altimeter calibration
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Yes I was aware of Warren Stutt's work, decoding the last frame of the FDR file, and thus proving Pilots for 9/11 Truth wrong in their assertion that the FDR file did not match the official flight path.
I did not know about the 588 pages of Boeing manual. Unfortunately it does not contain the altimeter calibration graph. Still hoping someone can send it.
I did not know about the 588 pages of Boeing manual. Unfortunately it does not contain the altimeter calibration graph. Still hoping someone can send it.
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The recorded "altitude" of Dulles was 40 feet. Using standard atmosphere this calculates to 29.88 inch Hg. Then using known actual atmosphere this converts to 312.8 feet, which seems about right for Dulles.
Similarly at the end of the flight the raw altitude, -99 feet, calculates to 180 feet. The radio altitude is about 56 feet thus the difference is 124 feet.
Similarly at the end of the flight the raw altitude, -99 feet, calculates to 180 feet. The radio altitude is about 56 feet thus the difference is 124 feet.
Depending on sampling rate/ dfr frame rate position it is possible that the the last RA altitude was at a later point than the barometric altitude.
Dont know if the vertical rate was high enough for this to matter but is another point to consider.
Pardon in advance if this has already been accounted for.
BTW: I agree that trying to convince the "true believers" is futile but also know that lack of response can lead semi rational people to the wrong conclusions so this is not wasted effort.
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gravity32,
I have my doubts that you will find the data you are looking for in the aircraft and pilot manuals.
I did find this document which lists all the required standards for altimetry calibration and error estimation.
Unfortunately they are not free and cost about US$65 per standard..
http://acast.grc.nasa.gov/wp-content...vey_report.pdf
I have my doubts that you will find the data you are looking for in the aircraft and pilot manuals.
I did find this document which lists all the required standards for altimetry calibration and error estimation.
Unfortunately they are not free and cost about US$65 per standard..
http://acast.grc.nasa.gov/wp-content...vey_report.pdf
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Yes you are right CJ, gravity32 wants the specific aircraft calibration data.
He has said that Boeing will not supply him with this data.
My thinking is that Boeing must surely adhere to the published standards and if the aircraft was within spec then the standards would at least provide an indication of the possible calibration error margins.
Pitot static altimetry is error prone and one of the standards deal with how to estimate the associated error which may (or may not) assist gravity32.
He has said that Boeing will not supply him with this data.
My thinking is that Boeing must surely adhere to the published standards and if the aircraft was within spec then the standards would at least provide an indication of the possible calibration error margins.
Pitot static altimetry is error prone and one of the standards deal with how to estimate the associated error which may (or may not) assist gravity32.
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Thanks, guys, for a "nice link" to a conspiracy theory.
I haven't read it all yet, but it looks highly "promising" (if you see what I mean....)
I'm familiar with RadAlts, ADCs, pitot static calibration issues, FDRs recording issues (been there, done that, even designed test equiment for that), so I'm having fun at the moment.
CJ
I haven't read it all yet, but it looks highly "promising" (if you see what I mean....)
I'm familiar with RadAlts, ADCs, pitot static calibration issues, FDRs recording issues (been there, done that, even designed test equiment for that), so I'm having fun at the moment.
CJ
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Hi CJ and gravity32,
Have you seen this?
The Pentagon Attack on 9/11: A Refutation of the Pentagon Flyover Hypothesis Based on Analysis of the Flight Path
By Frank Legge
(B.Sc., Ph.D., Chemistry)
and
David Chandler
(B.S. Physics, M.S.,Mathematics)
September 2011
Have you seen this?
The Pentagon Attack on 9/11: A Refutation of the Pentagon Flyover Hypothesis Based on Analysis of the Flight Path
By Frank Legge
(B.Sc., Ph.D., Chemistry)
and
David Chandler
(B.S. Physics, M.S.,Mathematics)
September 2011
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gravity32,
What puzzles me is what evidence will the calibration chart reveal.
I ask this question because the accuracy of the pressure altimeter is subject to so many variables. The only evidence I can see is evidence that the barometric measurement of altitude subject to an abundance of errors.
The only time that the inherent error is insignificant is when the instrument is at high altitude and every other altimeter at that altitude has a standard reference datum.
Here is a link to a document which illustrates the errors measured by a barometric altimeter. Although the study uses mountain climbing and altimeters to illustrate the problems, it applies equally well to any pressure altimeter.
The use of altimeters in height measurement
What puzzles me is what evidence will the calibration chart reveal.
I ask this question because the accuracy of the pressure altimeter is subject to so many variables. The only evidence I can see is evidence that the barometric measurement of altitude subject to an abundance of errors.
The only time that the inherent error is insignificant is when the instrument is at high altitude and every other altimeter at that altitude has a standard reference datum.
Here is a link to a document which illustrates the errors measured by a barometric altimeter. Although the study uses mountain climbing and altimeters to illustrate the problems, it applies equally well to any pressure altimeter.
The use of altimeters in height measurement
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skwinty,
Thanks for that Legge/Chandler link, gave me a better view on the 'moonbats', and it's got a nice lot of bookmarks/references to explore tomorrow.....
I'm not a 'believer' in conspiracy theories, just fascinated by how they evolve.
CJ
Thanks for that Legge/Chandler link, gave me a better view on the 'moonbats', and it's got a nice lot of bookmarks/references to explore tomorrow.....
I'm not a 'believer' in conspiracy theories, just fascinated by how they evolve.
CJ
gravity32,
Just back from pub where two other ex B757 captains and a very senior (but not B757) TC cannot recollect any TAS correction to altimeter published in our manuals. (Plenty of cold air and mountain wind corrections but not what you are looking for)
I guess that the ADC (Air Data Computer) took all that into account and the standby instruments may not have done so but, if we were down to those, it was considered better not to complicate an already difficult situation.
I wonder if the FDR was taking raw data or corrected data?
Sorry not to be of any help.
Just back from pub where two other ex B757 captains and a very senior (but not B757) TC cannot recollect any TAS correction to altimeter published in our manuals. (Plenty of cold air and mountain wind corrections but not what you are looking for)
I guess that the ADC (Air Data Computer) took all that into account and the standby instruments may not have done so but, if we were down to those, it was considered better not to complicate an already difficult situation.
I wonder if the FDR was taking raw data or corrected data?
Sorry not to be of any help.
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skwinty,
Unfortunately that link to altimeters does not apply to the case in question as it does not cover barometers inside planes. That is where the errors of interest arise. They arise due to the difficulty the plane designers have in providing the instrument with true outside air pressure. The best they can do is experiment to find the best place for the static source. The problem is that the best place varies with speed. The calibration chart would reveal the speed beyond which they did not measure the errors.
Basil,
As there is no sign of steps in the data at the transition level of all 11 prior flights, it appears that the recording ignored the pilot's adjustment of the reference pressure. However the ADC may well have made adjustments relating to airspeed, but any such adjustment would fail at speeds for which the errors had not been measured.
Unfortunately that link to altimeters does not apply to the case in question as it does not cover barometers inside planes. That is where the errors of interest arise. They arise due to the difficulty the plane designers have in providing the instrument with true outside air pressure. The best they can do is experiment to find the best place for the static source. The problem is that the best place varies with speed. The calibration chart would reveal the speed beyond which they did not measure the errors.
Basil,
As there is no sign of steps in the data at the transition level of all 11 prior flights, it appears that the recording ignored the pilot's adjustment of the reference pressure. However the ADC may well have made adjustments relating to airspeed, but any such adjustment would fail at speeds for which the errors had not been measured.
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Gravity32,
The static port can only be mounted on the fuselage side where the air is relatively undisturbed.
If it were mounted on the nose of the aircraft then the speed of the aircraft would have a huge effect on the altimeter reading.
The point of the link I provided was to indicate the following sources of error for the instrument and that they are inherently
error prone and in the case of an aircraft, specifically when standard operating procedures are not adhered to.
The link shows the equation variables which apply to any barometric altimeter.
All this aside, lets say that Boeing gives you a calibration chart for the altimeter and this chart shows that the final reading
was incorrect by a factor of ten.
What significance would this have in relation to the aircraft crashing into the Pentagon or overflying the Pentagon or any other factor
regarding the crash.
My opinion is that the 757 crashed into the building and this was deliberate. The hijackers would probably not have reset the reference
datum to local conditions on the descent.
What will confirmation of the calibration data prove?
That is in my opinion the million dollar question.
ETA: You may find this interesting.
luizmonteiro - Online Simulators - Altimeter Errors Simulator
The static port can only be mounted on the fuselage side where the air is relatively undisturbed.
If it were mounted on the nose of the aircraft then the speed of the aircraft would have a huge effect on the altimeter reading.
The point of the link I provided was to indicate the following sources of error for the instrument and that they are inherently
error prone and in the case of an aircraft, specifically when standard operating procedures are not adhered to.
The link shows the equation variables which apply to any barometric altimeter.
All this aside, lets say that Boeing gives you a calibration chart for the altimeter and this chart shows that the final reading
was incorrect by a factor of ten.
What significance would this have in relation to the aircraft crashing into the Pentagon or overflying the Pentagon or any other factor
regarding the crash.
My opinion is that the 757 crashed into the building and this was deliberate. The hijackers would probably not have reset the reference
datum to local conditions on the descent.
What will confirmation of the calibration data prove?
That is in my opinion the million dollar question.
ETA: You may find this interesting.
luizmonteiro - Online Simulators - Altimeter Errors Simulator
Last edited by skwinty; 16th Sep 2011 at 06:33.
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skwinty,
The complexity of finding an appropriate spot for the static port, and the remaining errors due to changes in speed, angle of attack and sideslip, may be seen here:
http://www.spaceagecontrol.com/nasa-tm-104316.pdf
You ask: "What will confirmation of the calibration data prove? "
We are interested only in errors due to excessive speed. We are pretty sure that the discrepancy between the radalt altitude and the pressure altitude is due to excessive speed, and that speed being outside the calibration envelope for the plane. To be "pretty sure" is one thing, but to have documentary evidence is another. The calibration data would go to some speed and no further. Beyond that speed, no one can know what the errors would be. If we had the calibration data we would "know" to what speed the calibration was done, and we would "know" whether or not the plane was flying in an uncalibrated region.
MurphyWasRight,
You ask whether the time delay between recording the radalt and the pressure altitude could have caused the discrepancy. The pressure altitude is recorded in words 29 and 30, and the radalt is recorded in words 31 and 32, thus they are 2 words apart. Each word takes 1/256 th of a second so these readings are only 7.8 milliseconds apart. No significant error would be possible from that cause. Good thought though.
The complexity of finding an appropriate spot for the static port, and the remaining errors due to changes in speed, angle of attack and sideslip, may be seen here:
http://www.spaceagecontrol.com/nasa-tm-104316.pdf
You ask: "What will confirmation of the calibration data prove? "
We are interested only in errors due to excessive speed. We are pretty sure that the discrepancy between the radalt altitude and the pressure altitude is due to excessive speed, and that speed being outside the calibration envelope for the plane. To be "pretty sure" is one thing, but to have documentary evidence is another. The calibration data would go to some speed and no further. Beyond that speed, no one can know what the errors would be. If we had the calibration data we would "know" to what speed the calibration was done, and we would "know" whether or not the plane was flying in an uncalibrated region.
MurphyWasRight,
You ask whether the time delay between recording the radalt and the pressure altitude could have caused the discrepancy. The pressure altitude is recorded in words 29 and 30, and the radalt is recorded in words 31 and 32, thus they are 2 words apart. Each word takes 1/256 th of a second so these readings are only 7.8 milliseconds apart. No significant error would be possible from that cause. Good thought though.
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Gravity32,
Fair enough. You are concerned about the speed affecting the altimeter reading.
Now once again, lets say the calibration data shows that the aircraft was flying at speeds exceeding the calibration envelope.
What does the FDR say about the speed of the aircraft and what would be the significance of this excess speed and barometric altitude error in relation to the weight of evidence?
This is what I am trying to understand. What will this data refute or confirm.?
Fair enough. You are concerned about the speed affecting the altimeter reading.
Now once again, lets say the calibration data shows that the aircraft was flying at speeds exceeding the calibration envelope.
What does the FDR say about the speed of the aircraft and what would be the significance of this excess speed and barometric altitude error in relation to the weight of evidence?
This is what I am trying to understand. What will this data refute or confirm.?
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Hi gravity32,
Have you considered the lag between the time of the pressure felt at the static port, and the delay taken as the pressure wave moves along the length of the narrow plumbing to the pressure transducer (which generates the altitude signal) with the rate of descent? The Rad Alt suffers no lag.
Edit. (The lag is the reason why they developed Instantaneous / Inertial VSIs.) What altitude difference would 1 second make at the recorded ROD?
Have you considered the lag between the time of the pressure felt at the static port, and the delay taken as the pressure wave moves along the length of the narrow plumbing to the pressure transducer (which generates the altitude signal) with the rate of descent? The Rad Alt suffers no lag.
Edit. (The lag is the reason why they developed Instantaneous / Inertial VSIs.) What altitude difference would 1 second make at the recorded ROD?
Last edited by rudderrudderrat; 16th Sep 2011 at 11:34.
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gravity32,
The perplexing thing about the idea of speed affecting the operation of a barometric altimeter is this:
The calibration of an altimeter is of the form
z=cT log(Po/P)
where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and Po is the pressure at sea level. The constant c depends on the acceleration of gravity and the molar mass of the air.
There is no v for velocity in the equation.
If there was, then you would expect any supersonic aircraft to use special altimeters.
Now, I am not arguing that the position of the static port is insignificant and that shockwaves and a whole variety of conditions cannot cause errors in the readout.
Those are the fundamentals of barometric altimetry.
The 757 clearly wasn't supersonic and it clearly collided with the Pentagon.
There are many reasons, most of them discussed already, that explain why there is a differential between presssure alt and rad alt without requiring the velocity to be considered.
The g-forces the aircraft experienced when pulling up out of the dive could have introduced even more error.
The perplexing thing about the idea of speed affecting the operation of a barometric altimeter is this:
The calibration of an altimeter is of the form
z=cT log(Po/P)
where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and Po is the pressure at sea level. The constant c depends on the acceleration of gravity and the molar mass of the air.
There is no v for velocity in the equation.
If there was, then you would expect any supersonic aircraft to use special altimeters.
Now, I am not arguing that the position of the static port is insignificant and that shockwaves and a whole variety of conditions cannot cause errors in the readout.
Those are the fundamentals of barometric altimetry.
The 757 clearly wasn't supersonic and it clearly collided with the Pentagon.
There are many reasons, most of them discussed already, that explain why there is a differential between presssure alt and rad alt without requiring the velocity to be considered.
The g-forces the aircraft experienced when pulling up out of the dive could have introduced even more error.
Edit. (The lag is the reason why they developed Instantaneous / Inertial VSIs.)
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the standard VSI uses a restricted tube to deliberately introduce a calibrated lag into the instrument
I believe modern ADCs sense static pressure and compare with a time base to produce the VSI signal. Apparently there is still a lag, and IRS vertical motion is incorporated to improve the response time.
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A static port on an aircraft is basically just a small hole in the fuselage, connected to the altimeter by a tube.
Because of the airflow around the fuselage, the pressure at the location of that little hole is not the same as the free-stream static pressure at a sufficient distance from the plane.
Even if a lot of care is taken to find a location where the pressure is 'nearly' the same, you still need an f(v) or f(M) correction term, generally obtained from wind tunnel and flight tests, in the form of a graph (or 'look-up table') of a correction term k against v or M.
It's that graph that gravity32 is after.
CJ
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CJ,
Even if the calibration is done at speed electronically, there would still be an inherent error due to external and conversion influences.
The error introduced by the difference in the velocity of the airstream and the boundary layer is not significant.
How is this evidence going to prove or refute any altitude related conspiracy theory?
Even if the calibration is done at speed electronically, there would still be an inherent error due to external and conversion influences.
The error introduced by the difference in the velocity of the airstream and the boundary layer is not significant.
How is this evidence going to prove or refute any altitude related conspiracy theory?