## Re: [Flightgear-devel] Is something amiss with altimeter settings?

 Re: [Flightgear-devel] Is something amiss with altimeter settings? From: Ron Jensen - 2010-05-30 18:44:25 ```Indeed. We are currently running atmosphere code written by John Denker, et al, that takes lapse rate into account. The reason pressure-sea-level-inhg doesn't match the altimeter setting is because it doesn't in real life. The altimeter setting number is chosen so the altimeter reads the field elevation when on the field. As David points out, if the sea-level temperature is not 15 degC. In your example[1]: 2010/05/29 15:54 KIND 291554Z 36004KT 10SM SCT250 27/18 A2994 RMK The local temperature is 27 degC (METAR) and local pressure is 28.9367 (/environment). KNID is 797 ft. MSL [2] and I assume your tower is 50 feet high, so local altitude is 847 ft. The equation for altimeter setting (QNH) is: QNH = (local_pressure^0.1903 + 1.313e−5•local_elevation)^(1/0.1903) [3] So: 29.84 = (28.94^0.1903 + 1.313e−5•847)^(1/0.1903) If you were in the UFO and descended to sea-level you should see /environment/pressure-inhg == /environment/sea-level-pressure-inhg when /position/altitude-ft == 0 Thanks, Ron [1] http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg [2] http://www.airnav.com/airport/KIND [3] http://www.av8n.com/physics/altimetry.htm#eq-qnh Other links on this subject: http://www.aviationbanter.com/showthread.php?t=16985 http://www.airliners.net/aviation-forums/tech_ops/read.main/44629/ On Saturday 29 May 2010 12:24:19 David Megginson wrote: > It's been a long time, but in addition to any issues with your > altimeter setting I *think* I might have added code ~8 years ago to > simulate the effect of temperature on altimeter readings. In real > life, even if you have exactly the correct altimeter setting for the > ground below you, and the altimeter is perfectly calibrated, your > altitude can be off by hundreds or thousands of feet from what you see > on the altimeter. > > Generally speaking, you'll be lower than you think if the temperature > is below ISA, and higher than you think if the temperature is above > ISA, because temperature affects how much the air pressure changes > with altitude. The altimeter is calibrated to assume 15 degC at sea > level and a perfect consistent temperature gradient above that. > > > All the best, > > > David > > > In Canada, we're required to increase minimum altitudes for > approaches, etc., when temperatures are cold. We also have designated > mountainous regions where you have to fly higher in the winter. > > On Sat, May 29, 2010 at 1:02 PM, Rob Shearman, Jr. wrote: > > Hi there -- > > > > Recently while flying with the MD-81 at cruise levels of FL330 or so, I > > noticed there were some significant discrepancies between the displayed > > altitude and the altitude found in the property tree at > > /position/altitude-ft.  I was able to observe the same problem in the > > 777-200ER -- I flew a flight at 33000 and the Flight Tracker reported my > > altitude at close to 35000.  So the discrepancy at cruise is as much as > > 1000-2000 feet sometimes, even when using the altimeter setting reported > > in the METAR (which, of course, you're not supposed to do above 18,000 in > > the U.S., but for testing purposes I did so to see if it accounted for > > the error). > > > > However, today loading up my custom ATC scope I observed a discrepancy in > > altimeter readings which may account for the problem. > > > > http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg > > > > Notice in the shot that the METAR is reporting 29.94, but the property > > tree and the scope panel are arriving at 29.90.  I presume at high > > altitude this discrepancy would account for differences in the gauge > > reading. > > > > This is with the 25 April CVS build, so if altimeter code has changed > > since the migration to git, I apologize in advance. > > > > Thanks, > > -R. (MD-Terp) > > > > Robert M. Shearman, Jr. > > Transit Operations Supervisor, > > University of Maryland Department of Transportation ```

 [Flightgear-devel] Is something amiss with altimeter settings? From: Rob Shearman, Jr. - 2010-05-29 17:02:10 Attachments: Message as HTML ```Hi there -- Recently while flying with the MD-81 at cruise levels of FL330 or so, I noticed there were some significant discrepancies between the displayed altitude and the altitude found in the property tree at /position/altitude-ft. I was able to observe the same problem in the 777-200ER -- I flew a flight at 33000 and the Flight Tracker reported my altitude at close to 35000. So the discrepancy at cruise is as much as 1000-2000 feet sometimes, even when using the altimeter setting reported in the METAR (which, of course, you're not supposed to do above 18,000 in the U.S., but for testing purposes I did so to see if it accounted for the error). However, today loading up my custom ATC scope I observed a discrepancy in altimeter readings which may account for the problem. http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg Notice in the shot that the METAR is reporting 29.94, but the property tree and the scope panel are arriving at 29.90. I presume at high altitude this discrepancy would account for differences in the gauge reading. This is with the 25 April CVS build, so if altimeter code has changed since the migration to git, I apologize in advance. Thanks, -R. (MD-Terp) Robert M. Shearman, Jr. Transit Operations Supervisor, University of Maryland Department of Transportation also known as rmsjr@... ```
 Re: [Flightgear-devel] Is something amiss with altimeter settings? From: Alex Perry - 2010-05-29 17:16:07 ```On Sat, May 29, 2010 at 10:02 AM, Rob Shearman, Jr. wrote: > Hi there -- > > Recently while flying with the MD-81 at cruise levels of FL330 or so, I > noticed there were some significant discrepancies between the displayed > altitude and the altitude found in the property tree at > /position/altitude-ft.  I was able to observe the same problem in the > 777-200ER -- I flew a flight at 33000 and the Flight Tracker reported my > altitude at close to 35000.  So the discrepancy at cruise is as much as > 1000-2000 feet sometimes, even when using the altimeter setting reported in > the METAR (which, of course, you're not supposed to do above 18,000 in the > U.S., but for testing purposes I did so to see if it accounted for the > error). > > However, today loading up my custom ATC scope I observed a discrepancy in > altimeter readings which may account for the problem. > > http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg > > Notice in the shot that the METAR is reporting 29.94, but the property tree > and the scope panel are arriving at 29.90.  I presume at high altitude this > discrepancy would account for differences in the gauge reading. I don't understand the test you're doing. Did you manually set the instrument altimeter setting as 29.94 (copying the metar to the knob) and are now noticing that, despite doing that, FGFS internally reports what you did as 29.90? Or are you expecting FGFS to automatically copy the metar setting into the instrument, and are disappointed that it isn't doing so? > This is with the 25 April CVS build, so if altimeter code has changed since > the migration to git, I apologize in advance. > > Thanks, > -R. (MD-Terp) > > Robert M. Shearman, Jr. > Transit Operations Supervisor, > University of Maryland Department of Transportation > also known as rmsjr@... > > > ------------------------------------------------------------------------------ > > > _______________________________________________ > Flightgear-devel mailing list > Flightgear-devel@... > https://lists.sourceforge.net/lists/listinfo/flightgear-devel > > ```
 Re: [Flightgear-devel] Is something amiss with altimeter settings? From: David Megginson - 2010-05-29 18:32:12 ```It's been a long time, but in addition to any issues with your altimeter setting I *think* I might have added code ~8 years ago to simulate the effect of temperature on altimeter readings. In real life, even if you have exactly the correct altimeter setting for the ground below you, and the altimeter is perfectly calibrated, your altitude can be off by hundreds or thousands of feet from what you see on the altimeter. Generally speaking, you'll be lower than you think if the temperature is below ISA, and higher than you think if the temperature is above ISA, because temperature affects how much the air pressure changes with altitude. The altimeter is calibrated to assume 15 degC at sea level and a perfect consistent temperature gradient above that. All the best, David In Canada, we're required to increase minimum altitudes for approaches, etc., when temperatures are cold. We also have designated mountainous regions where you have to fly higher in the winter. On Sat, May 29, 2010 at 1:02 PM, Rob Shearman, Jr. wrote: > Hi there -- > > Recently while flying with the MD-81 at cruise levels of FL330 or so, I > noticed there were some significant discrepancies between the displayed > altitude and the altitude found in the property tree at > /position/altitude-ft.  I was able to observe the same problem in the > 777-200ER -- I flew a flight at 33000 and the Flight Tracker reported my > altitude at close to 35000.  So the discrepancy at cruise is as much as > 1000-2000 feet sometimes, even when using the altimeter setting reported in > the METAR (which, of course, you're not supposed to do above 18,000 in the > U.S., but for testing purposes I did so to see if it accounted for the > error). > > However, today loading up my custom ATC scope I observed a discrepancy in > altimeter readings which may account for the problem. > > http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg > > Notice in the shot that the METAR is reporting 29.94, but the property tree > and the scope panel are arriving at 29.90.  I presume at high altitude this > discrepancy would account for differences in the gauge reading. > > This is with the 25 April CVS build, so if altimeter code has changed since > the migration to git, I apologize in advance. > > Thanks, > -R. (MD-Terp) > > Robert M. Shearman, Jr. > Transit Operations Supervisor, > University of Maryland Department of Transportation > also known as rmsjr@... > > > ------------------------------------------------------------------------------ > > > _______________________________________________ > Flightgear-devel mailing list > Flightgear-devel@... > https://lists.sourceforge.net/lists/listinfo/flightgear-devel > > ```
 Re: [Flightgear-devel] Is something amiss with altimeter settings? From: syd adams - 2010-05-29 19:39:20 Attachments: Message as HTML ```Hi Rob , Im not sure if I understand what you mean. The 777 uses the instrumentation/altimeter/indicated-altitude-ft to run the altimeter settings .... I havent checked to see how it deals with the reported MET pressure ... but it looks like you already know that above 17999 ft you set the altimeter to 29.92 ... and ATC should be receiving the aircraft's altitude from its transponder ... its not very likely that your at the reported altitude unless you have a perfect ICAO day ... Or am I missing the point entirely ? Cheers On Sat, May 29, 2010 at 10:02 AM, Rob Shearman, Jr. wrote: > Hi there -- > > Recently while flying with the MD-81 at cruise levels of FL330 or so, I > noticed there were some significant discrepancies between the displayed > altitude and the altitude found in the property tree at > /position/altitude-ft. I was able to observe the same problem in the > 777-200ER -- I flew a flight at 33000 and the Flight Tracker reported my > altitude at close to 35000. So the discrepancy at cruise is as much as > 1000-2000 feet sometimes, even when using the altimeter setting reported in > the METAR (which, of course, you're not supposed to do above 18,000 in the > U.S., but for testing purposes I did so to see if it accounted for the > error). > > However, today loading up my custom ATC scope I observed a discrepancy in > altimeter readings which may account for the problem. > > http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg > > Notice in the shot that the METAR is reporting 29.94, but the property tree > and the scope panel are arriving at 29.90. I presume at high altitude this > discrepancy would account for differences in the gauge reading. > > This is with the 25 April CVS build, so if altimeter code has changed since > the migration to git, I apologize in advance. > > Thanks, > -R. (MD-Terp) > > Robert M. Shearman, Jr. > Transit Operations Supervisor, > University of Maryland Department of Transportation > also known as rmsjr@... > > > > > ------------------------------------------------------------------------------ > > > _______________________________________________ > Flightgear-devel mailing list > Flightgear-devel@... > https://lists.sourceforge.net/lists/listinfo/flightgear-devel > > ```
 Re: [Flightgear-devel] Is something amiss with altimeter settings? From: Ron Jensen - 2010-05-30 18:44:25 ```Indeed. We are currently running atmosphere code written by John Denker, et al, that takes lapse rate into account. The reason pressure-sea-level-inhg doesn't match the altimeter setting is because it doesn't in real life. The altimeter setting number is chosen so the altimeter reads the field elevation when on the field. As David points out, if the sea-level temperature is not 15 degC. In your example[1]: 2010/05/29 15:54 KIND 291554Z 36004KT 10SM SCT250 27/18 A2994 RMK The local temperature is 27 degC (METAR) and local pressure is 28.9367 (/environment). KNID is 797 ft. MSL [2] and I assume your tower is 50 feet high, so local altitude is 847 ft. The equation for altimeter setting (QNH) is: QNH = (local_pressure^0.1903 + 1.313e−5•local_elevation)^(1/0.1903) [3] So: 29.84 = (28.94^0.1903 + 1.313e−5•847)^(1/0.1903) If you were in the UFO and descended to sea-level you should see /environment/pressure-inhg == /environment/sea-level-pressure-inhg when /position/altitude-ft == 0 Thanks, Ron [1] http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg [2] http://www.airnav.com/airport/KIND [3] http://www.av8n.com/physics/altimetry.htm#eq-qnh Other links on this subject: http://www.aviationbanter.com/showthread.php?t=16985 http://www.airliners.net/aviation-forums/tech_ops/read.main/44629/ On Saturday 29 May 2010 12:24:19 David Megginson wrote: > It's been a long time, but in addition to any issues with your > altimeter setting I *think* I might have added code ~8 years ago to > simulate the effect of temperature on altimeter readings. In real > life, even if you have exactly the correct altimeter setting for the > ground below you, and the altimeter is perfectly calibrated, your > altitude can be off by hundreds or thousands of feet from what you see > on the altimeter. > > Generally speaking, you'll be lower than you think if the temperature > is below ISA, and higher than you think if the temperature is above > ISA, because temperature affects how much the air pressure changes > with altitude. The altimeter is calibrated to assume 15 degC at sea > level and a perfect consistent temperature gradient above that. > > > All the best, > > > David > > > In Canada, we're required to increase minimum altitudes for > approaches, etc., when temperatures are cold. We also have designated > mountainous regions where you have to fly higher in the winter. > > On Sat, May 29, 2010 at 1:02 PM, Rob Shearman, Jr. wrote: > > Hi there -- > > > > Recently while flying with the MD-81 at cruise levels of FL330 or so, I > > noticed there were some significant discrepancies between the displayed > > altitude and the altitude found in the property tree at > > /position/altitude-ft.  I was able to observe the same problem in the > > 777-200ER -- I flew a flight at 33000 and the Flight Tracker reported my > > altitude at close to 35000.  So the discrepancy at cruise is as much as > > 1000-2000 feet sometimes, even when using the altimeter setting reported > > in the METAR (which, of course, you're not supposed to do above 18,000 in > > the U.S., but for testing purposes I did so to see if it accounted for > > the error). > > > > However, today loading up my custom ATC scope I observed a discrepancy in > > altimeter readings which may account for the problem. > > > > http://i289.photobucket.com/albums/ll209/rmsjr1974/altimeter-discrep.jpg > > > > Notice in the shot that the METAR is reporting 29.94, but the property > > tree and the scope panel are arriving at 29.90.  I presume at high > > altitude this discrepancy would account for differences in the gauge > > reading. > > > > This is with the 25 April CVS build, so if altimeter code has changed > > since the migration to git, I apologize in advance. > > > > Thanks, > > -R. (MD-Terp) > > > > Robert M. Shearman, Jr. > > Transit Operations Supervisor, > > University of Maryland Department of Transportation ```
 Re: [Flightgear-devel] Is something amiss with altimeter settings? From: Rob Shearman, Jr. - 2010-05-31 03:11:39 Attachments: Message as HTML ```Ron Jenson: > The reason pressure-sea-level-inhg doesn't match the altimeter setting is because it doesn't in real life. > The altimeter setting number is chosen so the altimeter reads the field elevation when on the field. Even descending to ground level, it's not matching. So either something's still off or I'm just not understanding. In either case, I take it from your explanation that it's completely normal for an aircraft's altimeter, even if the "local" altimeter setting happens to be 29.92, to vary as much as a few thousand feet at cruise from their actual height above sea level -- and that the air temperature is the primary driving factor behind how much variation this will be. In the meantime, I do see that the property I ought to be getting the displayed number from (which is supposed to represent the local altimeter setting, for controllers to announce over the ATC channels at appropriate times) is not /environment/pressure-inhg, but /environment/metar/pressure-inhg. I originally copied a bunch of that code from the ATC-sets in CVS (now git), but I don't have those installed locally anymore... so I wonder if the same corrections need to be made in those as well. Thanks for the explanation / clarification... -R. (MD-Terp) Robert M. Shearman, Jr. Transit Operations Supervisor, University of Maryland Department of Transportation also known as rmsjr@... ```