> On Sun, 2007-02-11 at 00:22 -0500, John Denker wrote: >> Both the Weather Conditions popup and the atis.cxx code rely >> on the "pressure-sea-level-inhg" property and use it in ways >> that the altimeter setting should be used.
>> This is at least a misnomer, and probably a misconception. >> The altimeter setting is not the same thing as the sea-level >> pressure. The altimeter setting is something else; it is >> properly called the altimeter setting. On 02/11/2007 10:02 AM, Dave Perry wrote: > > The altimeter setting in the Kollsman window adjusts the displayed > altitude that the AC is from that standard pressure level. I can't parse that. There are numerous things that "from" could mean in that sentence. Certainly it doesn't mean "altitude relative to", which is what it seemed to mean the first time I read it. Successive readings have not > Quoting from the asa Pilot's Manual #3: > "If you want to measure the altitude of the airplane above the > 30.10 in. Hg pressure level, then you set 30.10 in the pressure > window. If 30.10 in. Hg happens to be the current MSL > barometric pressure, then the altimeter will indicate the > altitude of the airplane above MSL." OK ... assuming by "altitude" they mean "pressure altitude" not "true altitude" or "absolute altitude" or ....... > So the altimeter setting is the same thing as the mean-sea-level > barometric pressure. Huh? That does not follow at all. The quoted passage does not say that at all. Do you do all of your flying in an atmosphere with a standard temperature profile? In mountainous country it is extremely common to have stations that differ hardly at all in position to have greatly different altimeter settings, because they differ in altitude, and there is a non-standard dP/dh in the column between them. Maybe your theory says this cannot happen, but the fact is it does happen. This is discussed in detail at http://www.av8n.com/how/htm/atmo.html#sec-h-a-l-t The magnitude of the effect is easily computed using: http://www.av8n.com/how/htm/atmo.html#eq-HALT Ed Williams added this formula to his formulary. > That said, there may still be bugs. I noticed in working through the > details of altAlert in kap140.nas that once the altimeter setting > deviates far from 29.92 in. Hg that the the target altitude differs from > the altimeter display even when the baro setting and the altimeter > setting agree. > > Have you looked for bugs? When you get a chance, please read the main part of my previous message, the first part, the part not quoted above. It describes in detail how anyone can demonstrate the bug. It takes only a minute. If you've lost that message, you can get the same information from http://wiki.flightgear.org/flightgear_wiki/index.php?title=Bugs Another minute of analysis shows that the bug resulted from the fact that altimeter.cxx was using an algorithm that bears no discernible relationship to the one the FAA uses for computing QNH values, and which in all probability altimeter manufacturers are required to use, and which is consistent with the ASA passage mentioned above. A patch to make the altimeter conform to FAA standards can be found at http://www.av8n.com/fly/fgfs/altimeter.diff Right now it uses transcendental functions, which may or may not be somewhat inefficient. It would not be hard to build suitable interpolation tables, but that's not my top priority at the moment. There are lots of broken things that take precedence over merely slightly inefficient things. ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier. Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
