and diminish any speed increase. But I was simply addressing the fact that the engines could not push the aircraft to those speeds and since we see a relatively shallow approach it is unlikely that much advantage was due to gravity.


767, my mistake.

Why don't you register and join the debate . . . with your credentials this could be fun!!!!

 

[George B]

Interesting comments . . .

I'm a pilot, commercial, with 1000's of hours in jet, air carrier aircraft.


These aircraft were apparantly fully operational with the autoflight and FMS systems working. If you have a working knowledge of how to program the FMS and also couple it to the autopilot, then yes, the WTC would be fairly easy to hit.


The Pentagon is another story. If they knew how to build a circling approach with a VNAV descent, then that could also be possible. But you gotta know what you're doing. Especially at that speed.
http://www.godlikeproductions.com/forum1/message2141354/pg5#lastpost

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[MikeC]

No, they don't compress regardless of density. When you are dealing with higher density air it becomes increasingly difficult to compress.

Gasses have bucketloads of "room to compress" - it is not an issue unelss you get to much, much higher densities than ar present on earth.

You will have to dump the extra mass somehow.

No you do not - the more airmass passes through the engine the more thrust you produce. Older engines used to have surge valves to handle unexpected pressures that might threaten their design limits, but newer engines have done away with those through superior design.

You've have the same Area, same Velocity and different Density so your mass-flow rate is higher - 3x higher. Everything is interdependent and they are designed to have maximum performance at their cruise altitude.

You cannot possibly claim that and also claim any expertise in the field - jet engines get their maximum performance - thust - at low altitude - not at cruise altitude!!

You can't just take the engines down to 700 or 1000 ft and have them perform at 3x the thrust levels. However is claiming that doesn't understand how jet engines work.

yes, jet engines DO generate 3-4-5 times as much thrust at sea level as they do at high altitude - here is somone who has done the caluculations for your on a 747-200 engine:

747-287 - Engine P&W JT9D-7Q
xxx
Maximum rated thrust sea level = 52,700 lbs
TSFC thrust specific fuel consumption = .378
Fuel flow at takeoff = 19,915 lbs (to get 52,700 lbs of thrust)
xxx
So if in cruise, FF is 4,000 lbs, thrust is about 10,500 lbs

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And some more figures from further down the thread:

So, cruise thrust in percent of rated

B752 18,0%
B773 14,7%
MD80 21,4%
B744 14,9%
B763 15,0%
A319 19,6%
A340 16,8%

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These aircraft can get as much as 7 times as much thrust at sea level as at cruise altitude - depending on conditions of course.

I didn't copy and paste I have a Masters in Aerospace Engineering and I have taken jet propulsion courses, rocket propulsion, etc.

then you really have no excuse for writing such nonsense and/or should ask for a refund of your coure fees!

The aircraft could dive but it would only gain performance benefits while in the dive. What we have from videos do not show a steep dive. As the aircraft encounters the increased air density it will slow down and diminish any speed increase.

do you know what the physical limit for speed for a 767 actually is?? I'll give you a hint - the Vne of a 767 is 590 mph.......that is the speed the pilots are not allowed to exceed. Do you think, just possibly, that the aircraft might be able to fly faster than that??

But I was simply addressing the fact that the engines could not push the aircraft to those speeds and since we see a relatively shallow approach it is unlikely that much advantage was due to gravity.

Flight 175 dove at almost 5000 fpm for 5 minutes before striking the WTC - that is not a "relatively shallow approach"!

You say you study aerodynamics, then you should know that the a/c in cruise configuration, with all its gear and flaps up, is very "slick" - it has low drag. Maintaining 500 mph for a little more time after that would be no trouble at all.


Here is MIT's analysis of the impact velocities based upon geometry of observations at the time.

 

[Rico]

Perhaps the word dive is misleading here, so let's use steep descent. The aircraft would maintain some degree of performance benefit while it continues a steady descent. Speed doesn't just bleed off immediately when one levels a plane either.

Check the mode C returns here: http://www.ntsb.gov/doclib/foia/9_11/Flight_Path_Study_UA175.pdf

Using that information, the aircraft was steadily "flying" towards the ground. Well, skyscraper sticking out of the ground to be precise.

 

[George B]

http://www.airliners.net/aviation-forums/

I'd recommend them.

I took a quick look . . . I couldn't tell if polls were allowed or possible . . . in their paid membership details I didn't notice a poll capability?? Are you a member and know if polls are possible??

This is the only reference to a poll I found . . .

SE-KGV (cn SH3670) After loading up all my Shorts commuter aircraft pictures on airliners.net, I remember there was a poll about two years ago on this site about the most popular commuter aircraft type. As far as I remember the Shorts 330/360 made it to the last rank in this competition. Don't have any idea why! I voted for Shorts!

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[Rico]

Interesting comments . . .

I'm a pilot, commercial, with 1000's of hours in jet, air carrier aircraft.


These aircraft were apparantly fully operational with the autoflight and FMS systems working. If you have a working knowledge of how to program the FMS and also couple it to the autopilot, then yes, the WTC would be fairly easy to hit.


The Pentagon is another story. If they knew how to build a circling approach with a VNAV descent, then that could also be possible. But you gotta know what you're doing. Especially at that speed.
http://www.godlikeproductions.com/forum1/message2141354/pg5#lastpost

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In a sense, that is correct. Long+lat coordinates can be entered into the FMS, and the aircraft are capable of lateral and vertical navigation.

But I have my reservations because quite frankly, the descent profiles of both flight 77 and 175 don't match that of automation. Despite hitting their targets, it was sloppy flying.

As for the last line, I don't really think they put much care to building a VNAV descent profile. You've seen the FDR video. They lined up, then they descended towards the pentagon. Control inputs include pitch and trim down while keeping their target on their windscreen. Small corrections to control roll. It's not a baffling concept.

Edit: Also, to my knowledge, the autopilot doesn't command anything more than about 30 degrees angle of bank. United 175 hit it's tower at 38 degrees angle of bank. I can see them using the autopilot to get to the general area of their target, but more than likely they flew it the rest of the way--that would be prudent.

 

[George B]

In a sense, that is correct. Long+lat coordinates can be entered into the FMS, and the aircraft are capable of lateral and vertical navigation.

But I have my reservations because quite frankly, the descent profiles of both flight 77 and 175 don't match that of automation. Despite hitting their targets, it was sloppy flying.

As for the last line, I don't really think they put much care to building a VNAV descent profile. You've seen the FDR video. They lined up, then they descended towards the pentagon. Control inputs include pitch and trim down while keeping their target on their windscreen. Small corrections to control roll. It's not a baffling concept.

Edit: Also, to my knowledge, the autopilot doesn't command anything more than about 30 degrees angle of bank. United 175 hit it's tower at 38 degrees angle of bank. I can see them using the autopilot to get to the general area of their target, but more than likely they flew it the rest of the way--that would be prudent.

So because of the 38 degree bank you feel a standard auto pilot could not have been engaged on U175?? Thanks . . . that is new to me. . .

 

[TWCobra]

That is correct..... Both the amount of roll and the rate of application is above the capabilities of the 767 auto-flight system.

 

[Jazzy]

I am very skeptical of a 757 being able to achieve approximately 500 mph at 1,000 ft.

I think you should argue with the radar people. Several sets of radars independently plotted the 767s at that overland speed, as they were diving. The last two points indicated 565 mph, which you can work out for yourself from plot positions and timings. This is within the error range of the system, and not the true speed at that instant, of course, which was over 500 mph.

In the Flight 1771 situation I would like to know how they determined it broke the sound barrier

The flight data recorder showed it - but probably off-scale. The cockpit voice recorder recorded increasing wind noise only. If you watched the vid you would see they calculated the deceleration the plane endured during impact (-5000G!), for which they must first have determined its speed. Given the crater depth, you can work back from that.

even if it did it was in a dive-bomb

So? I thought your point was about whether airplanes can remain intact at lower speeds. The answer is "yes".

The engine is likely to cut-off if you can't maintain the proper compression and expansion ratios.

According to witness reports the engines were described as "flat out", so it didn't appear to happen.

I have a much bigger problem with the lack of the tip and bound vortex during impact.

The second plane was pulling nearly 2G as it struck, increasing its wake and tip vortex strengths as it did so. Wake and tip vortices show clearly on a couple of the vids. Part of the fuel explosion backtracked a tip vortex, as it would, and braided wake vortices are plainly visible in the smoke and dust close to the face of a tower - for a brief moment.

[video=youtube_share;eI8u-I0GWs4]http://youtu.be/eI8u-I0GWs4[/video]

The final few frames are really impressive.

all of the info stream was being centrally controlled.

There were more than a single feed at that time, I believe.

You should consider how difficult placing all that "evidence" would have been if those impacts hadn't taken place. All those explosions, fragments, body parts, impacts down on the streets?

How many people would have to slip through crowds to start fires, place shafts, wheels, parts? How did they ignite two 24,000 lbs fuel explosions up the towers? Where was that placed?

How long would it have taken to carry out? How many people would have been needed?

The impacts needed none of that manpower. Moreover, each impact left a plane-shaped hole which could only have been made by impact (column ends bent inward). Too silly...

 

[George B]

That is correct..... Both the amount of roll and the rate of application is above the capabilities of the 767 auto-flight system.

I assume this is an example of a safety feature. . . .turning over the aircraft automatically for the pilot to intervene . . . Not unlike an automobile's cruise control disengaging when you put on your break . . . or is it a logic and mechanical limitation of the computer - aircraft interface?

 

[TWCobra]

The autopilots on airliners are designed to give Aunt Martha in seat 46b a smooth ride. They are generally smoother in operation, particularly with the application of lateral G forces, than the average pilot.
If you want to take evasive action, such as a TCAS climb or descent, the first memory item is "autopilot -disengage" because the AP simply does not react quickly enough.