altimeter calibration
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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.
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.
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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.
Do they suffer from serious altitude increases due to velocity induced pressure errors?
After all, aircraft have been going supersonic for years without computers.
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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.
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.
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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?
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?
Last edited by gravity32; 16th Sep 2011 at 16:43. Reason: Added last sentence
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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).
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.
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
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?
After all, aircraft have been going supersonic for years without computers.
Also, most of those aircraft carried a calibrated pitot-static 'boom'. Airliners don't.
CJ
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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
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
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
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
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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.
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.
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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
and here...
Evidence Strengthens To Support WTC Aircraft Speed Analysis
But the aircraft was well within the Pitot-Static calibration parameters rated to .86 Mach.
9/11: World Trade Center Attack Speed Analysis
and here...
Evidence Strengthens To Support WTC Aircraft Speed Analysis
But the aircraft was well within the Pitot-Static calibration parameters rated to .86 Mach.
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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.
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.
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My back-of-the-envelope says M=0.74, assuming ISA.
Seems to be too far below M=0.86 to really matter....
Seems to be too far below M=0.86 to really matter....
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
Here are their list of members.
Pilots For Truth List Of Members
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/v...Demolition.pdf
Frank Legge (Ph D)
Logical Systems Consulting
Perth, Western Australia.
[email protected]
http://www.journalof911studies.com/v...Demolition.pdf
Frank Legge (Ph D)
Logical Systems Consulting
Perth, Western Australia.
[email protected]
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As they are limited to 350 knots it seems unlikely that they would be calibrated much beyond that at any altitude.
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.
Warren Stutt Decode Shows Altitude too high to Impact Pentagon
http://pilotsfor911truth.org/forum/index.p...&p=10778240
Warren Stutt's admitted lack of expertise with respect to FDR Investigation
http://pilotsfor911truth.org/forum/index.p...&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
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
Calculations based on Stutt Theory with respect to RA Tracking Capability, proving Stutt's theory false.
http://pilotsfor911truth.org/forum/index.p...&p=10794159
Last edited by A320Slave; 16th Sep 2011 at 16:57.
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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.
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.
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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?
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?
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You appear to be using Pilots for 9/11 Truth as your authority
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
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
This group asserts that the plane could not have hit the Pentagon.
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
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.
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.
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If there was a conspiracy theory sub forum here, I would say this thread is heading in that direction.
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".
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
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
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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.
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.
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A320,
I notice you did not provide your reason for preferring the pressure to the radalt,
I notice you did not provide your reason for preferring the pressure to the radalt,
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
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
Calculations based on Stutt Theory with respect to RA Tracking Capability, proving Stutt's theory false.
http://pilotsfor911truth.org/forum/index.p...&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.