Baro Pressure and Autopilot

[Millingp 40]

Folks

Firstly, I'm a real pilot, so I know how this should operate in the real world.   Is there a fundamental problem with FS2020 with altitude management and baro pressure?  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.  However, the sim appears to climb, descend or hold the altitude that I dialed in at the outside baro pressure - not the baro pressure that I set.   Therefore the actual altitude acquired or held does not agree with the altitude that I selected.  I've checked this behavior in the Baron and C172.

This is a major flaw in FS2020.

[lzamm 143]

I believe it's already been commented upon, that the autopilot uses the actual plane altitude above MSL rather than the indicated altitude.  Without defending Asobo in any way, it's an easy enough mistake to make in a sim where the "real" values are available as they would not be in the real world.  In FSX/P3D XML code the difference is using the variable (PLANE ALTITUDE,feet) instead of (INDICATED ALTITUDE, feet).

[Millingp 40]

Thanks for your comment Izamm, however, I'm afraid I don't agree.  Unfortunately it demonstrates that FS2020 was not tested by real pilots.  Its an absolute must for a pilot to ensure that the baro pressure is set to that provided by ATC until reaching transition level where 29.92in or 1013hpa is set and to fly the altitude as stipulated by ATC for the baro pressure provided.  FS2020 fails to model this fundamental requirement correctly.

[JRBarrett 3,071]

2 hours ago, lzamm said:

 

I believe it's already been commented upon, that the autopilot uses the actual plane altitude above MSL rather than the indicated altitude.  Without defending Asobo in any way, it's an easy enough mistake to make in a sim where the "real" values are available as they would not be in the real world.  In FSX/P3D XML code the difference is using the variable (PLANE ALTITUDE,feet) instead of (INDICATED ALTITUDE, feet).

 

This is puzzling because this is not what  happens on my MSFS installation. The autopilot in the two aircraft I fly most often (the G-1000 C172 and the TBM) both capture based on indicated altitude. Since I do not use the built-in ATC, I reset the altimeter by pressing the B key at regular intervals while in cruise (if flying below 18,000 feet), and if doing so causes an increase or decrease in displayed altitude because the pressure has changed, the autopilot always slowly climbs or descends to re-acquire the altitude set in the autopilot based on the indicated altitude shown on the altimeter.

Edited October 8, 2020 by JRBarrett

[zihmer001 67]

1 hour ago, JRBarrett said:

This is puzzling because this is not what  happens on my MSFS installation. The autopilot in the two aircraft I fly most often (the G-1000 C172 and the TBM) both capture based on indicated altitude. Since I do not use the built-in ATC, I reset the altimeter by pressing the B key at regular intervals while in cruise (if flying below 18,000 feet), and if doing so causes an increase or decrease in displayed altitude because the pressure has changed, the autopilot always slowly climbs or descends to re-acquire the altitude set in the autopilot based on the indicated altitude shown on the altimeter.

Correct - I'm not defending the work around by the "B" key does have the required effect.  Granted, it doesn't allow you to set altimeter to a real value (I fly in VATSIM) but it gets you closer so as not to get yelled at by ATC too much.

[BerndB 349]

5 hours ago, Millingp said:

Folks

Firstly, I'm a real pilot, so I know how this should operate in the real world.   Is there a fundamental problem with FS2020 with altitude management and baro pressure?  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.  However, the sim appears to climb, descend or hold the altitude that I dialed in at the outside baro pressure - not the baro pressure that I set.   Therefore the actual altitude acquired or held does not agree with the altitude that I selected.  I've checked this behavior in the Baron and C172.

This is a major flaw in FS2020.

Maybe i got something wrong, but how exactly does the "realworld" autopilot know in which altitude the ac actually is. If this informations doesn´t comes from the altimeter of the ac, which depends on the actual baro setting, from where does the AP get it? Does a Cessna or Beach have an independent radar altimeter?🤔

Edited October 8, 2020 by BerndB

[JRBarrett 3,071]

46 minutes ago, BerndB said:

Maybe i got something wrong, but how exactly does the "realworld" autopilot know in which altitude the ac actually is. If this informations doesn´t comes from the altimeter of the ac, which depends on the actual baro setting, from where does the AP get it? Does a Cessna or Beach have an independent radar altimeter?🤔

Radar altimeters only work below 2500 feet AGL. No autopilot has the ability to determine the aircraft’s altitude independently - the “sole source of truth” for current altitude in a real aircraft is the altimeter, and to get the accurate MSL altitude, the baro setting on the altimeter must be set to the current ambient pressure.

The only problem at the moment (when flying in Vatsim in particular) is that the barometric pressure in the Vatsim environment, based in current METARS, is often different than the barometric pressure in MSFS, which comes from the predictive MeteoBlue forecast model, so yes, you can’t use the altimeter setting that a Vatsim controller might give you. But, as long as you sync the altimeter to whatever actually exists in the MSFS environment with the B key, a MSFS aircraft will be at the same altitude as other players using a different weather source.

This is not due to any bug in the MSFS altimeter or autopilot, it’s a simple side-effect of the MSFS Live Weather pressure being different than the METAR-based pressure that other Vatsim pilots (or controllers) are using.

[Ray Proudfoot 8,955]

2 hours ago, BerndB said:

Maybe i got something wrong, but how exactly does the "realworld" autopilot know in which altitude the ac actually is. If this informations doesn´t comes from the altimeter of the ac, which depends on the actual baro setting, from where does the AP get it? Does a Cessna or Beach have an independent radar altimeter?🤔

It doesn’t. Hence the purpose of the Transition Altitude. It varies from country to country but once you’re cleared by ATC to a flight level as opposed to an altitude you set the altimeter to standard pressure - 29.92” or 1013.2hPa.

That isn’t the actual air pressure but that doesn’t matter as all aircraft above the Transition Altitude will have the same pressure set.

The reverse happens on descent. You’ll be cleared down to a lower Flight Level and then to an altitude. When cleared to an altitude set your altimeter to QNH or local pressure.

Easy peasy! 😁

[BerndB 349]

2 hours ago, JRBarrett said:

Radar altimeters only work below 2500 feet AGL. No autopilot has the ability to determine the aircraft’s altitude independently - the “sole source of truth” for current altitude in a real aircraft is the altimeter, and to get the accurate MSL altitude, the baro setting on the altimeter must be set to the current ambient pressure.

The only problem at the moment (when flying in Vatsim in particular) is that the barometric pressure in the Vatsim environment, based in current METARS, is often different than the barometric pressure in MSFS, which comes from the predictive MeteoBlue forecast model, so yes, you can’t use the altimeter setting that a Vatsim controller might give you. But, as long as you sync the altimeter to whatever actually exists in the MSFS environment with the B key, a MSFS aircraft will be at the same altitude as other players using a different weather source.

This is not due to any bug in the MSFS altimeter or autopilot, it’s a simple side-effect of the MSFS Live Weather pressure being different than the METAR-based pressure that other Vatsim pilots (or controllers) are using.

exactly that´s what my mind is too. 

Of course i exspect, that if i have tuned the altitude in my autopilot to a certain alt, he will command the ac to climb or descent if the altitude changes on the baro altimeter even if it´s only because i changed the QNH. (Well in my A320 this is not the case, because the ap needs my approval to do so, but that´s just how Airbus handle this...)

but i´am quite sure i missed the true meaning of the raised thread, because of lingual discrepancys😔 ...on my side of course

Edited October 8, 2020 by BerndB

[Ray Proudfoot 8,955]

I should have qualified my earlier post by saying this is how it works in the real world. It sounds from the OP’s post that pressure can’t be set correctly in the aircraft. Should be a reasonably quick fix for the developers.

[BerndB 349]

25 minutes ago, Ray Proudfoot said:

It doesn’t. Hence the purpose of the Transition Altitude. It varies from country to country but once you’re cleared by ATC to a flight level as opposed to an altitude you set the altimeter to standard pressure - 29.92” or 1013.2hPa.

That isn’t the actual air pressure but that doesn’t matter as all aircraft above the Transition Altitude will have the same pressure set.

The reverse happens on descent. You’ll be cleared down to a lower Flight Level and then to an altitude. When cleared to an altitude set your altimeter to QNH or local pressure.

Easy peasy! 😁

Hi Ray, i know all that too. I have to excuse myself to be a german, non native english tounged, fool. Now, after reading the starting post 5 times again i see that i missunderstood the 

"...it should do so (hold or acquire the selected altitude) regardless of the baro pressure that I set" by not getting into it that he meant the invironmental baro pressure and not the one he has set on the altimeter baro.😏 

Edited October 8, 2020 by BerndB

[Ray Proudfoot 8,955]

@BerndB, sorry if you already knew that. Years of using Radar Contact have drilled it into me. 😀

Don’t criticise your English. Any non-native language is never as easy to understand. If the altimeter cannot be set correctly that is a bug.

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.

[JRBarrett 3,071]

3 minutes ago, Ray Proudfoot said:

I should have qualified my earlier post by saying this is how it works in the real world. It sounds from the OP’s post that pressure can’t be set correctly in the aircraft. Should be a reasonably quick fix for the developers.

I’m still a bit confused by the original post, because I have had no issues setting the altimeter, either automatically (by using the B key), or manually by rotating the setting knob.

Nor do I see any issues capturing a selected altitude with the autopilot. If I set e.g. 11,000 feet as my target in a V/S climb, the autopilot does capture and hold 11,000 feet as shown on the altimeter.

I prefer the B auto-sync method because there is no easy way to know what the current in-game altimeter setting should be in flight when using Live Weather. One can listen to ATIS when on the ground, but I’m not sure that is entirely trustworthy.

The only way to set a specific pressure in the sim weather environment is to use the Custom Weather dialog, or to use the Clear Sky preset which will set the environmental pressure to 29.92 everywhere.

[sd_flyer 6,042]

6 hours ago, Millingp said:

Thanks for your comment Izamm, however, I'm afraid I don't agree.  Unfortunately it demonstrates that FS2020 was not tested by real pilots.  Its an absolute must for a pilot to ensure that the baro pressure is set to that provided by ATC until reaching transition level where 29.92in or 1013hpa is set and to fly the altitude as stipulated by ATC for the baro pressure provided.  FS2020 fails to model this fundamental requirement correctly.

In contrary, you assumption is incorrect MSFS was  tested by many real pilots! What was fixed or not is another question.

Currently autopilot reads indicated altitude  in altimeter settings and if anything it's minor issue which really not a big killer of immersion. By default you can press "B" to adjust pressure in kollsman window based with altimeter setting. Since whole ATC was modeled after US default transition level is at 18000 ft

[Ray Proudfoot 8,955]

17 minutes ago, JRBarrett said:

I’m still a bit confused by the original post, because I have had no issues setting the altimeter, either automatically (by using the B key), or manually by rotating the setting knob.

Nor do I see any issues capturing a selected altitude with the autopilot. If I set e.g. 11,000 feet as my target in a V/S climb, the autopilot does capture and hold 11,000 feet as shown on the altimeter.

I prefer the B auto-sync method because there is no easy way to know what the current in-game altimeter setting should be in flight when using Live Weather. One can listen to ATIS when on the ground, but I’m not sure that is entirely trustworthy.

The only way to set a specific pressure in the sim weather environment is to use the Custom Weather dialog, or to use the Clear Sky preset which will set the environmental pressure to 29.92 everywhere.

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.