27 minutes ago, Ray Proudfoot said:

Jim, the B key is a hangover from FSX & P3D and isn’t a correct way of setting the altimeter pressure. You don’t say where in the world you were flying but in Europe there’s no such altitude as 11,000ft. It would be FL110 and you would change altimeter pressure to 1013.2 on passing the Transition Altitude.

In the US you would stay on QNH throughout If below 18,000ft.

I don’t have MFS so all my posts are based on how things happen in the real world and are mirrored in RC4. 

I appreciate that without a separate weather or ATC program knowing actual pressure may be difficult. You should set what the ATIS announces and if you get problems report back to ASOBO.

Without any way of determining actual ambient pressure at the aircraft position in flight, hitting the B key is the only way to accurately set the altimeter to whatever pressure currently exists in the sim environment as provided by Live Weather.

MSFS ATIS does give the current pressure at airports, but the only way to get that information in flight would be to use ATC, and I have absolutely NO interest in using the built-in ATC in MSFS (or any other sim for that matter).

I would of course set the altimeter in MSFS manually (rather than by hitting B), if there was some way to determine what the actual pressure is at a given in-flight location, but their is not. 

I realize of course that many parts of Europe use a much lower transition altitude than the standard 18,000 feet in the US, and in either case, setting 29.92 /1013 would be the proper technique when climbing above TA.

@JRBarrett, Jim, you're absolutely right. Until there is something similar to Active Sky that can calculate pressure between airports along your route and display it in a banner you're stuck with that method. Not very satisfactory but that's all you have sadly.

11 hours ago, Millingp said:

Therefore the actual altitude acquired or held does not agree with the altitude that I selected

With some autopilots like the KAP-140 using a remote baro reading is only an installation option.  If it's not installed that way you're required to set it on the unit itself.  If there's a baro button on the autopilot have you tried verifying or setting it there?

Please forgive me, but I'm getting confused. Excellent points have been made (especially that about the B key), but when flying single engine aircraft in real life, there's no B key anywhere. One gets the local sea level barometric pressure from an ATC facility (or flight service station) within VHF range and dials that into the altimeter window. Of course, at the transition altitude, one sets the altimeter to standard pressure.

Yes, there are potential errors, but that's part of the challenge of flying. If you look closely at some instrument approach plates, you'll see that the descent minimum will vary depending on whether or not the destination airport's control tower is open to give you the local barometer reading. And in really cold climates, low ambient temperatures can impact the MDA/DA as well. See

https://www.aopa.org/news-and-media/all-news/2016/october/05/faa-simplifies-altitude-corrections-for-cold-temperature-airports

Be safe out there and have fun.

39 minutes ago, jrw4 said:

 

Please forgive me, but I'm getting confused. Excellent points have been made (especially that about the B key), but when flying single engine aircraft in real life, there's no B key anywhere. One gets the local sea level barometric pressure from an ATC facility (or flight service station) within VHF range and dials that into the altimeter window. Of course, at the transition altitude, one sets the altimeter to standard pressure.

 

That’s fine, but in MSFS there is no way to know what the ambient barometric pressure is at any given moment while in flight. With Live Weather it will definitely change while enroute. You could potentially tune the ATIS of an airport that lies near your flight plan route, but many parts of the MSFS ATIS system are broken, and I would not trust the ATIS altimeter report to be accurate for any location other than on the ground at the initial departure airport. 

I’m all for realistic procedures, and setting the altimeter manually to the current barometric pressure while in flight, but without having a way in-game to know what the current pressure actually is, what would you set it to? 

2 hours ago, BrianW said:

With some autopilots like the KAP-140 using a remote baro reading is only an installation option.  If it's not installed that way you're required to set it on the unit itself.  If there's a baro button on the autopilot have you tried verifying or setting it there?

Using the B key to sync the altimeter to the current pressure works in any aircraft in MSFS.

This thread is drifting from the OP’s original post, and I am still not sure I understand what exact issue it is that he is seeing. I have had no problems in MSFS getting the autopilot to capture and hold a selected altitude. If I dial in 8000 feet on the altitude selector and climb in VS mode, when the altimeter reads 8000 feet, the aircraft will level off, capture and hold 8000 feet indicated.

 

Every time you get handed off between AI ATCs they give you a new baro number.  You can either press 'B' to update at that time, or manually dial it into the control panel.  Both produce the same results.  In VFR if you ask for flight following, when you get handed off they give you a new value.

As far as I can tell this works as expected.  The OP seems to be describing a system where the AP magically knows the baro value at all times, so the AP altitude is always magically correct.  Maybe there are some aircraft that have a remote baro capability that can do this, but that's certainly now how I've ever seen it work....

10 minutes ago, JRBarrett said:

That’s fine, but in MSFS there is no way to know what the ambient barometric pressure is at any given moment while in flight. With Live Weather it will definitely change while enroute. You could potentially tune the ATIS of an airport that lies near your flight plan route, but many parts of the MSFS ATIS system are broken, and I would not trust the ATIS altimeter report to be accurate for any location other than on the ground at the initial departure airport. 

I guess my point was that when one is flying IRL, one doesn't know what the atmospheric pressure is either. There can be 100 miles between airports with reporting capability. Everyone is (in principle) using the same sea level barometric pressure, so if you're all off by the same 50 feet, no problem. On instrument flights, there is sufficient ground clearance to ensure that small errors in barometric pressure don't create a situation where flight into the terrain might be possible.

There's no such thing as an "automatic barometer". The control tower doesn't give you the actual pressure at ground level, which is can be much less than the sea level pressure (think KDEN, for example). Rather, an barometer measures the ambient pressure and someone converts it to the equivalent pressure at sea level. In order to do that, one needs to know the actual altitude of the airport, hence this can't be done from an airplane. I actually had a grant from one of the aircraft manufacturer in the 1990s to develop techniques which would independently determine altitude other than by barometry. Not so easy......

14 hours ago, Millingp said:

 

If I set an altitude for the autopilot to hold, or climb or descend to, it should do so (hold or acquire the selected altitude) regardless of the baro pressure that I set. 

I know you said you'r a real pilot and all that, and i should say im not, but i believe the statement above to be false. AP sets itself based on the readings he gets from the altimeter, wich itself sets itself up based on the baro you dialed in...so, no, the AP DOESNT behave regardless of the baro you gave to the altimeter. Actually its the opposite, the AP doesnt care/know, what the real outside baro pressure is. So maybe there no real need for dramatic hyperbole like "major flaw", or "real pilot didnt test it".

32 minutes ago, JRBarrett said:

I am still not sure I understand what exact issue it is that he is seeing

My comment was only that some real autopilots reference an internal barometer, not the altimeter. If the baro setting in the autopilot doesn't match the pressure setting on the altimeter the altitude captured will be different from the altimeter reading.  I don't own MSFS yet so I don't know if they've simulated it or not, but maybe worth a check specifically on the analog 172.   

45 minutes ago, JRBarrett said:

That’s fine, but in MSFS there is no way to know what the ambient barometric pressure is at any given moment while in flight. With Live Weather it will definitely change while enroute. You could potentially tune the ATIS of an airport that lies near your flight plan route, but many parts of the MSFS ATIS system are broken, and I would not trust the ATIS altimeter report to be accurate for any location other than on the ground at the initial departure airport. 

I’m all for realistic procedures, and setting the altimeter manually to the current barometric pressure while in flight, but without having a way in-game to know what the current pressure actually is, what would you set it to? 

But there is tho. By getting the METAR from nearby stations (granted you'r gonna have to tab out a few sec to check it on any metar website, so its not really an "ingame" feature). It would actually be more realistic to not use the B method , and have a slight difference beetween the outside baro and the metar/atc/atis one. Using the B method means that there are never any discrepancies beetween real outside pressure in flight, and the baro used by the altimeter, wich in reality never happens as stated by @jrw4.

 

By the way, i also use the B method lol. 😄

6 hours ago, Ray Proudfoot said:

 

I wonder if MFS has the Transition Altitude built into it. If it has then it would be a pity if it doesn’t change country by country rather than the global 18,000ft.

Nope, regardless of the altitude, the B method always sets the "real" or at least the weather engine oustide baro pressure. So unfortunately pressing B above 18000ft doesnt set it to 29.92 automatically. I kind of recall that FSX used to do it tho?

6 minutes ago, BrianW said:

My comment was only that some real autopilots reference an internal barometer, not the altimeter. If the baro setting in the autopilot doesn't match the pressure setting on the altimeter the altitude captured will be different from the altimeter reading.  I don't own MSFS yet so I don't know if they've simulated it or not, but maybe worth a check specifically on the analog 172.   

So would there be a dial or a knob allowing you to set the baro specificaly for the AP?

2 minutes ago, leprechaunlive said:

So would there be a dial or a knob allowing you to set the baro specificaly for the AP?

Correct, on a KAP-140 it's the top right button labeled BARO.

From the KAP-140 manual:

"11. BARO SET (BARO) BUTTON - When pushed and released, will change the display from the altitude alerter selected altitude to the baro setting display (either IN HG or HPA) for 3 seconds. If pushed and held for 2 seconds, will change the baro setting display from IN HG to HPA or vice versa. Once the baro setting display is visible the rotary knobs may be used to manually adjust the baro setting if automatic baro correction is not available."

Automatic baro correction is only available if it was installed with the remote barometer input option, other wise you need to adjust the setting in the AP every time the altimeter setting is changed.

2 minutes ago, BrianW said:

Correct, on a KAP-140 it's the top right button labeled BARO.

From the KAP-140 manual:

"11. BARO SET (BARO) BUTTON - When pushed and released, will change the display from the altitude alerter selected altitude to the baro setting display (either IN HG or HPA) for 3 seconds. If pushed and held for 2 seconds, will change the baro setting display from IN HG to HPA or vice versa. Once the baro setting display is visible the rotary knobs may be used to manually adjust the baro setting if automatic baro correction is not available."

Automatic baro correction is only available if it was installed with the remote barometer input option, other wise you need to adjust the setting in the AP every time the altimeter setting is changed.

Oh ok, so you would have to set the baro twice, one for the plane, one for the AP. Thanks god we dont have that KAP thingy in MSFS2020 😄 (yet, maybe on future third party planes)