Why Flight 93?

This is why the AOA is interesting, and if you have a good AOA you can use it for neat things instead of looking up the darn numbers. I always think of the AOA as a source of DATA and I could skip looking in the big book of graphs to find max endurance, just fly it on the AOA, or max range, just fly it on the AOA. Take off for the KC-135 was tied to the AOA through the Collins ADI and helped us reach the correct attitude for takeoff. It could be used for max lift on approach, and thus the correct approach airspeed. There was a AOA readout to show us on speed for approach.

The AOA would not be zero at cruise speed. I am not even sure if the reading on the AOA probe is linear. Who is the real aero guy around here. The KC-135 AOA was normalized and we had a read out in the cockpit .2 was about max range, .4 endurance, .6 approach speed.

The AOA on 93 was reading -16 for level cruise, and -6 for takeoff. I think it was -20 near impact. But those are not angles, those are readings from the AOA. The readout was decoded to a scale of 5 to –25, take off was –6, and cruise (bet it was max range) –16. I think we need a table or charts to decode the raw numbers to angles and/or normalized readings us pilots need to decide to push or pull, Please another banana please.

If I did not say earlier, many pilots critiqued my using the AOA for max range, they said it was not very accurate at that end of the scale. But it was very close, and I used it to check my speed, I wanted to fly .81 mach anyway, it was close to 99 percent max range, and on the fast side.

I think if you take the readings and know how the instruments work you can figure out angles or normalized readings. Our KC aoa was not an angle, just normalized with marks for approach speed, endurance, and max range. Oh, the big stall was in the red zone.
 
Engineers always make things like this too complicated. Crips, AOA is merely the angle between the chord line of the wing and the relative wind or slipstream. The readout in the cockpit or FDR is probably different for different aircraft. I'm mostly from the old school same as rwguinn with a vane on the nose somewhere providing the indications to the cockpit. Although I have flown early versions of a glass cockpit, the AOA indicator was an analog gauge in degrees. On some aircraft there is a light indicator that shows on speed for an approach. I've never used one like that and I also don't remember the T-38 having an AOA indicator. I flew it a few years before Beachnut, so there may have been a modification.

I've primarily only used it to determine close proximity to a stall condition during high G maneuvering. For example, in the F-111 prior to a later flight control modifications the aircraft would enter a post stall gyration (from which no one has ever recovered) at about 21 degrees AOA. I do recall checking it on approach to ensure the proper speed/wing sweep for fuel weight was in the ball park. I never used it for anything else.

It is only remotely related to pitch attitude. For example, you can be flying straight and level and enter a stall condition (high AOA) by simply "yanking" (ape like) on the control stick very rapidly. How much the pitch attitude changes, of course, depends on airspeed. It would be in an upward direction, but an unknown amount. The reverse condition would apply inverted.

The cockpit readout or the FDR input should not differ if the aircraft is upright or inverted.

I don't understand how the AOA would tell us much of anything regarding UA93. Well, it would indicate trends, but it seems to me pitch would tell the primary story (and we know it was pointed downward toward the ground). It would tell us nothing about the roll axis.

I suspect that the inertial system in a B-757 could provide AOA, but whether it's inertial or vane we'd still need to know the meaning of the numbers specific to that aircraft before it would tell us anything useful.

Inertial systems are supposed to be reliable in all flight conditions, but they aren't. They could certainly tumble in a condition such as UA 93 experienced which was well outside the normal flight envelope for that aircraft.

I hope this helps to understand it better, but I do not believe it's important in discussing anything at all about UA 93.
 
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Reheat said:
To what age do you expect to live? :D
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I guess a 7606 grad may not make it.

You 111 guys fly way too fast. I think one of the birds (111) out of depot in SAC made a dump light north of SAC, the space guys saw it as a thermal event. They called me, asking me about our beale air space north of beale one day when I was chief of current ops. I do not think I have ever seen a dump, and I did a lot of refuelings. We did have an f-4 roll around us, but I did not see that as a great event in my 300,000 pound tricycle, the boom loved it. A barrel roll around the tanker. That call was strange.
 
Yea, that's known as "Torching". The fuel dump outlet was between the two tail pipes, so dumping fuel in conjunction with lighting the afterburners would ignite that stream of fuel and creating a very long stream of flame behind the aircraft.

One evening in England a friend inadvertently "Tourched" during a weather recall. He was low level and heard the recall, pulled up to altitude, and dumped with the afterburners going. The weather conditions were such that it created an eery glow over East Anglia. There were numerous calls to Police Agencies throughout that part of England, some as far away as London reporting a UFO!:jaw-dropp

Needless to say, he received "counseling" over that incident....
 
I would have been in trouble a lot in a fighter, if I lived long enough. A lot of tankers booms have had the show.
 
rwguinn said:
I'm not quite following here...
Let us assume a coordinate system, such that
+ X axis (FS) is aft.
+Z axis (WL) is up, i.e., vertical tail is +WL (Z axis).
+Y (+BL) is then out the left wing.
These references do not change, regardless of aircraft orientation.
The vane (fairly typical--test aircraft used 4 holes in a ball-end probe) has as its reference the axis of the tube it is mounted on (or aircraft x-axis)
Thus, a nose-up attitude (+AoA) would indicate a + rotation about the Y axis, regardless of the aircraft attitude in 3D space, right? Or am I totally hosed, here?
Then we can get into pitch attitude...
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OK, now we are getting somewhere. The AoA is the angle between the wings chord line and the direction of air flow. The wing chord line is fixed relative to the X axis (i.e. it is a fixed wing aircraft). Flaps may change this somewhat. The air flow is always 180 degrees from what I have called the velocity vector. I should qualify this by saying it is the velocity relative to the air, not the ground. Thus the AoA is also relative to the velocity.

Beachnut, what AoA is relative to and how it is measured are two different things, so don't hold your breath assuming I will go looking for a velocity vector instrument or sensing device.
 
Gregory, are you trying to say that you were able to determine the exact direction of "plowing" from the provided pictures? You do realize that the "plowing" direction doesn't have to be perpendicular to wing imprints?

This is the crater I get, running a rigid 3D 757 model through the ground. Plane model was aligned according to last recorded FDR data.



I'm too lazy to do the calculations at the moment, I just took two plane models, one half burried, other just above the ground (look at the gif below), aligned them on the same trajectory (I guess that would be velocity vector central through the fuselage, tail to nose; theoretical AoA = 0), drew lines through wing tips and the center of the fuselage and checked where they intersect with the terrain. I got the white area seen on the GoogleEarth snapshot above. Red dots show the positions of wingtips and the center of the fuselage.

For nitpickers - this graphics is only intended to quickly show the orientation of ground imprints made by a plane the size of 757, flying with the same parameters as provided in the NTSB CSV file; I didn't bother to line up the crater with the actual point of impact. GU seems most perplexed by the crater orientation, not by its location. Also keep in mind that this crater outline was produced with parameters from a plane that was still in the air at the time. Since the plane was still turning after the FDR data got recorded, the actual crater would probably also be turned slightly more in the clockwise direction. And, as mentioned, I didn't account for any AoA or plane breaking up.

Last FDR CSV data used:
Pitch: -41.1°
Roll: 142.0°
True Heading: 177.4° (from magnetic 187°)

Animated gif with the plane (due to plane's attitude, it may not appear so, but this is a straight down view of the area)


So, even though the plane is flying due south (177.4°), it can still leave a skewed imprint ("30 degrees east of south") and the angle of the wing imprints doesn't have much to do with direction of the "plowing" action.

More importantly, notice the angle between the imprint of the vertical stabilizer and wing imprints. It's not symetrical or perpendicular to the wings. This appears to look the same in the actual crash site photos. As for the plowing action, yes, I can imagine some of it on the photos, but it looks like it's directed towards south, just as the plane's mass was at the time of impact. I don't see any anomalies here.
 
celestrin said:
Gregory, are you trying to say that you were able to determine the exact direction of "plowing" from the provided pictures? You do realize that the "plowing" direction doesn't have to be perpendicular to wing imprints?

This is the crater I get, running a rigid 3D 757 model through the ground. Plane model was aligned according to last recorded FDR data.



I'm too lazy to do the calculations at the moment, I just took two plane models, one half burried, other just above the ground (look at the gif below), aligned them on the same trajectory (I guess that would be velocity vector central through the fuselage, tail to nose; theoretical AoA = 0), drew lines through wing tips and the center of the fuselage and checked where they intersect with the terrain. I got the white area seen on the GoogleEarth snapshot above. Red dots show the positions of wingtips and the center of the fuselage.

For nitpickers - this graphics is only intended to quickly show the orientation of ground imprints made by a plane the size of 757, flying with the same parameters as provided in the NTSB CSV file; I didn't bother to line up the crater with the actual point of impact. GU seems most perplexed by the crater orientation, not by its location. Also keep in mind that this crater outline was produced with parameters from a plane that was still in the air at the time. Since the plane was still turning after the FDR data got recorded, the actual crater would probably also be turned slightly more in the clockwise direction. And, as mentioned, I didn't account for any AoA or plane breaking up.

Last FDR CSV data used:
Pitch: -41.1°
Roll: 142.0°
True Heading: 177.4° (from magnetic 187°)

Animated gif with the plane (due to plane's attitude, it may not appear so, but this is a straight down view of the area)


So, even though the plane is flying due south (177.4°), it can still leave a skewed imprint ("30 degrees east of south") and the angle of the wing imprints doesn't have much to do with direction of the "plowing" action.

More importantly, notice the angle between the imprint of the vertical stabilizer and wing imprints. It's not symetrical or perpendicular to the wings. This appears to look the same in the actual crash site photos. As for the plowing action, yes, I can imagine some of it on the photos, but it looks like it's directed towards south, just as the plane's mass was at the time of impact. I don't see any anomalies here.
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Excellent model. You do not have to worry much about the AOA and other factors. Your work is in the ball park (better), if not better than needed if this was an accident and we needed to know how the impact looked. Gregory has no thesis, he is wondering around with no stated goal but a nebulous want the truth; No real goal, no progress.

Good job on the graphics. Do you have a data file of the FDR, vs the graphs? Can I get a copy? I think your model shows clearly the impact relative to the ground with the data. Nice job again.
 
Good job on the graphics. Do you have a data file of the FDR, vs the graphs?
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Thanks, Beachnut. What exactly do you have in mind? SLOB's FOIA request has provided a spreadsheet file and what seems to be a raw FDR file (.dlu). Spreadsheet files can be imported into OpenOffice or Excel, but don't contain as much parameters as the AA77's one, while the raw FDR file probably requires some propriety software by FDR manufacturer. Here are the spreadsheet CSV files. If you want the raw file, I'll have to think of something else (it's a bit chunkier at about 35MB).
 

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celestrin said:
Thanks, Beachnut. What exactly do you have in mind? SLOB's FOIA request has provided a spreadsheet file and what seems to be a raw FDR file (.dlu). Spreadsheet files can be imported into OpenOffice or Excel, but don't contain as much parameters as the AA77's one, while the raw FDR file probably requires some propriety software by FDR manufacturer. Here are the spreadsheet CSV files. If you want the raw file, I'll have to think of something else (it's a bit chunkier at about 35MB).
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I can not decode the raw data, but this looks good. If it contains the data you used, it is great. The graphs had offsets as much as 15 degrees on headings on the different graphs. Thanks.

it is perfect - I have been looking for the files like this - It looks like this FDR did not loose any data, and it is a different make than Flight 77's FDR. Thanks again.
 
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GregoryUrich said:
Sorry, you don't get it. A plane can be diving towards the ground with 0 degrees angle of attack.
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Sure. But the FDR numbers Gravy posted shows the aircraft had a -20° AoA at the moment of impact. Which is why I raised the issue of AoA in the first place.
 
celestrin said:
I didn't bother to line up the crater with the actual point of impact. GU seems most perplexed by the crater orientation, not by its location. Also keep in mind that this crater outline was produced with parameters from a plane that was still in the air at the time. Since the plane was still turning after the FDR data got recorded, the actual crater would probably also be turned slightly more in the clockwise direction. And, as mentioned, I didn't account for any AoA or plane breaking up.

Last FDR CSV data used:
Pitch: -41.1°
Roll: 142.0°
True Heading: 177.4° (from magnetic 187°)
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I think you came very close to the actual point of impact celestrin, as anyone can se from the attached FBI aerial below. Job well done in visualizing the impact.

As I mentioned in an earlier post it is possible to measure the roll angle of the aircraft form the impact crater and the direction of the debris/ aircraft heading. In the figure below the red arrow is aligned from the center of the crater in the main direction of the debris/aircraft heading. While the black line is aligned along the wing imprint. When I measure the angle in the picture below I get an angle of about 144 deg. You can also do this exercise with the aerial exhibit photos or in the figure provided by celestrin above. When I do it, I get angles in the area of 140 - 150 degrees.
 

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Corsair 115 said:
Sure. But the FDR numbers Gravy posted shows the aircraft had a -20° AoA at the moment of impact. Which is why I raised the issue of AoA in the first place.
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Corsair, those AoA numbers from the FDR can not be in DEGREES. It is a set of numbers correlating to AoA, but we don't know what the correlation is. We need additional information to translate those numbers into degrees.
 
Both the debris ejected from the crater and the plowing action were aligned along the aircrafts heading on impact. As celestrin says there is no anomalies here.

The key to understanding the look of the impact crater is hidden in the fact that the aircraft was rolled on impact and in the internal construction of the fuselage. That explains why a lot of earth was thrown up to the left and in the front of the plowing action, while almost nothing on the right hand side of the crater when you look in the direction of the aircrafts heading on impact.

Since its way to late, I am going to pull an Apollo on you and leave you with this. But I will come back tomorrow with some figures and why I think so. Actually it is quite simple and can easily confirmed by a simple experiment.
 
Reheat said:
Corsair, those AoA numbers from the FDR can not be in DEGREES.
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I'm just going by what Gravy posted, and he listed the number as degress. Since he's usually one to get the facts correct, I took the data in the post at face value. :)

Perhaps Gravy can answer whether the AoA value from the FDR that he posted was actually in degrees or not.
 
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Here is the best example of all - a B-757

The cruise angle of attack for a large airliner like the Boeing 757, for example, is around 1° or 2°. Aircraft fly at a much higher angle of attack during takeoff and landing because it is during these stages of flight when speed is the lowest and a high angle of attack is required to generate the needed lift.
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http://www.aerospaceweb.org/question/conspiracy/q0274.shtml
 
Gravy said:
The FDR data are graphed here:
http://www.gwu.edu/%7Ensarchiv/NSAEBB/NSAEBB196/doc04.pdf

My list is from a flight sim designer who posts on pprune:
http://www.pprune.org/forums/showpost.php?p=2778627&postcount=145
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OK, those charts are a pain to read, so I'll take some time to do that tomorrow. Beachnut passed me some additional AoA parameters last evening so we'll discuss those and resolve it. The numbers he passed DO NOT correlate to reasonable or proper AoA for any aircraft and the AoA range didn't correlate. It's possible that Gravy is not the culprit here.

Assuming Mr. Morten @ pprune is correct with a -20 degree AoA they were pushing some pretty hefty -G at impact. Based on 1-2 degrees at cruise I'd estimate at least 2-3 or more negative G's to achieve that kind of negative AoA.

ADD:I have found the AoA parameter on the FDR printout and although it says degrees on the printout, that simply can not be. The aircraft is cruising at a -16 on the charts, but we know to generate a positive lift component the AoA must be positive. Based upon the Boeing statement that the B-757 cruises at 1-2 degrees AoA and assuming the scale is linear the aircraft would have had -3 to -4 degrees AoA at impact. That's a guess as some type of correction factor must be applied to the FDR data to achieve the correct number of degrees.

ADD: That's what beachnut said several posts ago. Why did I waste my time doing this?:rolleyes:
 
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I would go over the charts myself if I thought this whole issue was anything but an attempt by Gregory to get lost in details, to find smaller and smaller "anomalies," in order to avoid the mountains of evidence that prove beyond all doubt that flight 93 crashed there, killing real people in the real world.

I will not waste my time on 9/11 "no-planers."
 
Gravy said:
I would go over the charts myself if I thought this whole issue was anything but an attempt by Gregory to get lost in details, to find smaller and smaller "anomalies," in order to avoid the mountains of evidence that prove beyond all doubt that flight 93 crashed there, killing real people in the real world.

I will not waste my time on 9/11 "no-planers."
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An excellent suggestion! I just glanced at a couple of other parameters and find no difference to what Mr. Morten listed. Heck, you can even see the hijacker pitching the aircraft as the passengers were attempting to enter the cockpit. I'm sure you can see the roll too, but I didn't look. That gave me cold chills anyway, so I'll leave it be.....
 
Reheat said:
An excellent suggestion! I just glanced at a couple of other parameters and find no difference to what Mr. Morten listed. Heck, you can even see the hijacker pitching the aircraft as the passengers were attempting to enter the cockpit. I'm sure you can see the roll too, but I didn't look. That gave me cold chills anyway, so I'll leave it be.....
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I don't want to suggest that it's not interesting, or that Gregory is being dishonest about not knowing the answer. It's tricky, and I have great respect for people who have the intellectual curiosity and ability to solve problems like this. I just don't think that's what's driving Gregory in this case. Just my $.02.
 
Isn't the "indicated AoA" on 757s actually some sort of an interpreted value? It doesn't show the actual AoA but a calculated suggestion, an envelope, boundary, where one could still fly at current parameters (airspeed) without going in a stall? Could the -19° actually mean "Hey, we're going so fast and generating so much lift, we could raise our pitch by 19° and still not stall." 19 degrees below stall? Positive values would indicate stall? Is that it? FDR data appears to support this.

The only instance where the iAoA went into the positive, was shortly after the autopilot was disengaged and Jarrah (or whoever the hijacker pilot was) took the plane into +20° pitch, pulling over 3 g (@ 09:59:53).

AA77's data doesn't go positive at all, right until the last recorded value, where it hits a whopping +71.5°. Have any CT[]s latched onto this anomaly yet?
 
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The AOA of -20 in a high speed dive going fast, is relative to a cruise AOA of -16 and a takeoff AOA of -6 on this 757. Think about being on a 757 at cruise the AOA on this model with this AOA would read -16. Think about your pitch angle on the cruise it would be only 3 degrees. I do not know what units the AOA is in on the FDR. "Pilots" are use to having the AOA presented in a form of approach speed bug, or a normalized instrument to 1, with .2 as cruise and .6 as approach speeds and a red zone of stall near .9. The neat thing about AOA, it covers all weights, I mean a pilot has to know the weight of an aircraft to calculate speeds for landing and approach, etc. But the AOA reading shows the pilot the speeds, just check the AOA as you slow to a approach speed and it indicates on speed with a reading. You set .6 AOA in a KC-135, and that is the approach speed for your configuration, you do not have to look it up in Charts. (but you pilots need to know the speed incase the instrument is bad, a good check both ways)

The -20 AOA is relative and could be used to calculate a velocity vector from pitch, as speed increases the angle of AOA would decrease. As seen the high speed dive is -20, cruise is -16. The AOA could be in degrees, but you need to know where zero angle with the cord line is. Good thing the aircraft does not care about all this crap. It would not be too out of line for AOA to be in degrees, but you must think about relative to what. So it may not be far off to think the AOA would be -4 degrees from cruise for a high speed dive.

So at take off you pitch up to lift off, the AOA is -6, then you level off at cruise and it is -16. Not bad, take off pitch is about 10 degrees, and cruise is about 10 degrees less. AOA angle in the text books would be smaller at high speed. At a real fast speed, in level flight, I would not be surprised if the cabin deck would appear to be nose down slightly. As a plane slows down with out flaps the AOA would approach -6, and the deck of the aircraft would appear to be nose up. The AOA can not be added or subtracted from the pitch to get the velocity vector unless you know some more information.

http://www.aerospaceweb.org/question/aerodynamics/q0165.shtml Shows the AOA stuff. Have I messed this up enough?

Gregory is not worth it. If he was a "real" engineer, he could do this himself.
 
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Reheat said:
You can find the answer for yourself here:

http://en.wikipedia.org/wiki/Angle_of_attack

and here:

http://www.aerospaceweb.org/question/aerodynamics/q0165.shtml

and here:

http://www.av8n.com/how/htm/aoa.html

Look at the numbers in specific examples and you'll note that the FDR number range of 5 to - 25 for UA93 absolutely can not be degrees. Plus, the other numbers that Gravy posted can not possibly be degrees
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You are likely correct. If the cruise at -16 as quoted by beechnut is correct, then they are referenced to something other than degrees.
I know that some flat-bottom airfoils will generate lift at negative AoA (The Clark-Y springs to mind), but -16 degrees is way outside the positive lift range. I also seriously doubt that any commercial carrier aircraft has the pitch authority to induce a +20 degree AoA at 500Kt (Which would, without a doubt stall the wing). It would require very rapid and very large elevator deflections to accomplish. With the way most commercial aircraft are set up, any attempt would result in a "zoom" climb, trading airspeed for altitude, culminating in a stall when the airspeed bled of.

Sorry I started this mess--AoA is not particularly important, other than to determine stall status. Pitch angle will be important, if by "pitch" we mean "angle of the aircraft X axis with respect to the gravity vector".
 
celestrin said:
Isn't the "indicated AoA" on 757s actually some sort of an interpreted value? It doesn't show the actual AoA but a calculated suggestion, an envelope, boundary, where one could still fly at current parameters (airspeed) without going in a stall? Could the -19° actually mean "Hey, we're going so fast and generating so much lift, we could raise our pitch by 19° and still not stall." 19 degrees below stall? Positive values would indicate stall? Is that it? FDR data appears to support this.
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This is the best possibility that's been suggested. This is very possible and a cursory glance at the numbers it seems to be in the ballpark. I'd guess the wing would stall at approximately +16-18 degrees AoA, so the -16 at cruise with this interpolation would roughly correlate to the Boeing published cruise AoA for a B-757. Using this same interpolation, a -6 for TO would also be in the ballpark as well. For the climb the AoA would be near Max lift over drag (Max L/D), but it's so compressed by time in the chart it's difficult to determine numbers, but it is still in the ballpark as well.

Remember this is not what the pilot is seeing, it is merely the FDR data. There is likely some sort of light display in the cockpit, not a actual AoA indicator of real numbers.

This idea also accounts for the range of +5 to -25, as well. I'd say this is very close to what the FDR is showing.

ADD - I don't want to speculate what the FDR for AA77 shows as I haven't looked at it. As I understand they were made by different manufacturers, so may show parameters differently.
 
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beachnut said:
Gregory is not worth it. If he was a "real" engineer, he could do this himself.
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This is the best idea of all.

I think we need to put this to bed and leave it be. One thing is for sure. If the FDR data were real AoA in degrees we'd be seeing POSITIVE numbers for the entire flight except for the level off at altitude and final decent. We can not determine the real AoA numbers in degrees for the final descent prior to impact until we know what the FDR numbers are.

Celestrin's suggestion is the best, so far, as it does correlate to reality. Until such time as someone knowledgeable of this FDR model stops by we don't know for certain.
 
While steering clear of a discussion of UA93's FDR AoA indications it might be worthwhile for those interested to learn more about these modern computerized (fly-by-wire) flight control systems with regard to a stall condition.

In about 1981-82 the F-111 flight control system was modified to NOT allow the aircraft to stall in a conventional manner. In other words, it would not exhibit the traditional wing drop, post stall gyration or other typical stall characteristics of traditional aircraft. The fly-by-wire system would not allow the AoA to reach a stall point (out of control). The aircraft would merely enter a high rate of descend (sink rate) still in a very controllable attitude. These same modifications were included in the flight control system in the F-16. Even tho' these aircraft were manufactured by General Dynamics, I suspect Boeing used similar technology in the 757 and 767 fly-by-wire flight controls. Consequently, the 757 and 767 likely will not stall in a conventional sense. Of course, the high sink rate will still allow the aircraft to crash, but (if you will) in a controllable manner. Just a $ 0.01 or two for those interested.
 
Gregory is not worth it.
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Probably true, but the problem is, this is not about Gregory at all. I've been casually wondering about AoA for months, never bothering to really get to the bottom of it. It's time to clear it up once and for all :)

I've found a Boeing article about the AoA.
http://www.smartcockpit.com/pdf/flightops/aerodynamics/0039/
It would appear that our mystery FDR values are actually a part of what's called the Pitch Limit Indicator (PLI).

"Because stall warning is primarily a function of AOA, the PLI shows AOA margin to stall warning, even though it is part of the pitch attitude display (fig. 11). The distance from the airplane symbol to the PLI is calculated from the difference between the AOA of the airplane and the AOA at which stall warning will occur."
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That's the behaviour displayed in the FDR data, so I'm pretty sure that this is what "indicated AoA" actually is. The numbers are not the AoA itself, it's the difference between the actual AoA and the calculated stall AoA, which is Mach number dependent.

AA77's data appears to follow the same principles, so it's even more likely to be a Boeing feature rather than a feature of the FDR manufacturer. And it would be measured in degrees.
 
Gravy said:
The 757 and 767 aren't FBW, BTW.
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Correct! Fly-by-wire was a poor choice of words on my part when applied to 757/767. Let's change it to computerized flight control system and then we'd be correct.

I guess the military paves the way as fly-by-wire has been a viable concept since the 60's, 70's at the latest. I did not realize Commercial Airline design was so far behind the technology curve.
 
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celestrin said:
The numbers are not the AoA itself, it's the difference between the actual AoA and the calculated stall AoA, which is Mach number dependent.
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By George, you've got it! :eye-poppi

Everything now fits near perfectly with the FDR Data. So, in conclusion the actual AoA of UA93 near impact was as I said previously, in the ballpark of -2 to -4 degrees, not -20 degrees as previously stated.
 
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Reheat said:
Correct! Fly-by-wire was a poor choice of words on my part. Let's change it to computerized flight control system and then we'd be correct.

I guess the military paves the way as fly-by-wire has been a viable concept since the 60's, 70's at the latest. I did not realize Commercial Airline design was so far behind the technology curve.
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The Concorde used analog FBW computers, and the A320 was introduced with digital FBW in 1984. I think all the modern Airbus planes are FBW, as is the Boeing 777. As Apathoid points out in this excellent paper, the 757/767 engines are essentially FBW.
 
celestrin said:
Probably true, but the problem is, this is not about Gregory at all. I've been casually wondering about AoA for months, never bothering to really get to the bottom of it. It's time to clear it up once and for all :)

I've found a Boeing article about the AoA.
http://www.smartcockpit.com/pdf/flightops/aerodynamics/0039/
It would appear that our mystery FDR values are actually a part of what's called the Pitch Limit Indicator (PLI).



That's the behaviour displayed in the FDR data, so I'm pretty sure that this is what "indicated AoA" actually is. The numbers are not the AoA itself, it's the difference between the actual AoA and the calculated stall AoA, which is Mach number dependent.

AA77's data appears to follow the same principles, so it's even more likely to be a Boeing feature rather than a feature of the FDR manufacturer. And it would be measured in degrees.
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Good find, Celestrin. I completely forgot about this and I think that solves our little AoA mystery. I forgot about it because you hardly ever see it on the display(on the 757 - it shows up as a horizontal yellow bar on the EADI, over the various pitch reference lines in the "sky") . I wonder if there is another AoA parameter for vane angle? If not, I guess a stall active/inactive discrete would tell investigators what they wanted to know if there was a suspected high AoA...
 
Why Flight 93?
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Why flight 93? Why flight 93? Why flight 93? Sheesh, always with the flight 93, you guys. Look, you stick to operations and let us handle planinng, mmmmk? When you need to know something, we'll tell you. If you don't know it, you don't need to know it.

K?
 
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apathoid said:
I wonder if there is another AoA parameter for vane angle? If not, I guess a stall active/inactive discrete would tell investigators what they wanted to know if there was a suspected high AoA...
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BTW, there is a vane on the 757 as one of the parameters on the FDR is for AoA heat on/off. (I believe you were in one the previous discussion about the source of the AoA data).

I see no need for measuring vane angle. What purpose would that serve when the differential data is already recorded on the FDR?
 
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