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altimeter calibration

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altimeter calibration

Old 13th Sep 2011, 14:56
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altimeter calibration

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?
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Old 13th Sep 2011, 15:10
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Why do you want to be flying a 757 at high speed (i.e. outside calibration envelope) below 2,500 ft agl?
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Old 13th Sep 2011, 15:33
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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.
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Old 13th Sep 2011, 15:48
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The cause is that they are two different types of instruments that operate on totally different principles and measure two distinctly different parameters.
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Old 13th Sep 2011, 17:02
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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.
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Old 13th Sep 2011, 17:23
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gravity32, try reading this link. It will have the same and more data that you are seeking.

Pitot-Static Instrument Calibration

Your questions kept referring to the differences between radalt and pressure altitude so I just wanted to remind you to not conflate the two.
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Old 13th Sep 2011, 17:34
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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.
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Old 13th Sep 2011, 17:42
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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.

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Old 13th Sep 2011, 17:47
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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.
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Old 13th Sep 2011, 17:56
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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..
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Old 13th Sep 2011, 17:57
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Originally Posted by Lightning Mate View Post
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.
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Old 13th Sep 2011, 17:59
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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.
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Old 13th Sep 2011, 18:20
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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?
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Old 13th Sep 2011, 18:31
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That is exactly why you should use your pressure altimeter at altitude and your radalt for ground proximity at low altitude.

Horses for courses.
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Old 13th Sep 2011, 18:46
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Originally Posted by gravity32 View Post
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".
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Old 13th Sep 2011, 21:55
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a Boeing 757, was travelling at very high speed at low altitude
... another 9/11 conspiracy thing, then .
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Old 13th Sep 2011, 23:10
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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 ?
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Old 14th Sep 2011, 01:50
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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.
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Old 14th Sep 2011, 02:18
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@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 ....
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Old 14th Sep 2011, 03:28
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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.
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