JohnDoeX: "I'm a real boy!"

jaydeehess said:
I am also told that a 757 cannot exceed VMO, that the computer will not allow it. Anyone know if the computer will counter pilot commands and reduce power as Vmo is exceeded?
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Welcome jaydeehess:)

You were misinformed. The 757/767 isn't fly-by-wire; there are no hard or soft limits for bank angle/AoA/VS/speed/etc... Even with the airplane on autopilot, if the crew decides to conduct a fast descent past Vmo, using autothrottle - the airplane won't correct it. Autopilot or none, if a pilot exceeds Vmo - all that will happen is a red "OVERSPEED" warning message will appear on the upper engine display, an aural warning will sound(the typical Boeing EICAS "beep-beep-beep-beep")and the overspeed clacker will sound. Its entirely up to the crew to correct the overspeed.
 
apathoid said:
Welcome jaydeehess:)

You were misinformed. The 757/767 isn't fly-by-wire; there are no hard or soft limits for bank angle/AoA/VS/speed/etc... Even with the airplane on autopilot, if the crew decides to conduct a fast descent past Vmo, using autothrottle - the airplane won't correct it. Autopilot or none, if a pilot exceeds Vmo - all that will happen is a red "OVERSPEED" warning message will appear on the upper engine display, an aural warning will sound(the typical Boeing EICAS "beep-beep-beep-beep")and the overspeed clacker will sound. Its entirely up to the crew to correct the overspeed.
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Now ya see, after some research concerning 757's I came to that conclusion and told the CT in question that and my resaoning for saying so including citing Boeing pages to back it up. He would not believe it AND stated that had flight 77 (what we were discussing at the time) had exceeded Vmo it would have suffered airframe damage. That ellicited a loud guffaw from me after which I pointed out that the plane did not exceed Vmo until it was only a few seconds from impact(30 or so IIRC). I then asked , over and over again, how long it would take for a 757 to start breaking up if Vmo was exceeded, imediatly, 30 seconds, 1 minute etc. Never did get an answer but one thing is for sure as far as I am concerned, it would suffer no damage in the few seconds it was over Vmo that would even be discernable by the pilot and that would be far outshadowed by the damage that would occur as a result of hitting a big friggin concrete wall at the end of that short time period.:)

But thanks for confirming what this non-pilot had concluded.
 
jaydeehess said:
but one thing is for sure as far as I am concerned, it would suffer no damage in the few seconds it was over Vmo that would even be discernable by the pilot and that would be far outshadowed by the damage that would occur as a result of hitting a big friggin concrete wall at the end of that short time period.:)

But thanks for confirming what this non-pilot had concluded.
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Perhaps he thinks when an aircraft suffers structural damage it immediately and spontaneously explodes in all directions with violent force.

-Gumboot
 
jaydeehess said:
But thanks for confirming what this non-pilot had concluded.
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No problem.

About Vmo, and I'm no expert at all on this BTW(I'm not a pilot), its not a "magic" number, like stall speed, for airframe stress. I'd think you could fly all the way across the Pacific at 1-2% above Vmo and not have any problems(other than an annoying clacker). The speed at which the airframe starts to buffet or flutter is a good deal higher than max operating speed. How much higher? I'd have to research, but I'd say 5% higher or more than Vmo.

Where is Billzilla when you need him?:)
 
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Anti-sophist said:
If anyone cares, the G forces are recorded in the FDR data, and none of it is even remotely abnormal. I can look it up later but I think the vertical G never gets about about 1.7 or 1.8. The latteral is even less interesting.
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I had calculated averages given the radius of the turn, max speed, elapsed time in the turn, height through which it desended. Come pretty close to what you state above, I think I had 1.9 g vert. I don't post at that forum anymore. It had one or two 9/11 threads and I had been pointed to it because a poster in another forum had used it as a reference.

CT's discount the DFDR data stating that it could easily have been faked but they do agree that the aircraft did a 270 to 330 degree turn through 7000 feet vert in 2.5 minutes and that the max speed of the a/c was 400 -500 MPH.

One can then calc the radius of the turn for 400 MPH and for 500 MPH and thus calc the centrepetal force on the plane.
One can also calc the average desent rate of 47 fps and calculate the vert g's involved if one pulls out of that desent over a time of 2 seconds, 5 seconds etc.

One calc'd in this way the CT has to find a way by which the parameters could have been in error but these numbers are used by CT sites all over the place so that gets difficult for them.
 
jaydeehess said:
CT's discount the DFDR data stating that it could easily have been faked but they do agree that the aircraft did a 270 to 330 degree turn through 7000 feet vert in 2.5 minutes and that the max speed of the a/c was 400 -500 MPH.
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One of the most odd things about the AA77 CTs...

In the early days it was all about the "270 degree turn". OR 300 or whatever they were currently claiming it was. As if being able to turn 270 degrees was a phenomenal achievement only a fighter jet could achieve.

One of the most ridiculous things I had ever heard.

What use would a vehicle of ANY sort be if it was incapable of turning around?

What would you do? Have giant rotating runways, line the plane up on its destination, and let her rip?

What happens if you miss ur approach for whatever reason?

"Sorry folks, we missed the runway, we'll have to continue on to the next airfield we're lined up on, which is the Number Three at Cairo International. ETA is 10 hours"

:confused:

-Gumboot
 
Having been told that Flight 77 had pulled a 5 g turn I responded with:



530 mph =240 meters/sec

Centrepital force
F=(1/r)mv^2

if the g force on this plane was 5gs then
(1/r)v^2 = 49meter/sec^2
r = (1/49)(240)^2
r=1175 meters or a radius of about 3/4 of a mile.

In order to do a 330 degree turn in a radius of 3/4 miles it would do so in 30 seconds.

Of course this is the first I have heard of it turning 330 degrees. Instead all the sites I have seen have it turning 270 degrees. 270 at 5g's would take 25 seconds.

Did it take that short a time??

Not according to billstclair.com/911timeline/main/flight77.html
Quote:
(9:33 - 9:38 a.m.) Radar data shows Flight 77 crossing the Capitol Beltway and headed toward the Pentagon. But the plane, flying more than 400 mph, is too high when it nears the Pentagon at 9:35, crossing the Pentagon at about 7,000 feet up. [CBS News, 9/21/01, Boston Globe, 11/23/01] The plane then makes a difficult high-speed descending turn. It makes a "downward spiral, turning almost a complete circle and dropping the last 7,000 feet in two-and-a-half minutes.


2 1/2 minutes!!! That's 150 seconds
What's more here we have the plane at 400mph not 530.


Giving your source the benefit of the doubt on the velocity and heading change, let's say it still is 530 mph though and let's say it still is 330 degrees of heading change.

530 mph for 150 seconds means it travels 22 miles.

(360/330)22 = circumference = 24 miles
r=3.8 miles = 6000 meters
(v^2)/r = (240^2)/6000
9.6 meters/sec^2
That's 0.98 g


Very uncomfortable, very scary for the normal passenger but less than one fifth your 5 g accelleration!!!!!!!!!!!!!!!!!

Make it slower(400mph) and/or less change of heading(270 degrees) and that g force drops even more!!!!!!!!!!!!!!!


At 500 MPH that drops to 0.86 g
At 400 MPH its only 0.43 g
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Just out of curiosity, and as comparison, how many Gs would an airliner pull on a typical steep departure?

-Gumboot
 
gumboot said:
One of the most odd things about the AA77 CTs...

In the early days it was all about the "270 degree turn". OR 300 or whatever they were currently claiming it was. As if being able to turn 270 degrees was a phenomenal achievement only a fighter jet could achieve.

One of the most ridiculous things I had ever heard.

What use would a vehicle of ANY sort be if it was incapable of turning around?

What would you do? Have giant rotating runways, line the plane up on its destination, and let her rip?

What happens if you miss ur approach for whatever reason?

"Sorry folks, we missed the runway, we'll have to continue on to the next airfield we're lined up on, which is the Number Three at Cairo International. ETA is 10 hours"

:confused:

-Gumboot
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I dont think our CT friends were amazed by the airplanes ability to turn around as much as they were amazed that an amateur commerically-rated pilot was able to peform the turn........equally braindead IMO.
 
In the early days it was all about the "270 degree turn". OR 300 or whatever they were currently claiming it was. As if being able to turn 270 degrees was a phenomenal achievement only a fighter jet could achieve
Click to expand...

This was one of the first things that glared out at me when CT's started talking about AA77 so I did the calcs and could not believe that CT's were not bothering to do the math and still make the claims.

I assumed they knew one could turn a plane and keep turning it forever ( fuel constrained of course) if you wished to.
What they really wanted was big numbers to scare the illterate who would see 270 degrees and go "whoa, that's a big number".
 
gumboot said:
Just out of curiosity, and as comparison, how many Gs would an airliner pull on a typical steep departure?
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Well, you could do a vertical departure, like an F16, and hardly pull more than 1G. At a constant rate of ascent, no matter how steep, it's just that 1G due to gravity. The G forces come when the rate of descent/ascent changes.
 
I belive that a 2G vert-acc during a turn or take-off is about the upper limit of what commercial planes do. In practice, that is. Not by design, etc.
 
Perhaps someone should point out that the "standard rate turn" used for IFR approaches is two minutes: that is, two minutes to do a 360 degree turn.
So doing a 270, or even a 330 degree turn in two and a half minutes is not a particularly quick turn.
Admittedly, you'd generally be making the approach at less than 400 kts.
 
Anti-sophist said:
I belive that a 2G vert-acc during a turn or take-off is about the upper limit of what commercial planes do. In practice, that is. Not by design, etc.
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A two-G turn would be a 60 degree bank angle. That bank angle would probably scare the bejeezus out of the passengers. I've had power pilots up for rides in the glider that were noticeably uncomfortable with a 45 degree bank. (Which you just have to do sometimes, to stay in some of the small thermal cores.)

If jaydeehess' calculations are right, then the flight 77 turn would have been made at about a 45 degree bank angle, and in coordinated flight, that would be about 1.4 G, which is consistent with his 1 G horizontal acceleration. 1G horizontal and 1G vertical adds to 1.4G along the diagonal. Having done about a bajillion 360 degree turns at 45 to 60 degrees in thermals, I can tell you that it's not hard, it's not uncomfortable, and if you don't have a suicidal idiot at the controls, it's not dangerous.
 
Anti-sophist said:
I belive that a 2G vert-acc during a turn or take-off is about the upper limit of what commercial planes do. In practice, that is. Not by design, etc.
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Thanks. So at most the G's experienced in the turn are comparable to the experience of a rapid departure in an airliner (all be it in a different direction).

Hardly throw-about-plane stuff.

My best friend was in an Air NZ 737 that hit wind shear across the Southern Alps and dropped many thousand feet in a very very small space of time. That threw people into the air and broke bones. A rather more severe aircraft movement than AA77 experienced prior to impact. And of course the 737 didn't disintegrate into pieces.. ;)

-Gumboot
 
I looked up the FDR data and here's what I got:

Max Vertical G: 1.753
Max Lateral G: 0.094
Max Long. G: 0.363

Max/Min Roll : -19, 41.8 (degrees)

The "turn" was spent mostly between 25 and 35 degrees, and between 1.1 and 1.4 Gs. Only for brief moments (on the order of a second or two) does the G force poke above 1.4G.

This doesn't seem to me to be all that scary.

It starts around 9:34:03 and ends around 9:36:52. That's from scrolling through the data waiting for the roll to back to "normal".

--
ETA

I think the reason for the G forces to come out lower than back-of-the-envelope calculation is because the plane was losing altitude. This relaxes the need for lateral g forces altogether.
 
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Anti-sophist said:
I think the reason for the G forces to come out lower than back-of-the-envelope calculation is because the plane was losing altitude. This relaxes the need for lateral g forces altogether.
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Why? A plane in a constant descent would have the same forces as a plane flying straight and level.

My back-of-the-bar-napkin figures, with a 169-second turn and a speed of 600 ft/s (this is around 400 mph - is that about right?) yield a horizontal acceleration of around 22.5 ft/s2, or 0.7 g. Combined with the downward force of gravity, this would be an average of 1.22 g during that turn.

Oh- are you trying to explain why the FDR's lateral g number is only 0.094, and not the 0.7 number I get? It's because lateral g's in the plane are not the same as horizontal g's, because the plane banks in turns. You'd expect the lateral g number to stay pretty close to zero, and would be exactly zero in a perfectly coordinated turn.
 
I apologize if this has already been posted, but everytime I see the new Flight Simulater X commercial, I can't help but think of JDX.

For those that haven't seen it, go here.
 
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