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. <rmsjr74@yahoo.com> 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@umd.edu



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