[vic365]

This is something I've been dealing with for many, many years and always said I would ask about it "some day". Today is that day. Why is it that you can be flying along above the transition altitude with the altimeter set to 29.92...and you are suddenly high or low by 200 feet? This seems to happen a lot in western europe, but does occur elsewhere. Why would your altitude suddenly change, requiring your aircraft to climb or descend, when you are flying "standard" at 29.92?

[TheDude71]

I'll take a stap at this.

Yes. This happens to me also. I believe this differnce of a couple hundred feet is due to air pressure difference. All aircraft above FL180 have altimeters set to 29.92. So any aircraft in your area will be effected by the same difference in airpressure. Meaning you will be at the same altitude.

Some easy reading. http://www.madsci.org/posts/archives/au ... .Ph.r.html

Hope this answered your question.

[vic365]

Brett, your explanation only works below Transition altitude, as far as I know? Above TA you set the altimeter to 29.92" and any real change in barometric pressure should be ignored. You are basically telling the altimeter that the pressure is "locked" at 29.92", no matter what the true pressure is. You should not see the sudden altitude swings I'm seeing above TA.

From your link:

Above 18,000 feet, aircraft can cover 100 miles every 10 minutes or so. To preclude having to update the altimeter setting many times an hour, the practice is for all pilots intending to fly above 18,000 feet to adjust their altimeter setting to 29.92 inches of Hg (1013 millibars), the barometric pressure corresponding to seal leval pressure in a standard atmosphere. This setting is used REGARDLESS of the true barometric pressure. As a result, the altimeter no longer reads actual height above sea level, but "pressure altitude" -- the altitude corresponding to the current air pressure in a standard atmosphere. Pilots set their altimeters to 29.92 upon climbing through 18,000 feet and to local barometric pressure upon descending through 18,000 feet.

[TheDude71]

Right.

Above 18k, we know to set our ALT. to 29.92. I guess what I was trying to say, is maybe, and I could be way off here, but the change in your altitude could be caused by a change in bar pressure. As I write this, Im doing a flight from KBOS to KMIA. Only change I have noticed is maybe 20 ft or so. Maybe this is a flightsim issue. Not sure.

Does it only happen is one spot??

[vic365]

It happens more in western Europe than anywhere else, so far, especially heading accross the Channel, but I have also run across it in the central US. I believe you are right in that this is an FS9 issue. Once my altimeter is set to "Standard" (29.92") above TA...I should not see any sudden changes in altitude. This has been going on for many years and it's one of those "one day I'm going to ask about this". I finally asked.

[Thumper_KJFK AS.381]

I used to ponder this very same thing.

But what I think is going on in our flight Simulator is the fact our aircraft is flying a Straight Line from Point A to B. But our Simulator Earth is Somewhat Round, I Think our aircraft is making adjustments at some point in it's Plot from straight line A to Straight line B to C to D exc... just try to put short sticks in a line to encircle a globe. Our simulator aircraft has to Correct height above or below in order Not to fly off into space. Strange, it is always aprx + - 200ft. depending on air speed

I also think that those of us who always fly LONG overseas flights, notice this anomaly more often, and when I fly the Concorde at FL650, that anomaly is happening more frequently, now that I think of it. our flight path is constantly being adjusted with reference to the center of the simulator Earth. I read somewhere our computers don't do such a good job on drawing perfect circles. Again I am just guessing at this. I am just taking what I know about things in general. I will be following up on this Now my curiosity is peaked

That is a great brain teaser vic

[vic365]

What you bring up, Greg, has me thinking also. I'd never considered "round earth vs. flat flightplan" possibilities. The only problem I have with your theory is that I don't have these at regular intervals, which I would expect? What I see is constant up 200 ft. then down 200ft, over and over (or some such number...can be as low as 20 ft.), every few minutes, for 20 to 50 miles...and then it just stops for hundreds of miles? If it's an "adjustment" I would think it would happen once, in one direction, and be done until the next adjustment? HMMmmm :?

[Thumper_KJFK AS.381]

you know. your right...

I sat here thinking of all the strange things that I have noticed over all the years from as far back as FS6.

Getting Lost when you ran out of Earth. ie: don't go to far north or south. or the holes in space time you would seem to fly into an abyss of White, LOL no up down or anything. But as you pointed out, that random gain and loss of altitude has always been there. but not every where. yep... Hhmmmm


Oh by the way. It worked... I changed All my B777's. I pulled a reference to my PSS Pro series aircraft. Man everything is nice and smoth. fuel is being burned a a good rate. I did a short flight NZAA to YSSY. and now I am flying a long one YSSY to KLAX, Just to get a good Base burn rate.

[vic365]

The only difference I show between the 200LR's and 300ER, in PSS models, is .950 for -200 and .955 for -300, if you want to fine tune the different models. Anyway, glad that's working better for you!