United Flight 93, What actually happened ? [somewhat edited by JT]
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Originally Posted by chornedsnorkack
Which is why aerodynamic forces cannot change very fast. The lift has the maximum upward value at stall - AoA about 15...20 degrees - and maximum downward value at inverted stall - AoA about minus 15...20 degrees.
Turning a 50 m long plane by 30 degrees of pitch axis takes some time...
Turning a 50 m long plane by 30 degrees of pitch axis takes some time...
page 17. (view the plot with 400% or 800% zoom by acrabat reader)
http://www.gwu.edu/%7Ensarchiv/NSAEB...B196/doc04.pdf
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Originally Posted by SUPERMNNN
Actually we missed a force, centrifugal force can also give you a high vertical g. 
acceleration (vertical to the acf longitudal axis, not the horizon), it's just a
figure of speach. It has nothing to do with the real g force except
that we call 9.81 m/s^2 = 1 g.
We are really talking about "sentripetal acceleration a(s)" (not sure about the english word)
a(s) = v^2/r
v= speed = meters/sec
r = radius = meters
So, if you loop (or turn) the aircraft in a radius of 1000 m, at a speed of 200 m/s,
you will be pulling;
200^2/1000 = 40 m/s^2 = 40/9,81 = 4,08 g's
M
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Originally Posted by chornedsnorkack
Which is why aerodynamic forces cannot change very fast. The lift has the maximum upward value at stall - AoA about 15...20 degrees - and maximum downward value at inverted stall - AoA about minus 15...20 degrees.
Turning a 50 m long plane by 30 degrees of pitch axis takes some time...
Turning a 50 m long plane by 30 degrees of pitch axis takes some time...
Given that the alpha change required to get from 1'g' to 3-4'g' at high speed is probably closer to 10 degs, I would estimate that changing 'g' by that much in 1 sec is practical with aggressive control application. Essentially, the response time of the aircraft in the pitch axis is charactised by the SPPO (short period pitch oscillation) response, which is of the order of 1-2 seconds.
Again, if one were actually investigating this 'properly' one would be running a simulation with appropriate control inputs and evaluating all the responses against the data. We can't do that here, but the response 'looks' about right.
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Originally Posted by XPMorten
G is always constant. When we talk about "pulling several" G's VERTICAL
acceleration (vertical to the acf longitudal axis, not the horizon), it's just a
figure of speach. It has nothing to do with the real g force except
that we call 9.81 m/s^2 = 1 g.
We are really talking about "sentripetal acceleration a(s)" (not sure about the english word)
a(s) = v^2/r
v= speed = meters/sec
r = radius = meters
So, if you loop (or turn) the aircraft in a radius of 1000 m, at a speed of 200 m/s,
you will be pulling;
200^2/1000 = 40 m/s^2 = 40/9,81 = 4,08 g's
M
acceleration (vertical to the acf longitudal axis, not the horizon), it's just a
figure of speach. It has nothing to do with the real g force except
that we call 9.81 m/s^2 = 1 g.
We are really talking about "sentripetal acceleration a(s)" (not sure about the english word)
a(s) = v^2/r
v= speed = meters/sec
r = radius = meters
So, if you loop (or turn) the aircraft in a radius of 1000 m, at a speed of 200 m/s,
you will be pulling;
200^2/1000 = 40 m/s^2 = 40/9,81 = 4,08 g's
M
The problem is that when you do a roller coaster ride, the "r" is changing gradually, therefore you should have a "g" curve, gradually increase to high g, then to negative g, not jump to 3.7g in a second, then negative 2 g in the other.
And when you do that, your pitch angle change first, before the acceleration occurs, not after.
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It isn't "jumping" - it's changing at a rate faster than the plot is capable of showing other than as a jump, especially when combined with the discrete sampling of the FDR data aquisition system.
There's nothing in the data in that FDR plot which is inconsistent with a series of pushes and pulls on the column, generating alternately positive and negative 'g' in the sense of a 'rollercoaster ride'.
Which, incidentally, is perfectly consistent with the CVR transcript that I believe was released at one point.
There's nothing in the data in that FDR plot which is inconsistent with a series of pushes and pulls on the column, generating alternately positive and negative 'g' in the sense of a 'rollercoaster ride'.
Which, incidentally, is perfectly consistent with the CVR transcript that I believe was released at one point.
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Ok, let's exam the peaks closely starting from the last one before crash.
page 17. (view the plot with 400% or 800% zoom by acrabat reader and you may want to rotate the page 90 degree's clockwise.)
http://www.gwu.edu/%7Ensarchiv/NSAEB...B196/doc04.pdf
If we zoom in 400% with acrobat reader, look at the last peak of "longitudial acceleration" and "pitch angle" (one minute before crash. two red line 2nd and 4th plots from the top):
After the last peak (at about 10:02:00), the longitudial acceleration started to drop (with a curve), half minutes later passed 0.0, then keep droping, before crash there was a "sharp" change to a almost straight line drop (negative g).
At the same time, after the peak, the pitch angle drop pass the 0.0 almost at the same time as the logitudial acceleration passed 0.0. Pitching angle continue to drop following the curve, then there was a "sharp" change to an almost straight line drop. (This last minute looks like "missing engine" or engine lost power to me, am I right?)
This actually matched eyewitness accounts:
The plane suddenly came out of the cloud, flying sideways over head, made a spiral, turned belly up, then dive to the ground.
The last pitch angle was about -45 degrees.
But if you look at the Computed airspeed (bottom of the plots the red curve) for the last minute, the plane's air spead soared from 300 knots to 500 knots. As the longitudial acceleration drop so quickly, how can the air spead increase 200 knots in the last minute? (I believe the speed and presure altitude plot is incorrect, at the last minute at least.)
If you look at the last minute of the "longitudial acceleration" and "pitch angle" plots (ignore the air speed), this looks like both engine were gone, and the plane was doing a decending glide with remaining momentum. The turning point (before a close to straight line drop for both longitudial acceleration and pitch angle), seemed to be the "belly up" point, before dived to the ground at 45 degrees.
Did the plane's both engines shut down one minute before crashing? Or the plane simply lost both engines?
page 17. (view the plot with 400% or 800% zoom by acrabat reader and you may want to rotate the page 90 degree's clockwise.)
http://www.gwu.edu/%7Ensarchiv/NSAEB...B196/doc04.pdf
If we zoom in 400% with acrobat reader, look at the last peak of "longitudial acceleration" and "pitch angle" (one minute before crash. two red line 2nd and 4th plots from the top):
After the last peak (at about 10:02:00), the longitudial acceleration started to drop (with a curve), half minutes later passed 0.0, then keep droping, before crash there was a "sharp" change to a almost straight line drop (negative g).
At the same time, after the peak, the pitch angle drop pass the 0.0 almost at the same time as the logitudial acceleration passed 0.0. Pitching angle continue to drop following the curve, then there was a "sharp" change to an almost straight line drop. (This last minute looks like "missing engine" or engine lost power to me, am I right?)
This actually matched eyewitness accounts:
The plane suddenly came out of the cloud, flying sideways over head, made a spiral, turned belly up, then dive to the ground.
The last pitch angle was about -45 degrees.
But if you look at the Computed airspeed (bottom of the plots the red curve) for the last minute, the plane's air spead soared from 300 knots to 500 knots. As the longitudial acceleration drop so quickly, how can the air spead increase 200 knots in the last minute? (I believe the speed and presure altitude plot is incorrect, at the last minute at least.)
If you look at the last minute of the "longitudial acceleration" and "pitch angle" plots (ignore the air speed), this looks like both engine were gone, and the plane was doing a decending glide with remaining momentum. The turning point (before a close to straight line drop for both longitudial acceleration and pitch angle), seemed to be the "belly up" point, before dived to the ground at 45 degrees.
Did the plane's both engines shut down one minute before crashing? Or the plane simply lost both engines?
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Originally Posted by SUPERMNNN
page 17. (view the plot with 400% or 800% zoom by acrabat reader and you may want to rotate the page 90 degree's clockwise.)
http://www.gwu.edu/%7Ensarchiv/NSAEB...B196/doc04.pdf
If we zoom in 400% with acrobat reader, look at the last peak of "longitudial acceleration" and "pitch angle" (one minute before crash. two red line 2nd and 4th plots from the top):
http://www.gwu.edu/%7Ensarchiv/NSAEB...B196/doc04.pdf
If we zoom in 400% with acrobat reader, look at the last peak of "longitudial acceleration" and "pitch angle" (one minute before crash. two red line 2nd and 4th plots from the top):
After the last peak (at about 10:02:00), the longitudial acceleration started to drop (with a curve), half minutes later passed 0.0, then keep droping, before crash there was a "sharp" change to a almost straight line drop (negative g).
At the same time, after the peak, the pitch angle drop pass the 0.0 almost at the same time as the logitudial acceleration passed 0.0. Pitching angle continue to drop following the curve, then there was a "sharp" change to an almost straight line drop. (This last minute looks like "missing engine" or engine lost power to me, am I right?)
At the same time, after the peak, the pitch angle drop pass the 0.0 almost at the same time as the logitudial acceleration passed 0.0. Pitching angle continue to drop following the curve, then there was a "sharp" change to an almost straight line drop. (This last minute looks like "missing engine" or engine lost power to me, am I right?)
And yet again, it's not a "straight line drop" except in a purely graphical sense due to the presentation of the data - it may well be (and I suspect is) just high negative pitch rate which *looks* instantaneous on such a small timescale.
There's no evidence of a missing engine on page 17 for me. Rather more conclusively, page 27 - as already mentioned by a previous poster - clearly shows engine N1 and EPR data valid to the end. The engines were on the plane to the end of the FDR data. No question about that.
But if you look at the Computed airspeed (bottom of the plots the red curve) for the last minute, the plane's air spead soared from 300 knots to 500 knots. As the longitudial acceleration drop so quickly, how can the air spead increase 200 knots in the last minute? (I believe the speed and presure altitude plot is incorrect, at the last minute at least.)
Airspeed and altitude both look perfectly valid. Altitude (blue) is dropping very quickly, airspeed is increasing rapidly, pitch is massively nose down, long accel is consistent with accelerating nose-down flight. There is NOTHING strange in the data.
If you want to try to understand FDR data and conclude things from it you're going to need a much greater understanding of how aerodynamics and flight mechanics works than you'll pick up in this forum. You could spend ten years and still not know all the tricks of the trade.
But on the basis of my 20 years, that data is valid, it shows the engines on wing to the end of data, it shows a plane manoeuvring violently in a pilot-controlled fashion and it shows what seems to be an intentional high speed dive towards the ground.
As previously stated, again, if there *was* anything suspect in the data the professionals would have noted it, and it would have been in the mainstream and specialist press. magazines like Aviation Week have no scruples about discussing 'black projects' and the like, so the idea that they wouldn't publish is nonsense, and when the FDR was made public enough interested professionals around the world would have looked at the data to spot anomalies. I know in my department its common, when an accident report gets published either officially or via the media for the watercooler talk to be about the results - even if it isnt our aircraft.
There is no mystery about what happened to Flight 93.
