Baro Pressure and Autopilot

[JRBarrett 3,048]

6 minutes ago, leprechaunlive said:

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

 

9 minutes ago, leprechaunlive said:

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?

It does in the MSFS TBM. (Which appears to indicate that it defaults to 18,000 which would be fine for the US and Canada, but not Europe). Not sure about other MSFS aircraft. In airliners like the A320, you would press the baroset button to set STD.

No reason not to set the altimeter manually when climbing through transition altitude. This is one time I would not use the B key in MSFS.

[leprechaunlive 1,852]

4 minutes ago, JRBarrett said:

 

It does in the MSFS TBM. (Which appears to indicate that it defaults to 18,000 which would be fine for the US and Canada, but not Europe). Not sure about other MSFS aircraft. In airliners like the A320, you would press the baroset button to set STD.

No reason not to set the altimeter manually when climbing through transition altitude. This is one time I would not use the B key in MSFS.

Actually i was already doubting myself as soon as i posted 😄 I rarely fly at those altitudes, but based on the fact that pressing B, say, 5 times in 5 minutes,slightly changes the pressure every single time, i assumed this behavior would also be true at any altitude. My bad 🙂 

Edited October 8, 2020 by leprechaunlive

[jrw4 624]

According to the user guide for the Bendix AP in the M20R, there is such a thing as an encoding altimeter, and it is that which the AP uses to reach/maintain a specific attitude. So there is no need to let the AP know what the sea level pressure is. It reads the atmospheric pressure compensated altitude from the analog altimeter which actually has a digital output even in a real airplane.

[JRBarrett 3,048]

3 minutes ago, leprechaunlive said:

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)

AFAIK, all the MSFS autopilots capture based on indicated altitude of the altimeter (or Air Data Computer in the case of an airliner). Certainly all the Garmin G-xxxx units do. I haven’t tried all the GA aircraft in the sim, so I don’t know if any of them emulate a KAP-140.

There are some basic GA autopilots with a simple altitude hold function that do use a stand-alone altitude sensor, but these are not capable of capturing a preselected altitude in either climb or descent. They simply hold whatever pressure altitude the aircraft happened to be at when the ALT button was pressed - and the pilot needs to first manually level off and trim the airplane before engaging the altitude hold function.

[fppilot 4,355]

9 hours ago, JRBarrett said:

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.

I confirmed to myself without any doubt today that MFS is driving the captured altitude based on standard pressure, not on altimeter settings.  Flights today in the 208 and in the G58.  Did a number of altitude changes, both up and down, and confirmed.  Used altimeter settings reported by ATIS/AWOS stations along my two routes.  At no point in either flight did either aircraft capture the target altitude based on altimeter.  If I adjusted the altimeter setting to 29.92 the indicated captured altitude moved to match the target altitude.  100% of the time. 

[leprechaunlive 1,852]

7 minutes ago, JRBarrett said:

AFAIK, all the MSFS autopilots capture based on indicated altitude of the altimeter (or Air Data Computer in the case of an airliner). Certainly all the Garmin G-xxxx units do. I haven’t tried all the GA aircraft in the sim, so I don’t know if any of them emulate a KAP-140.

There are some basic GA autopilots with a simple altitude hold function that do use a stand-alone altitude sensor, but these are not capable of capturing a preselected altitude in either climb or descent. They simply hold whatever pressure altitude the aircraft happened to be at when the ALT button was pressed - and the pilot needs to first manually level off and trim the airplane before engaging the altitude hold function.

yes, thats what im saying, we dont have to set a baro parameters on any of the aircraft's AP as far as i know. Wich is why i still dont understand the OP's problem.

[fppilot 4,355]

18 minutes ago, leprechaunlive said:

yes, thats what im saying, we dont have to set a baro parameters on any of the aircraft's AP as far as i know.

There in lies the flaw.  You may not be understanding the issue.  Perhaps read my recent post.  
 

Do this.  Set your altimeter at 29.92 (standard pressure).  Set a target altitude, say 5,000 ft.  Have the aircraft climb to and capture that altitude.  I will capture 5,000 ft based on my experience.

Now.  Assume the altimeter setting provided by ATIS/AWOS, or by ATC, is 30.17.  Set that pressure in the altimeter.  Set a target altitude of 5,000 and see where it captures.  It will not be indicated as 5,000 feet.  more like 5,250 or there about.  That is not right!  The altitude when captured should indicate 5,000 ft when the altimeter is set to the current pressure.  The issue pointed out by the OP indicates that in MFS it may be that the aircraft is actually be at a 5,000 ft altitude, but the indicated altitude may be the culprit.  There is a flaw somewhere.

Edited October 8, 2020 by fppilot

[viz 139]

I spotted an issue the last time I flew the Caravan, not sure if it's been fixed since:

With the autopilot disengaged, select an altitude. Now adjust baro. The selected altitude should NOT change, but it does.

[fppilot 4,355]

9 minutes ago, viz said:

With the autopilot disengaged, select an altitude. Now adjust baro. The selected altitude should NOT change, but it does.

Selected?  Or indicated?  One appears in the top right enunciations.  The other appears on the vertical scale.

[leprechaunlive 1,852]

19 minutes ago, fppilot said:

There in lies the flaw.  You may not be understanding the issue.  Perhaps read my recent post.  
 

Do this.  Set your altimeter at 29.92 (standard pressure).  Set a target altitude, say 5,000 ft.  Have the aircraft climb to and capture that altitude.  I will capture 5,000 ft based on my experience.

Now.  Assume the altimeter setting provided by ATIS/AWOS, or by ATC, is 30.17.  Set that pressure in the altimeter.  Set a target altitude of 5,000 and see where it captures.  It will not be indicated as 5,000 feet.  more like 5,250 or there about.  That is not right!  The altitude when captured should indicate 5,000 ft when the altimeter is set to the current pressure.  The issue pointed out by the OP indicates that in MFS it may be that the aircraft is actually be at a 5,000 ft altitude, but the indicated altitude may be the culprit.  There is a flaw somewhere.

[leprechaunlive 1,852]

21 minutes ago, fppilot said:

There in lies the flaw.  You may not be understanding the issue.  Perhaps read my recent post.  
 

Do this.  Set your altimeter at 29.92 (standard pressure).  Set a target altitude, say 5,000 ft.  Have the aircraft climb to and capture that altitude.  I will capture 5,000 ft based on my experience.

Now.  Assume the altimeter setting provided by ATIS/AWOS, or by ATC, is 30.17.  Set that pressure in the altimeter.  Set a target altitude of 5,000 and see where it captures.  It will not be indicated as 5,000 feet.  more like 5,250 or there about.  That is not right!  The altitude when captured should indicate 5,000 ft when the altimeter is set to the current pressure.  The issue pointed out by the OP indicates that in MFS it may be that the aircraft is actually be at a 5,000 ft altitude, but the indicated altitude may be the culprit.  There is a flaw somewhere.

So, once again, wheres the problem? i did what you said, pic above, and havent noticed th behavior you'r describing.

 

Edited October 9, 2020 by leprechaunlive

[viz 139]

12 minutes ago, fppilot said:

Selected?  Or indicated?  One appears in the top right enunciations.  The other appears on the vertical scale.

The selected altitude (top right of PFD) would show the selected altitude +/- the calculated feet as baro was changed, but only did it with autopilot disengaged. I just jumped in and checked it appears to have been fixed which is fantastic.

Edited October 9, 2020 by viz

[fppilot 4,355]

7 minutes ago, leprechaunlive said:

That is not what I experienced today.  Question. Do you have any G1000 mods installed?

[leprechaunlive 1,852]

Just now, fppilot said:

That is not what I experienced today.  Question. Do you have any G1000 mods installed?

Yep, forgot to mention it my bad, BUT, went thru the changelog, and that kind of change isnt mentioned. Its 3am here, so i'll test without the mod, when i wake up....at 7am 😄

[fppilot 4,355]

Just now, leprechaunlive said:

Yep, forgot to mention it my bad, BUT, went thru the changelog, and that kind of change isnt mentioned. Its 3am here, so i'll test without the mod, when i wake up....at 7am 😄

I have the mod in place.  

 

29 minutes ago, viz said:

With the autopilot disengaged, select an altitude. Now adjust baro. The selected altitude should NOT change, but it does.

Like Viz stated, this is what I observed today.  Adjusted the altimeter and observed the target altitude change.