altimeter calibration

[gravity32]

Early this year I started a discussion on another forum asking what might cause a discrepancy between altitude calculated from radalt plus ground elevation, and altitude as shown by an altimeter, correctly set on local barometric conditions. The plane in question, a Boeing 757, was travelling at very high speed at low altitude. The consensus was that radalt would not be affected by high speed but the altimeter would likely be out of its calibration envelope.

I have been told that there is a graph in the flight manual which shows errors that would be encountered under various flight conditions. Is there any way to get hold of this graph?

[rudderrudderrat]

Why do you want to be flying a 757 at high speed (i.e. outside calibration envelope) below 2,500 ft agl?

[gravity32]

I don't want to do it. I am looking at the FDR data file, and I see this discrepancy between radalt and pressure and would like to know the cause.

[skwinty]

The cause is that they are two different types of instruments that operate on totally different principles and measure two distinctly different parameters.:ok:

[gravity32]

Thanks skwinty, that has already become clear from comments on the other thread. What I am looking for now is documetary evidence of the calibration envelope. I want to see what Boeing says about speed and calibration errors. They would have done the measurements, but on their website they do not reveal such details to the public. I am hoping someone can provide the graph.

[skwinty]

gravity32, try reading this link. It will have the same and more data that you are seeking.

Pitot-Static Instrument Calibration (http://www.eaa1000.av.org/technicl/instcal/instcal.htm)

Your questions kept referring to the differences between radalt and pressure altitude so I just wanted to remind you to not conflate the two.:)

[Mad (Flt) Scientist]

Additionally, radalt measure the height above the actual ground. Barometric altitude is relative to a notional ground level or sea level and not necessarily relative to the local terrain.

There's no reason why the two should agree, and in many cases they do not, especially if you're looking at FDR data with a fine tooth comb.

[Lightning Mate]

Barometric altitude is relative to a notional ground level or sea level and not necessarily relative to the local terrain.

As a senior ATPL lecturer of aircraft instrumentation, may I comment?

An altimeter "measures" a vertical displacement from absolutely any pressure datum you wish to choose. It has nothing to do with a "notional ground level" or indeed "sea level".

An altimeter is merely an aneriod barometer, the measurement datum of which may be chosen as you wish.

:)

[gravity32]

Thaks skwinty, that is useful but it still does not provide me with Boeing's calibration data. I have been told there is a graph of it in the Flight Manual and that is what I am hoping for.

[skwinty]

No problem gravity32,
I hope that some one is able to come up with the graph you are looking for. I cannot help you with that..:)

[Mad (Flt) Scientist]

As a senior ATPL lecturer of aircraft instrumentation, may I comment?

An altimeter "measures" a vertical displacement from absolutely any pressure datum you wish to choose. It has nothing to do with a "notional ground level" or indeed "sea level".

An altimeter is merely an aneriod barometer, the measurement datum of which may be chosen as you wish.

:)

Sorry, but when calibrated as an ALTImeter, it is using pressure to sense altitude i.e. height. The reference pressure does indeed correspond to a notional ground level, in the case where you set to achieve the correct reading at touchdown, or a notional sea level, which is the higher altitude approach, using the ISA that gives a notional height above notional sea level.

The measurement data used for aeronautical purposes are to enable an instrument to provide a relative height to a reference.

Otherwise, the scale would be in Pa or Mbar, not feet and metres.

[gravity32]

Mad (Flt) Scientist and Lightning Mate,

The FDR data file shows that the radalt and the pressure agree very well while the plane is flying at 310 knots at 2500 feet. This was shown by adding the radalt to the ground elevation at each position report and using the known local baro and temp to calculate true altitude. As the plane descends and picks up speed the two diverge, with the pressure indicating a higher altitude than the radalt. The difference between them was about 120 feet at the end of the file. This is quite a big discrepancy.

Some pilots have been telling me that the altimeter is more accurate than radalt. I am looking for Boeing's data to refute them.

[STBYRUD]

Hmm.. Having no idea about the reasons for it I can report that my 737 happily calls 'twenty-five hundred' at 2300ft on the altimeter when coming in for landing over the sea... I guess it would be interesting to look at the accuracy of the RA as advertised by the manufacturer?

[skwinty]

That is exactly why you should use your pressure altimeter at altitude and your radalt for ground proximity at low altitude.

Horses for courses.;)

[Mad (Flt) Scientist]

Some pilots have been telling me that the altimeter is more accurate than radalt. I am looking for Boeing's data to refute them.

Its a meaningless statement so other than by logic its not refutable.

Radalt measures distance to an object giving a radar return.

Altimeter uses pressure data to infer height above a reference datum.

You really can't compare the two. they aren't even really measuring the same thing. Depending on what you actually want to know, either may be better/"more accurate".

[Checkboard]

a Boeing 757, was travelling at very high speed at low altitude

... another 9/11 conspiracy thing, then . :rolleyes:

[john_tullamarine]

If you have access to the FDR readout for analysis then, ergo, your involvement is such that you will have direct access to the relevant P/N AFM.

Something doesn't make a lot of sense here, methinks ?

[gravity32]

STBYRUD,
Apparently you are getting an 8% error in your radalt. That seems excessive. Don't manufacturers quote 2 or 3 % max? That error would diminish as you descend, as manufacturers quote a max error of 1 foot on the ground. In the FDR file in question the error does not diminish, it increasess as the plane descends, so does not seem to be due to radalt error.

Checkboard,
Yes, this is about flight AA77 which went into the Pentagon. The pilots who are saying the altimeter is more accurate than the radalt are trying to make the case that the plane flew over the Pentagon and that the damage was done by other means, thereby casting suspicion on the authorities. The pilots have an effective website and have convinced a lot of people, which I find pretty annoying.

john_tullamarine,
No, the FDR file was released to the public by the NTSB. Anyone can get it. What seems to be hard to get is the altimeter calibration data.

[Mad (Flt) Scientist]

@gravity32

Trying to convince a 9/11truther (or whatever they are0 is like trying to convince any other believer. The best you can hope for is to remove a piece of evidence, but you'll almost never shake their belief.

if you did manage to produce a convincing argument to show that, for example, Boeing has altimeter calibration charts above Vmo which demonstrate that the altimeter was overreading, they'd just turn round and say something like "how convenient that NOW this is available to aid in the cover up".

Unless you like tilting at windmills ....

[gravity32]

Mad (Flt) Scientist,
This is not about tilting at windmills. You are no doubt right that it will be impossible to persuade these pilots that they are wrong, but it will be possible to reduce their influence with the public by proving that these pilots are wrong. This can be done by publishing the proof. The proof must be available in the Boeing calibration data. That is what I would like help to locate, as a public service.

[mono]

Any graph in the AFM would be for the normal flight envelope. you may be able to extrapolate but the reationship would not be linear and the further away you get from VMo the worse any errors will be. As said, you won't convince the conspiracy theorists no matter how hard you try.

One thing to suggest however is that loads of people must have seen a 757 flying AWAY from the pentagon at 120 ft which I doubt.

[gravity32]

mono,
Yes, but the graph will provide the limits to the calibration envelope, and thus prove the plane was outside the envelope. That will be something. It will also be interesting to see in what direction the error may be trending at the boundary.

The argument that the plane would have been seen by many people if it flew over has already been used.

[skwinty]

It has been my experience that you will never ruin a good story with facts.

People who believe in conspiracy theories will not be swayed by a calibration graph, which is not surprising given that the graph will prove nothing with respect to the precision of the pressure altimeter on the day in question.

[gravity32]

skwinty,
You are apparently not involved in debate with people about these theories so you are not familiar with how it works. As you say there is no point in trying to convince a believer to change his mind, but there is a very much larger body of people making enquiries. These people are not believers. They are doing what everyone does these days, they are surfing the internet for information. They see both sides. They make up their minds on the basis of the weight of evidence. That graph I am hoping for will add to the weight of evidence.

[skwinty]

The weight of the evidence lay in the ruins of the Pentagon and WTC.

I have debated with enough twoofers to know that they are delusional.:ugh:

[Avtrician]

Speed doesnt have an effect on the altitude reading of an Altimeter, how ever altitude can have an effect on speed indication. At a higher altitude there is less "ambient " pressure on , so the pressure due to speed in the pitot probe will be less. This difference is compensated for by mixing in the static air pressure. Speed doesnt affect static pressure, so doesn't alter altitude readings.

An altimeter when calibrated in a lab is set to read a certain calculated standard height for a certain pressure. eg 10000ft = 1013mBar or hecta whosits.

The air pressure over an area changes, so if flying straight and level, over perfectly flat terrain, its possible that an altimeter that has been set at a reference pressure (or even zero feet at the departure point) will show ( and record) a change in height. In this instance ( hypothetical of course) the RAD alt which uses radar waves to measure its true height above the ground (and record it). On examination of the recorded data it would show a difference between the two readings.

Its not a question of accuracy of the calibration of either device , as they are measuring differnt things. the altimeter measures air pressure, and the radalt measures the time it takes for a radar pulse to return to it.

Note, that a radalt is only accurate when aimed vertically at the ground, any tilt in the aircraft axis will give an erroneous reading.

[gravity32]

Avtrician,
Speed does have an effect on the altimeter if the plane flies outside the calibration envelope. These issues have already been covered. The plane was nearly level at the end so radalt would not have been upset by angles. The True altitude was calculated from the recorded pressure using the known baro and temp on that day. Radalt plus ground elevation should therefore have been the same as True altitude. They were the same while the plane was travelling at 310 knots at 2500 feet, as expected. They were not the same after the plane descended and picked up speed, therefore one can deduce that the plane was flying outside the calibration envelope for the altimeter. What I am hoping for is documentary evidence that the plane was flying outside its calibration envelope, and for that I need the Boeing calibration graph. Can anyone provide a copy?

[skwinty]

gravity32,

Do you think that the terrorists flying the aircraft would have worried about resetting the reference datum of the altimeter en route to their target?

[gravity32]

skwinty,
This is a good point to check out. Naturally any adjustment will make a difference to what the pilot will see. However in the case at hand we are using the pressure data in the FDR file. There are 12 flights on the file and in no case is there a step in the pressure, either in ascent or descent. We can assume therefore that this is unadjusted data and will give correct True altitude when adjusted using the known baro and temp at the location on the day.

[skwinty]

gravity32,
have a look at this boeing 757 bulletin.

"
The amount of undershoot or overshoot depends on the amount of the barometric correction, when the altimeter is reset, and the elapsed time from altimeter reset to level-off. "

It may help you a bit in your quest.

Boeing 757 Boeing Bulletins :: FMC-16 : FMC VNAV Anomaly During Intermediate Level Off (http://www.biggles-software.com/software/757_tech/bulletins/fmc-16.htm)

[gravity32]

Thanks skwinty, but I don't think it is relevant as it refers to errors the pilot may encounter shortly after passing through the transition level if he makes a large adjustment. It therefore applies to what the pilot may see on his altimeter, but we are looking at unadjusted data.

[felixthecat]

Its worth remembering that the earth beneath you is very rarely flat. An extreme example is flying onto a runway in the mountains or on a cliff. The Rad Alt will read every bump and quarry but the altimeter will give a much smoother change since it measures pressure changes that generally are very gradual.

[gravity32]

felixthecat,
You are of course right. However this is not relevant to the problem as we used the ground elevation at every position report to produce a corrected radalt altitude. The result was almost as smooth as the calculated pressure altitude.

[skwinty]

I read the bulletin as the changing of the barometric setting on the altimeter having an effect on the VNAV signals and FMC systems.

Anyway, I am in uncharted territory here, so good luck in your endeavours.:ok:

[skwinty]

gravity32,

See link for analysis of a Boeing 757 altitude data being used in serious incident investigation. The altitude data used in this investigation is FDR QNH corrected altitude data.

Are you sure you are looking at uncorrected data? Perhaps the math you are applying to correct data is causing a problem.

I claim no experience on Boeing aircraft but I am interested in accident/incident investigation.

ETA: Here is another link about pitot static system errors wrt to Boeing.

http://www.boeing.com/commercial/aeromagazine/aero_23/aero_23.pdf

REPORT ON THE serious incident to icelandair BOEING 757 (http://911research.wtc7.net/cache/pentagon/analysis/theories/aaib_757incident.html)

[ChristiaanJ]

gravity32,

Sorry for barging in, but if I understand correctly, the issue is with the FDR record.
Doesn't that show the displayed baro alt ? (Most relevant in a crash analysis.)
And does the FDR show the reference datum set on the altimeter ?

Are you saying the FDR also shows the "raw" static pressure (in mbar or psi or whatever) as 'measured' by the ADC ?
I can think of several reasons for discrepancies between "baro alt" and "rad alt" records on an FDR, depending on where exactly the FDR gets its data. Your "outside the calibration envelope" is only one of them.

In the case of a "CFIT" such as this, I would first look at the RA data, and only then see to what extent they could be correlated with the baro alt data.
RA is pretty accurate, especially low down, which is exactly why it's there in the first place.

Have you got a link to the 'moonbats' site (PM me if you prefer)? First I hear of this particular controversy, so I'm curious to read the original "arguments".

CJ

[gravity32]

skwinty and ChristiaanJ

Yes the altitude data is unadjusted in the FDR file. We know it is unadjusted because in all 12 flights on the file there is no jump at the transition level on ascent and descent. The units of the data are feet, so two steps were used to arrive at True altitude, first converting the feet back to pressure then calculating True. On the ground at Dulles this gave the published elevation for Dulles, after adjusting for baro and temp, so we know it is based on standard atmosphere, and so should give correct altitude at destination by applying destination baro and temp. What it gave at the last position was 124 feet higher than radalt plus ground elevation.

The radalt appears to be reliable and accurate on the ground as all 12 flights show -6 feet while on the ground. If we add 16 to this we get wing height, allowing that the radalt should read zero on touchdown. The last radalt reading was 4 feet, giving a wing height of about 20 feet, which looks about right as the plane still had a little distance to go before impact.

The "moonbat" site is Pilots For 9/11 Truth (http://pilotsfor911truth.org/)

[skwinty]

gravity32,

How can the altitude data be "unadjusted" if it's units are feet?

A raw or unadjusted signal would be measured in pressure units and not feet.

The deviations to the data at transition levels or altimeter adjustments are very small in relation to the overall signal and therefore not easily discerned.

I would say that the data is FDR QNH corrected altitude as per the investigation I linked to earlier.

[gravity32]

The data is unadjusted in the sense that nothing has been done to it other than to convert pressure to feet, using standard atmosphere. If you can convert pressure to feet, you can convert backwards from feet to pressure.

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.

[skwinty]

Hi gravity32,

Have you seen these?

AA77 FDR Data, Explained - Page 108 - JREF Forum (http://forums.randi.org/showthread.php?t=66047&page=108)

108 pages of FDR altitude conspiracy theory debunking


Orbitfiles - Download "D226A101-3G.zip" - Free Online storage, Free Online File Backup, Free Online File Sharing (http://www.orbitfiles.com/download/id4067718242.html)


588 pages of Boeing 757 FDR manual.

:ok:

[gravity32]

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.

[MurphyWasRight]

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.

Have not looked at the DFR data (and dont have time to do so) but wanted to mention another factor that 'may' explain some of the apparent errorr:

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.

[skwinty]

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/uploads/acast/2007/09/28/mmda_relevant_standard_and_working_groups_survey_report.pdf

[ChristiaanJ]

skwinty,
Thanks, I'll read that first.
However, I'm not sure it's relevant to gravity32's original question.

CJ

[skwinty]

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.

:ok:

[ChristiaanJ]

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

[skwinty]

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 (http://stj911.org/legge/Legge_Chandler_NOC_Refutation.html)

By Frank Legge
(B.Sc., Ph.D., Chemistry)
and
David Chandler
(B.S. Physics, M.S.,Mathematics)


September 2011

[skwinty]

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 (http://www.biber.fsnet.co.uk/altim.html#References)

[ChristiaanJ]

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

[Basil]

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.

[gravity32]

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.

[skwinty]

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 (http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx)

[gravity32]

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.

[skwinty]

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.?

[rudderrudderrat]

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?

[skwinty]

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.

[Checkboard]

Edit. (The lag is the reason why they developed Instantaneous / Inertial VSIs.)
Nope - they developed IVSIs because the standard VSI uses a restricted tube to deliberately introduce a calibrated lag into the instrument, so that the lagged pressure may be compared to the current pressure - and the difference is of course related to the change of pressure.

[rudderrudderrat]

the standard VSI uses a restricted tube to deliberately introduce a calibrated lag into the instrument
I agree that the old mechanical VSI had such a restriction.

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.

[ChristiaanJ]

The calibration of an altimeter is of the form

z=cT log(Po/P)

There is no v for velocity in the equation.

There isn't because your formula is for calibrating a stationary altimeter in the lab (e.g., with a pneumatic test set)..

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

[skwinty]

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?

[gravity32]

skwinty,
you ask: "What does the FDR say about the speed of the aircraft"
The FDR shows a final speed of 488 knots.

"and what would be the significance of this excess speed and barometric altitude error in relation to the weight of evidence?"
We don't know the significance - that is the problem. We suspect the high speed is causing a serious increase in recorded altitude but we can't prove it.

"This is what I am trying to understand. What will this data refute or confirm.? "
If we had the graph we could see whether it reaches to 488 knots. If it does, we could read the error correction needed and provide a corrected altitude. If it does not, we could say the plane is operating outside its calibration envelope therefore recorded altitude cannot be trusted. In that case the recorded pressure altitude is meaningless. It cannot be used to make the case that the plane flew over the Pentagon. Pilots for 9/11 Truth will be shown to be making a false argument.

CJ and skwinty,
As I understand it, the reason why selection of the position of the static port is such an art is that at different speeds the plane will have different angles of attack, thus the airstream will be deflected in different ways and produce different pressures. There may be other powerful factors like changing turbulence patterns with speed. So an altimeter in a plane cannot be treated as an altimeter on land. There are two goals: to minimize the error between the pressure in the tube and outside, and to minimize the difference between the errors at different speeds. It appears that while wind tunnel tests may be used to determine the regions where pressure is close to outside pressure, it is necessary to use trial and error experiments to refine the position and to create error tables.

So, yes, the lookup table or graph of error correction due to speed is what I am after.

Rudderrudderrat,
Vertical speed may also require a correction factor due to lag, or may not if it is compensated in some way. It is not relevant to this discussion because the descent rate at the end was not unusual so would have been corrected by the ADC.

Similarly g-force and angle of attack, which are correlated to each other, could require correction of the altimeter, however the g-force was not excessive and the angle of attack would have been small.

We are left with just the excessive air speed as the suspicious factor.

[skwinty]

What happens then with supersonic aircraft?

Do they suffer from serious altitude increases due to velocity induced pressure errors?

After all, aircraft have been going supersonic for years without computers.;)

[skwinty]

If the computer performs the calibration in flight, then the speed envelope should cover at least Mach .8 so the graph would show that as the upper velocity range.

Obviously, at low altitude the density of the air is significantly higher than that of 33,000 ft.

I doubt if you will find a graph for 488 knots at 500 ft.

So any error at this altitude and speed would be more related to air density.

[gravity32]

skwinty,
I don't know the purpose of your question about supersonic aircraft and I don't know the answer. Here is what I think. It is quite likely that the form of the airstream around a supersonic plane would change as it went through the sound barrier. It seems quite likely that these planes would be provided with two static ports, one for subsonic, one for supersonic flight. Each would be provided with approprate error correction procedures.

In the case we are discussing, we have a radalt altitude and a pressure altitude which disagree by 124 feet. You say: "The error introduced by the difference in the velocity of the airstream and the boundary layer is not significant."

If you dont think this 124 foot discrepancy is caused by something to do with the flow of air changing with speed, to what do you attribute it?

Are you suggesting that it is not the altimeter but the radalt that is wrong?

[ChristiaanJ]

What happens then with supersonic aircraft?I don't have all the figures at hand, sorry.
But one well-known phenomenon on Concorde occurs when 'breaking the sound barrier', or better expressed, when accelerating past Mach 1.
A shock wave then moves from the nose along the fuselage past the static ports.
It's barely visible on the altimeter, but there is a very noticeable 'twitch' on the VSI (there are videos of that, and we've even managed to simulate it on the Brooklands Concorde simulator).

Do they suffer from serious altitude increases due to velocity induced pressure errors?As said, the "drilling of the little hole" is very much trial-and-error to find the location which produces the least errors. The final location may introduce altitude increases or decreases as a function of airspeed, depending on the exact effects of the airflow and pressure 'distribution' at that exact location.

After all, aircraft have been going supersonic for years without computers.;)Sure, but in that era nobody cared about a few hundred feet error in indicated altitude......
Also, most of those aircraft carried a calibrated pitot-static 'boom'. Airliners don't.

CJ

[A320Slave]

gravity32,

the static systems on large transport aircraft are rated to the Mmo of that aircraft and corrected by the air data computer. This is why you will read a calibrated airspeed on most large transports and not an Indicated Airspeed. So the staitic system would be rated to .86 mach. To understand the reasons for Vmo/Mmo click here...

Boeing 757 Boeing :: Vmo/Mmo Limitations Review (http://www.biggles-software.com/software/757_tech/flight_instruments/_popup_for_40_vmo_mmo.htm)

The reason you see the altitude diverge as the aircraft gets lower, is because the Radar altimeter only measures from an object underneath the airplane. Whether that be a building, etc. It is impossible to determine a True Altitude from a Radar altimeter unless you know exactly what object you are measuring from. The altitude readout from a Radar Altimeter is known as Absolute altitude. It does not necessarily mean your height above the ground, especially if there are many buildings along the approach corridor. You would expect to see the altitude diverge when descending towards a runway with obstacles along the approach.

When and if you measure from a known object, such as over a runway, the True Altitude lines up pretty well with the Radar Altitude when adding runway elevation. There is no more altitude divergence.

See here:
Debunking Fdr Debunking - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=7163&view=findpost&p=10798304)

[skwinty]

It is firstly obvious to me that the aircraft was completely outside of its normal flight envelope. The aircraft was deliberately flown into the building with no regard to normal flight procedures.
That cannot be disputed.

The factors that could have influenced the pressure altitude are numerous. The only thing I can say with confidence is that the error in question would be the sum of all total instrument errors.

I would not be able to isolate a single variable with great statistical certainty.

Even if you extrapolate from the original data, your next problem would be the low sample number and lack of repeatability.

[A320Slave]

The aircraft was operating well outside it's normal flight envelope for structural and stability purposes for a standard 757, I agree. See more here...

9/11: World Trade Center Attack Speed Analysis (http://pilotsfor911truth.org/wtc_speed)

and here...

Evidence Strengthens To Support WTC Aircraft Speed Analysis (http://pilotsfor911truth.org/wtc_speed_part2.html)

But the aircraft was well within the Pitot-Static calibration parameters rated to .86 Mach.

[ChristiaanJ]

gravity34,
What's the (raw) Mach number in the last seconds?
My back-of-the-envelope says M=0.74, assuming ISA.

Seems to be too far below M=0.86 to really matter....

CJ

[gravity32]

A320Slave and skwinty,
It seems we have a disagreement. A320 says the calibration should be OK to Mach 0.86, skwinty says "the aircraft was completely outside of its normal flight envelope".

While these aircraft may be permitted to fly at 0.86 Mach at high altitude, they are not permitted to fly that fast at low altitude. It seems reasonable to me that the altitude calibration at high altitude would be to 0.86 M but at low altitude they wouldn't need to calibrate to that speed and probably would not bother. As they are limited to 350 knots it seems unlikely that they would be calibrated much beyond that at any altitude.

But unlikely is not proof. Still looking for the documentary proof.

A320, you appear to be unaware that earlier in this thread it was pointed out that the ground elevation was determined at each position report, and added to the radalt height, so a proper radalt altitude was obtained. In the last few seconds before impact there are no buildings which can confuse the radalt.

[A320Slave]

My back-of-the-envelope says M=0.74, assuming ISA.

Seems to be too far below M=0.86 to really matter....I came up with about the same and I agree.

There is a discussion on the Pilots Forum regarding this as well. Groundspeed data computed by the FMC, when adjusted for wind is within 1-2 knots airspeed. Obviously, there weren't any problems with the Static System.

It appears the reason for this whole thread is that a man named Frank Legge, a supposed chemist from Australia has been trying to discredit pilots who have questions regarding 9/11. His paper was posted a few pages back in this thread. You can read more from the pilots here with respect to this topic.

Frank Legge Begging For Peer Reviewers For Pentagon Paper - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?showtopic=21569)

Here are their list of members.
Pilots For Truth List Of Members (http://pilotsfor911truth.org/core)

However, the same man (Frank Legge) who is trying to discredit pilots, without being a pilot himself, claims the WTC was destroyed by Controlled Demolition. He seems to trust the data provided by the NTSB, but doesn't trust anything from the NIST. Go figure.


9/11 – Acceleration Study Proves Explosive Demolition
http://www.journalof911studies.com/volume/200611/911-Acceleration-Study-Proves-Explosive-Demolition.pdf

Frank Legge (Ph D)
Logical Systems Consulting
Perth, Western Australia.
[email protected]

[A320Slave]

As they are limited to 350 knots it seems unlikely that they would be calibrated much beyond that at any altitude. The 757 cruises at 500+ knots at altitude. You should learn the definition of equivalent airspeed. Dynamic pressure does not effect static pressure. Mach shockwaves, can (as pointed out above re: The Concorde). This is why the static system is based on Mach number and calibrated (errors removed) through an air data computer.

A320, you appear to be unaware that earlier in this thread it was pointed out that the ground elevation was determined at each position report, and added to the radalt height, so a proper radalt altitude was obtained. In the last few seconds before impact there are no buildings which can confuse the radalt. You should check these threads.

Warren Stutt Decode Shows Altitude too high to Impact Pentagon
http://pilotsfor911truth.org/forum/index.p...&p=10778240 (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=18239&view=findpost&p=10778240)

Warren Stutt's admitted lack of expertise with respect to FDR Investigation
http://pilotsfor911truth.org/forum/index.p...&p=10799563 (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=21131&view=findpost&p=10799563)

RA - PA Correlation, proving the "Altitude Divergence" calculated by Legge/Stutt was due to RA measuring from an object higher than ground level. Fatal to the Legge/Stutt argument.
http://pilotsfor911truth.org/forum/index.p...&p=10794074 (http://pilotsfor911truth.org/forum/index.php?s&showtopic=20999&view=findpost&p=10794074)

If Legge/Stutt "Altitude Divergence" calculations were correct, Aircraft would be slamming into the ground. IAD ILS RWY 01R Approach Analysis, Instruments required for IFR Flight Based on Regulation.
http://pilotsfor911truth.org/forum/index.p...&p=10793490 (http://pilotsfor911truth.org/forum/index.php?s&showtopic=20960&view=findpost&p=10793490)

Calculations based on Stutt Theory with respect to RA Tracking Capability, proving Stutt's theory false.
http://pilotsfor911truth.org/forum/index.p...&p=10794159 (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=18239&view=findpost&p=10794159)

[skwinty]

gravity32,

The altitude calibration velocity would be at least Mach .8 at high altitude.
As I posted earlier.

488 knots at 500 ft is completely outside of the normal flight envelope.

This, as confirmed by A320slave, does not imply that the pitot static was out of its calibration envelope.

[gravity32]

A320

You appear to be using Pilots for 9/11 Truth as your authority. This group asserts that the plane could not have hit the Pentagon. As they base this belief largely on the high altitude shown in the FDR data file, and as they reject the validity of the radalt data, which shows the plane hitting the Pentagon, this thread has been discussing the question of which is more reliable.

Can you explain why you personally think the pressure altitude is more reliable than the radalt?

[A320Slave]

You appear to be using Pilots for 9/11 Truth as your authority

They are not "my authority". I understand the work because I am a pilot.

You appear to be using Frank Legge and Warren Stutt as your authority. Neither of which are a pilot and one who admits he doesn't have any expertise whatsoever in FDR Investigation.

"Some people have queried my credentials for investigating Flight Data Recorder (FDR commonly called "black box") data and any relevant affiliations I may have.

I do not have any specific credentials to investigate FDRs or aircraft accidents....

I am not affiliated with nor have I ever worked with or for neither the US National Transport Safety Board (NTSB) nor any other aircraft accident investigator.

I am neither a pilot nor an aircraft engineer and have never flown an aircraft. I am not affiliated with nor have I ever worked with or for any airline, any pilots’ organisation, any aircraft engineering organisation or any FDR manufacturer.

My investigation in to the events of September 11th 2001 is unofficial, independent and completely voluntary.

I was born in New Zealand and I have joint New Zealand and Irish citizenship. I lived in New Zealand up until June 2008 and since then have lived in Brisbane, Australia." - Warren Stutt (http://warrenstutt.com/AboutMyFDRWork/index.html)

Not even the NTSB has confirmed the "decode" done by Stutt. Have you?

As all the pilots here agree, the static system was not operating outside it's calibrated envelope of M0.86, standard for a 757. The airspeed - groundspeed analysis confirms this.

Another Question For Warren Stutt - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=18453&view=findpost&p=10781116)

This group asserts that the plane could not have hit the Pentagon.

I have not read that. Do you have a sourced quote?

What I have read is this,

We have never made such a claim. Matter of fact, i have repeatedly stated on interviews, right here on this forum, and elsewhere that the FDR files are not "proof" of anything. I have told this to Legge each time he has brought up such a strawman. Clearly Legge prefers Tactics Of Truth Suppression (http://pilotsfor911truth.org/forum/index.php?showtopic=21015)

What we do claim, is that the data provided by the NTSB does not support an impact. This is a factual statement, for either the NTSB decode, or Stutt's "additional" data.

<snip>

The FDR data does not support the govt story in many significant ways including but not limited to "AA77" impact with the Pentagon, nor is there any evidence linking the data to "AA77", N644AA. When contacted, the NTSB/FBI refuse to comment.



Source - A Response To Frank Legge And Warren Stutt - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=21025&view=findpost&p=10799418)

[skwinty]

If there was a conspiracy theory sub forum here, I would say this thread is heading in that direction.:suspect:

[A320Slave]

If there was a conspiracy theory sub forum here, I would say this thread is heading in that direction.Hopefully it won't go too far down that road.

To sum up and continue on topic.

gravity32 asks -

"Can you explain why you personally think the pressure altitude is more reliable than the radalt?"

MadFLT already answered this back on post 15.

You really can't compare the two. they aren't even really measuring the same thing. Depending on what you actually want to know, either may be better/"more accurate". http://www.pprune.org/6697454-post15.html

When Radalt is compared to True Altitude (MSL) over a known object (such as the runway), Radalt is within 2-8 feet of True Altitude once runway elevation is added to radalt, as would be expected.

Flight Data Expert Confirmation: No Evidence Linking Fdr Data To American 77 - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?showtopic=20999&st=80&p=10794074&#entry10794074)

For some reason, Frank Legge omitted this from his paper and continues to dodge such analysis. It is obvious why he does, as it is fatal to his argument with respect to an "altitude divergence" along the approach corridor for the approaches cited in his paper. It is clear the divergence between radalt and pressure alt was due to the radalt measuring from the tops of buildings, etc, till the aircraft got over the runway threshold.

Also for gravity32, the reason you do not see a step in the altitude data at FL180 from the FDR files, is because the FDR records Pressure Altitude only. ie. 29.92. You must adjust the Pressure Altitude data using local barometric pressure to get a True Altitude (MSL). The Baro Cor column in the FDR data has this information. A detailed explanation of this can be found here with respect to the CSV files supplied by the NTSB vs the Animation reconstruction done by the NTSB.

Flight Data Recorder Research Team Presents... - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=15&view=findpost&p=992727)

[skwinty]

I hope it doesn't head in that direction as well. The no planers theories are beyond comprehension..

gravity32's question was answered in post 4 already.:\

[gravity32]

A320,
I notice you did not provide your reason for preferring the pressure to the radalt, just attacked some authors. You quote Pilots for 9/11 Truth: "What we do claim, is that the data provided by the NTSB does not support an impact. This is a factual statement, for either the NTSB decode, or Stutt's "additional" data."

This is only "factual" if you belief the pressure but reject the radalt. So I ask again why do you reject the radalt? There are no buildings near the impact point to interfere with radalt.

[A320Slave]

A320,
I notice you did not provide your reason for preferring the pressure to the radalt, Where did I claim I prefer one over the other? It depends on what you are measuring as to which instrument to use. The Radalt isn't even required for IFR flight but a pressure altimeter is. Why does the FAA require the use of a Sensitive Pressure Altimeter over a Radalt for IFR flight? When you understand this question, you will understand the answers you have been given here from pilots.

Why do you prefer radalt over pressure altitude when the NTSB lists radalt as ":not working or unconfirmed", the radalt is traveling way outside it's tracking capability of 330 fps, and the objects it is measuring from is unknown?

Furthermore, the NTSB lists Pressure Altitude as "working and confirmed" and is operating well within it's calibration rated to M0.86.

Again, read here for a more thorough and detailed explanation.

RA - PA Correlation, proving the "Altitude Divergence" calculated by Legge/Stutt was due to RA measuring from an object higher than ground level. Fatal to the Legge/Stutt argument.
http://pilotsfor911truth.org/forum/index.p...&p=10794074 (http://pilotsfor911truth.org/forum/index.php?s&showtopic=20999&view=findpost&p=10794074)

If Legge/Stutt "Altitude Divergence" calculations were correct, Aircraft would be slamming into the ground. IAD ILS RWY 01R Approach Analysis, Instruments required for IFR Flight Based on Regulation.
http://pilotsfor911truth.org/forum/index.p...&p=10793490 (http://pilotsfor911truth.org/forum/index.php?s&showtopic=20960&view=findpost&p=10793490)

Calculations based on Stutt Theory with respect to RA Tracking Capability, proving Stutt's theory false.
http://pilotsfor911truth.org/forum/index.p...&p=10794159 (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=18239&view=findpost&p=10794159)

Those are not attacks, they are facts.

Do you feel there is an altitude divergence due to pressure altimeter errors at ORD, LAX and the other approaches as stated by Frank Legge in his paper? Legge is wrong and the above links prove it. Do you understand that the "altitude divergence" disappears when the aircraft enters over the touchdown zone? Do you know why this is? Once you understand all of the above, you will understand the answers you have been given.

[ChristiaanJ]

....the radalt is traveling way outside it's tracking capability of 330 fps....
Either you are suggesting the a/c was descending at 'way outside' 20 000 fpm, or you don't understand the notion of radalt 'tracking capability'.
I assume the latter.

The rest of your comments about pressure altimeters demonstrate you don't even understand that concept.

CJ

[A320Slave]

"Either you are suggesting the a/c was descending at 'way outside' 20 000 fpm, or you don't understand the notion of radalt 'tracking capability'.
I assume the latter."

If you are claiming the tracking capability is based on terrain closure, ie, in the vertical, when flying at 330 fps and level, anything with a rise in terrain of 45 deg slope will out run the tracking capability. This includes buldings which have 90 degree "slopes".

The faster you go, the less slope in terrain that will be required to outrun the tracking capability.

Again, it is all explained here.
Aal77 Fdr Decoder Program - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?showtopic=18239&st=200&p=10794159&#entry10794159)

You don't have to be descending for terrain to rise to you. The speed at which terrain rises to your altitude is a function of your forward speed.

The tracking capability is based on forward motion of the aircraft. These radalt's are used for low and slow approaches. They are not TFR radalts as used by Fighter and Attack aircraft following terrain.

As for pressure altimeters, which comment in particular are you referring to? That they are required for IFR flight and a radlat is not? Do you feel the static system in a standard 757 is not calibrated to .86 mach?

[gravity32]

A320,
"It depends on what you are measuring as to which instrument to use. The Radalt isn't even required for IFR flight but a pressure altimeter is. Why does the FAA require the use of a Sensitive Pressure Altimeter over a Radalt for IFR flight? When you understand this question, you will understand the answers you have been given here from pilots."

Well it is obvious - if you only have one instrument it has to be the altimeter as radalt does not work above 2500 feet above ground level.

"Why do you prefer radalt over pressure altitude when the NTSB lists radalt as ":not working or unconfirmed","

The radalt appears to be working as ground proximity warning was working. Only radalt provides that.

"the radalt is traveling way outside it's tracking capability of 330 fps,"

The vertical speed was much less than 330 fps.

" and the objects it is measuring from is unknown?"

You can see what is was measuring from by counting back from the impact point where the longitudinal acceleration went strongly negative. There are no buildings there to confuse it. I think you are confused. How do you account for the strong longitudinal acceleration if the plane did not hit?

"Furthermore, the NTSB lists Pressure Altitude as "working and confirmed" and is operating well within it's calibration rated to M0.86."

We have covered this. At high altitude the pressure altimeter would no doubt be calibrated to M 0.86 but at low altitude the plane is not allowed to fly that fast so why would it be calibrated to M 0.86? In fact, how could they do the low altitude tests to determine the calibration errors at speeds beyond which the plane is permitted to fly?

I notice that you are very adept at finding statements on the Pilots for 9/11 Truth website. I suspect you are a member. I suspect you personally think the plane flew over the Pentagon. Do you think the last radalt recording was from the roof of the Pentagon?

http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/statusicon/user_offline.gif http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/report.gif (http://www.pprune.org/report.php?p=6703114) http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/reply_small.gif (http://www.pprune.org/newreply.php?do=newreply&p=6703114&noquote=1)

[A320Slave]

"the radalt is traveling way outside it's tracking capability of 330 fps,"

The vertical speed was much less than 330 fps.Radalt tracking capability is not based on the vertical closure rate with terrain. It is based on the forward motion of the aircraft. Forward motion of the aircraft, combined with slope of terrain, determines aircraft closure rate with the ground. If your theory were correct, you would not get a GPWS on terrain sloping up to you which has a more than a 45 degree slope if you were flying level at 330 fps (roughly 195 knots).

If you were flying level at 400 knots, the terrain will outrun the radalt tracking capability on a mere 27 degree slope, if your theory were correct. It's not.

Furthermore, the usual angular limits (not tracking capability) of a radalt are 30 deg bank and 15 deg pitch. In other words, you will not get a Radlt reading if you are descending over 15 degree pitch or rolling into more than a 30 deg bank.

At 330 fps forward speed (~ 195 knots) and more than 5,200 fpm (roughly 85 fps descent rate) you will not get a radalt reading as this is more than a 15 degree angle and the radalt is no longer "painting" the ground, it is shooting off to the horizon due to descent angle.. Someone mentioned these limits earlier in this thread.

[Piltdown Man]

Gravity - I wouldn't bother with your mission. Because when dealing with conspiracy prats, they'll tell you that the data has been 'got at' or that someone else has snuck into Boeing and changed the calibration specs. Remember, when you start arguing with idiots, they'll drag you down to their level and beat you with experience. Instead, I'd set them off looking for the 'Extra-Terrestrials' who were really flying the plane or along the line that the whole thing was organised by those who had the most to gain - the security industry.

PM

[A320Slave]

gravity32,

You appear to be using much of the same arguments as Frank Legge and Warren Stutt.

Do you agree with Frank Legge that the WTC was destroyed by controlled demolition? Specifically thermite?

Is this you gravity32?
OpEdNews - Articles - Author's Page for Gravity32 (http://www.opednews.com/author/author5412.html)

Do you agree with Frank Legge that the pressure altimeter was in error when approaching ORD, LAX, MCO and IAD?

Have you figured out that when the aircraft is over the touchdown zone at the above airports, there is no "altitude divergence" (as described by Legge) between pressure altitude and Radalt?

[gravity32]

I have been aware of the paper by Legge and Stutt for some time. I found the paper to be logical. I saw that the discrepancy between the radalt and the pressure data was so large that even if they had made some error in calculation, there was no way the two readings could be in agreement. So it boils down to which is the more reliable.

Everyone on this thread confirms that the radalt will be accurate in this case because the plane is not tilted, is not closing too rapidly with the terrain and, close to impact, is not passing over significant buildings or steeply sloping ground.

Everyone on this thread assures me that there are numerous reasons why the pressure data could be corrupted. The plane is certainly flying faster that its permitted speed, and that is a likely cause of error.

So it seems rather odd that you favour the pressure altitude over radalt.

I remind you that the people on this thread are mostly experienced pilots. I suggest you reconsider the evidence, consult with pilots and mathematicians who are not members of Pilots for 9/11 Truth, and see what you can come up with if you start afresh.

[A320Slave]

I think you misunderstood the posts made on this thread with respect to radalt being more "accurate" than Pressure altitude.. Here they are again -

skwinty - post 4 -
The cause is that they are two different types of instruments that operate on totally different principles and measure two distinctly different parametersMadFLT - post 14 -
Radalt measures distance to an object giving a radar return.

Altimeter uses pressure data to infer height above a reference datum.

You really can't compare the two. they aren't even really measuring the same thing. Depending on what you actually want to know, either may be better/"more accurate". When you offered the speed on page 4 -

skwinty - post 63 -
If the computer performs the calibration in flight, then the speed envelope should cover at least Mach .8 so the graph would show that as the upper velocity range.ChristiannJ aka CJ - post 65 -
But one well-known phenomenon on Concorde occurs when 'breaking the sound barrier', or better expressed, when accelerating past Mach 1.
A shock wave then moves from the nose along the fuselage past the static ports.
It's barely visible on the altimeter, but there is a very noticeable 'twitch' on the VSI (there are videos of that, and we've even managed to simulate it on the Brooklands Concorde simulator).(added bold above)

Me - post 66 -
the static systems on large transport aircraft are rated to the Mmo of that aircraft and corrected by the air data computer. This is why you will read a calibrated airspeed on most large transports and not an Indicated Airspeed. So the staitic system would be rated to .86 mach. To understand the reasons for Vmo/Mmo click here...

Boeing 757 Boeing :: Vmo/Mmo Limitations Review (http://www.biggles-software.com/software/757_tech/flight_instruments/_popup_for_40_vmo_mmo.htm)Me - post 68 -
The aircraft was operating well outside it's normal flight envelope for structural and stability purposes for a standard 757, I agree. See more here...

9/11: World Trade Center Attack Speed Analysis (http://pilotsfor911truth.org/wtc_speed)

and here...

Evidence Strengthens To Support WTC Aircraft Speed Analysis (http://pilotsfor911truth.org/wtc_speed_part2.html)

But the aircraft was well within the Pitot-Static calibration parameters rated to .86 Mach. (added bold above)

CJ - post 69 -
My back-of-the-envelope says M=0.74, assuming ISA.

Seems to be too far below M=0.86 to really matter(added bold)

skwinty - post 73 -
488 knots at 500 ft is completely outside of the normal flight envelope.

This, as confirmed by A320slave, does not imply that the pitot static was out of its calibration envelope. (added bold)

Gravity32, if you feel the static system was experiencing error at M0.74, why wasn't the airspeed experiencing some type of error when Ram air is used to measure against static pressure for an airspeed indicator? According to you, the airspeed indicator should have disintegrated (or perhaps shown some type of error) at such high dynamic pressure. Unlike the Altimeter and VSI, the airspeed indicator has RAM AIR pushing directly into the system. But yet, it is working fine and correlates with groundspeed. It is working fine because the Pitot-static system is rated to M0.86.

Finally, you failed to answer my questions gravity32.

Please respond to these questions.

Do you agree with Frank Legge that the WTC was destroyed by controlled demolition? Specifically thermite? Yes or no.

Is this you gravity32? Yes or no.
OpEdNews - Articles - Author's Page for Gravity32 (http://www.opednews.com/author/author5412.html)

Do you agree with Frank Legge that the pressure altimeter was in error when approaching ORD, LAX, MCO and IAD? Yes or no.

Have you figured out that when the aircraft is over the touchdown zone at the above airports, there is no "altitude divergence" (as described by Legge) between pressure altitude and Radalt? Yes or no.

The above questions only require a Yes or no answer. Please respond to them and stop your tap dancing.

[gravity32]

A320,

Everyone but you on this thread confirms that the radalt will be accurate in this case because the plane is not tilted, is not closing too rapidly with the terrain and, close to impact, is not passing over significant buildings or steeply sloping ground. It was also proved reliable by checking every landing on the FDR file, and found to give -6 feet in every case while taxiing.

When you deal effectively with this, that the radalt is reliable, it will be time to discuss other matters.

[A320Slave]

Everyone but you on this thread confirms that the radalt will be accurate in this case because the plane is not tilted, is not closing too rapidly with the terrain and, close to impact, is not passing over significant buildings or steeply sloping ground. It was also proved reliable by checking every landing on the FDR file, and found to give -6 feet in every case while taxiing. You started this thread claiming that the Pressure altimeter would not be accurate operating "outside of it's envelope".

Many agreed. I would agree too.

Then you posted the actual speed on page 4. See the above quotes. The Pitot-Static system was operating well within it's capability without error and is confirmed with airspeed v. groundspeed analysis as pointed out in this thread.

Do you think that Radalt only measures from the ground? What happens if a building is on the ground? Does the Radalt measure through that building to the ground?

If you feel Radalt tracking capability is based on vertical speed, how does that apply when crossing over multiple obstacles with a 90 degree side? Do you know basic trigonometry?

Perhaps this is the reason you do not wish to answer my questions?
Frank Legge Begging For Peer Reviewers For Pentagon Paper - Pilots For 9/11 Truth Forum (http://pilotsfor911truth.org/forum/index.php?s=&showtopic=21569&view=findpost&p=10801255)

(also, if you want to talk amongst real pilots, try not to use the word "tilted".... rather bank, roll, pitch, yaw, crab... etc.... these are the terms real pilots use.)

[gravity32]

Do you think that Radalt only measures from the ground? What happens if a building is on the ground? Does the Radalt measure through that building to the ground?A320,
You clearly know enough about this crash to know that there were no buildings other than the Citgo service station in the vicinity. How high is that? It provides little more than a roof over the heads of customers. Your response is clearly an attempt to sidestep the reality. The reality is that the radalt can be trusted in this case.

If you feel Radalt tracking capability is based on vertical speed, how does that apply when crossing over multiple obstacles with a 90 degree side? Do you know basic trigonometry?You know enough about radalt to know that a plane passing over a city with many buildings with vertical sides has no problem providing correct guidance to pilots when they arrive at the airport. Again your response is just a sidestep.

if you want to talk amongst real pilots, try not to use the word "tilted".... rather bank, roll, pitch, yaw, crab... etc.... these are the terms real pilots use.) You don't have to be a pilot to know that "tilted" in one word covers all these angles. Again your response is a sidestep.

What we see is you making strenous efforts to sidestep the issue, that radalt can be trusted in this situation. It reminds me of the language I see at Pilots for 9/11 Truth. You should be careful who you copy.

[A320Slave]

graivity32,

For anyone who clicks my links combined with reading your posts here, they will understand you opened this thread to gain support of pressure altimeter error operating "outside pressure altimeter calibration".

After you disclosed the speed on page 4, every single pilot on this thread concluded that the pressure altimeter was not operating outside of it's calibrated envelope.

So let us put this into perspective..

The Radalt was operating more than 2x outside of it's tracking capability according to the data.

You seem to think that the tracking capability is based on vertical closure rate with terrain and/or any obstacles on the terrain. You are wrong. Trigonometry proves you wrong.

Here is another lesson in aviation terminology. When one talks of "tracking" or "track", it is the track of the aircraft over the ground. Not the vertical speed of the aircraft..

For example, according to your claim that radalt tracking capability is based on vertical speed, if the aircraft were traveling at 250 knots (422 fps forward speed) with a descent rate of 320 fps (19,200 fpm), you are implying that the radalt could track this measurement because the vertical descent rate is less than 330 fps tracking capability..

Pilots here will understand how absurd this notion is as the angle of descent is almost 50 degrees.

Given the links I have provided above, and the replies given by the pilots here, I guarantee you will not find one pilot to sign their name to your next paper, just like you have not been able to find one to sign their name to your last 3 papers over the past 3 years, including your papers prior which claim the WTC was demolished by the use of themate.

You will not gain "influence" among the circles you are attempting to influence. You lose credibility each time you attempt to discredit real and verified pilots. Keep up the good work!

[gravity32]

A320,For anyone who clicks my links combined with reading your posts here, they will understand you opened this thread to gain support of pressure altimeter error operating "outside pressure altimeter calibration". True, but I soon learned here that most of these experienced pilots agreed that there was no reason to suspect the radalt would be wrong, and therefore it must have been the pressure altitude that was wrong.


After you disclosed the speed on page 4, every single pilot on this thread concluded that the pressure altimeter was not operating outside of it's calibrated envelope.False. Obviously it would be calibrated at the observed speed, 488 knots, at high altitude, but at 50 feet above sea level the plane couldn't be legally flown, so couldn't be test flown, so one would think it couldn't be calibrated. Can you explain how it could be calibrated at a speed far above its legal maximum speed? What grounds are there for claiming that a calibration in thin high altitude air can be applied in dense sea level air?

So let us put this into perspective.. The Radalt was operating more than 2x outside of it's tracking capability according to the data.You have not provided any source for your claim that the manufacturer's limitation is for horizontal velocity. The plane only moves a tiny fraction of a millimeter in the time it takes for the signal to reach the ground and be reflected back. How can there be any limit to horizontal capability? One concludes this is a false claim.

For example, according to your claim that radalt tracking capability is based on vertical speed, if the aircraft were traveling at 250 knots (422 fps forward speed) with a descent rate of 320 fps (19,200 fpm), you are implying that the radalt could track this measurement because the vertical descent rate is less than 330 fps tracking capability..Pilots here will understand how absurd this notion is as the angle of descent is almost 50 degrees.

The plane was only descending at about 50 feet per second, 3000 feet per minute. I think pilots here will agree that they can safely depend on the radalt at that rate of descent and will find your discussion of an "almost 50 degree" angle irrelevant.

By the way, are you the same A320Slave who came to this site in 2008 to try to get people to agree that AA77 would not have been able to get its position reports correct if it started its flight without entering its exact position prior to moving? Did you not believe John Farmer when he provided on overlay of the radar track and the FDR track, which showed the two tracks converging after the plane had been in the air for a few minutes, and presumably in reach of DME and VOR signals?

[A320Slave]

Now that I have learned who "gravity32" actually is, It is pointless to continue this discussion beyond this reply.

Gravity32 -

You make the statement -

"You have not provided any source for your claim that the manufacturer's limitation is for horizontal velocity."

Gravity32, you have made the claim that 330 fps tracking capability is based on vertical speed and have not provided any source for your claim. 330 fps is roughly 20,000 fpm.

Please let us know when you find one real and verified pilot who will agree with you that a 757 radalt can accurately measure True Altitude, absolute altitude, any altitude... at anything below 20,000 fpm (330 fps) descent rate.

To keep this even more simple, please let us know when you find one pilot who will endorse your claims that a radalt can measure precisely at a descent rate of perhaps 10,000 fpm. Less than half the radalt tracking capability according to your vertical speed theory..

I believe someone above mentioned, "Never wrestle with a pig, they'll only drag you down to their level".

I agree wholeheartedly.

Gravity32, anytime you wish to answer my questions and/or get a real and verified pilot to support your absurd claims with respect to aviation, including your WTC claims of controlled demolition based on thermate, let us know. Until then, this argument is like (to a layman) a 45 year old arguing with a 12 yr old on how to drive a car.

Gravity32 just does not have the basic knowledge. "tilted"... LMAO!

[gravity32]

A320SlavePlease let us know when you find one real and verified pilot who will agree with you that a 757 radalt can accurately measure True Altitude, absolute altitude, any altitude... at anything below 20,000 fpm (330 fps) descent rate.There is a great deal I don't know about aviation and I get some things wrong. I am happy to ask for advice. That is why I am here. But I do not take advice from just anyone.

For instance I would be reluctant to take advice from someone who thought that AA77 would experience an unsurvivable wing load of 10.14g as it pulled out of its dive on approach to the Pentagon. That is what the experts at Pilots for 9/11 Truth are telling the public. Several other people have calculated a wing load of less than 2g. That is very different. I have checked the calculation myself and have found it could be as low as 1.7g depending on the actual course taken. I have informed Rob Balsamo of the error in his calculation but he makes no change to the site and makes no apology to the public for missleading them. That is why I seek advice elsewhere.

Please let me know when you find one real and verified pilot who thinks radalt will not be accurate for a plane at a descent speed of 50 ft per second and levelling to almost level at the last radalt reading, not flying over buildings, not at a steep bank, pitch, or yaw, when only a few feet above the ground.

[john_tullamarine]

We now appear to be going around in ever decreasing circles.