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.
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.
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.
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.
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.
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, 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.
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.
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.
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.
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".
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 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.
Last edited by gravity32; 15th Sep 2011 at 13:04.
Reason: correct typo - 312.8 ft