I just did a flight from Cheyenne, Wy in the Bonanza at several altitudes from field elevation (6160 feet) to 15000 feet with the altimeter set to 29.92 to give “indicated” pressure altitude. (Actual airport altimeter setting was 30.44 courtesy of Live Weather).
At each test altitude, I entered the simvar AMBIENT PRESSURE in the calculator linked above for the value “P” and the calculated pressure altitude was within +/- 25 feet (or better) of the altimeter at each test altitude.
Not sure where in the MSFS atmospheric model the simvar AMBIENT PRESSURE is derived, or whether it is old (or new) code, but when run through the FAA formula, it gives excellent results. I assume that when Live Weather is active, the ambient pressure for a given altitude is injected by the weather model, whereas when the clear sky theme is active, it would be derived from the standard ISA pressure lapse rate.
That’s close, but not exactly the accepted formula used by the aviation industry. Plus, there is a different formula for when above the tropopause (36089 ft.). Asobo has the proper formulas.
Matt,
Thanks for the reply! I guess the big question that most folks are trying to figure out (And probably the team too!) is if this is going to be something that will realistically be fixed in the next week or two or is this something we can expect to plague us for the next month or longer until another SU update is released?
I assume you are going to shelve the altimeter error due to temperature simulation? Seems to be too many issues with trying to implement it. (I was the one who originally raised the issue along the pressure altitude issue for non-ISA days and the airspeed conversion errors) . Would be fine with me if you shelved the temperature effect issue.)
Thanks for the confirmation! Always happy to see additional users try these things out.
The altimeter calc should be quite accurate now, indeed. Just need to get the pressure altitude simvar in line and I think things will be very, very good.
Since the majority of the error is solely due to non-ISA pressures, I think we’re going to keep it to that for now as far as inputs into the pitot static sim go. Still a pretty big jump in accuracy compared to before.
The tropopause at 36090 feet is the ISA standard, but in real world conditions it can vary widely based on latitude and season. I don’t know if MSFS simulates the tropopause when using the ISA-based clear weather theme, but Live Weather can give quite a range of temperatures. At the moment, the extreme high temp problem appears to be gone. I just did a flight in the Aerosoft CRJ at FL340 with Live Weather, and the injected temperature over North Carolina was -38C (ISA+12). Not unusual in summer and a close match for the NWS GFS upper level forecast for that location.
The ISA standard is 36089.24 feet. That’s all that matters when determining the pressure altitude as pressure altitude is based on the ISA standard.
Sorry if I’m coming across as condescending or unfriendly. I really don’t mean to be. I’m just a bit too sensitive on this topic as I identified these issues and supplied the necessary equations/data long ago. What I don’t have a good appreciation for is how difficult it can be to actually implement the stuff in a program as complex and as MSFS with plenty of legacy code from previous editions of FS.
Hey Matt,
Unfortunately, I don’t think that is the case unless you are talking about an internal build that hasn’t been released yet. I’m afraid there are still inaccuracies in the standard pressure vs altitude relationship you are using. I would say a bit more about this, but I can’t due to the NDA on the SU5 preview version.
The errors center around the altitudes of 20,000, 30,000, and 40,000 feet. @HalberQuacky didn’t notice these because he only went to an altitude of 15,000 feet. I initially missed this in my screenshots and messages to you on the WT Discord server (as donbikes) because I happened to do my check at 25,000 feet, where the MSFS pressure is correct for the altitude.
Here is data that I supplied previously to Asobo that still applies. It is an altitude sweep from sea level to 45,000 feet comparing the MSFS ambient pressure at each altitude (at ISA conditions) to the standard atmosphere table and to the equations for calculating that table (so naturally the calculated values agree with the table values. (Note the column labels aren’t correctly aligned, but I think you can make it out.)
Standard Day Conditions (Sea Level pressure: 1013.25 hPa, temperature: 15 deg C)
I hope you find this helpful and will use it to increase the accuracy of the MSFS standard pressure vs altitude relationship. This is what is used in the aviation industry.
This is also an excellent reference; explains various concepts and provides the equations (though some algebra required) without trimming significant digits.
Remember the issue is not the standard day, it’s the deviation. Hot day, warm air mass, low pressure, etc.
Actually, the issue here is the standard day pressure vs altitude relationship. And I could have provided a complete derivation of the equations I presented and gone into the fact that the pressure vs altitude relationship is based on geopotential altitudes, but I think it is better to simply give the equations in the form that are actually being used within the industry.
@Bishop398 Something that has not been mentioned here is that the indicated altitude is still way off, it does not work correctly.
If you set STD on the altimeter it should give you the pressure altitude which is not the case. Depending on sea level pressure the error is more or less but it’s there.
I wasn’t trying to say you were wrong. Sorry that it came across that way. I was only providing additional information.
Standard day behaves properly, right? That’s why clear skies setting doesn’t cause problem. When you add real world weather variance from standard, that’s when we end up with all this kooky stuff.
No problem. Just don’t want to see Matt get confused with too much information.
If I have it right, Matt is saying that the problems in the code for the pressure deviations are now understood and will be fixed. What I am presenting is that there is still a problem with the standard pressure vs altitude relationship, which forms the basis for both the standard day and non-standard pressure day altitudes.
The really kooky stuff comes from the live weather wild temperature exceedances, which are a separate problem. But at least removing the temperature effect from the sim reported altitudes should do away with this issue.
