@leprechaunlive, sounds like a bit of a nightmare trying to find the issue. If you can set pressure manually set it to a value well away from 29.92" / 1013.2. Very low (29.00") or very high (1050hPa) will result in a huge difference in altitude between QNH and Standard Pressure.

That might be easier to work with since values can be controlled.

On 10/8/2020 at 9:15 AM, 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.

I see the same. You can't set std pressure and have autopilot adhere to it. It seems only to follow QNH.

I reported it via Zendesk.

 

Stu

1 hour ago, Bunchy said:

I see the same. You can't set std pressure and have autopilot adhere to it. It seems only to follow QNH.

I reported it via Zendesk.

 

Stu

Isnt the Standard pressure just a QNH thats equal to 29.92? 

1 hour ago, Ray Proudfoot said:

@leprechaunlive, sounds like a bit of a nightmare trying to find the issue. If you can set pressure manually set it to a value well away from 29.92" / 1013.2. Very low (29.00") or very high (1050hPa) will result in a huge difference in altitude between QNH and Standard Pressure.

That might be easier to work with since values can be controlled.

Had a few minutes to test further, disabled the G1000 Mod, then went to the weather engine to set the baro at 1050, or 31.05 

 

Then set the target at 3000ft, deliberately leaving the QNH at 29.92 or 1013, it capture it perfectly, regardless of the real outside pressure, wich is still 1050

 

Then set the QNH to 1050, the real outside pressure, and went to recapture the 3000ft, wich it did, again perfectly

 

Deliberately left in the screens the backup altimeter. So yea, no problems here. If you guys having trouble could do the same. 

preset was:

Few Clouds

Nothing changed exept pressure at 1050

Location, departing LFLU RWY19, runway axis to 3000ft

Cessna 172, no mods, no add on liveries

Time 13:30 UTC

 

Yes it is in essence.  The QNH is pressure above sea level. Basically it is what you get when you press "B"

As you move across country, the QNH changes depending on whether you move from high to low pressure or vice versa.

The Standard pressure is used above the transition altitude.

In the UK, the transition altitude is 6000ft. Why? because the highest terrain we have in the UK is 5500ft approx.

Once you are safely above 6000ft, you can use standard pressure, which is 1013Hpa or mb.

In the US, and Canada I believe, the transition altitude is much higher at 18000ft, due to their terrain being much higher than ours.

 

Probably a verbose response to your simple and clear question but hope it's useful to you.

 

Stu

 

@leprechaunlive, does your aircraft have a button you can press to toggle between QNH and Standard Pressure? If so, try this but bear in mind the TA in MFS is 18,000ft worldwide.

Set Pressure in your wx engine to 29.92". Climb to 17,000ft.

Next, change pressure in the wx engine to 30.40". Your displayed altitude will now be significantly higher.

Climb to FL200. Passing 18,000ft press the button to switch to Std Press if you have one.

Does the aircraft level out at FL200?

Of course all this is meaningless if your aircraft doesn't have a button to toggle between QNH and Std. It should have if the real aircraft had one.

LATER: You can still set Altimeter Pressure to 29.92" / 1013.2 once above the TA and it should give the same result.

12 minutes ago, Bunchy said:

Yes it is in essence.  The QNH is pressure above sea level. Basically it is what you get when you press "B"

As you move across country, the QNH changes depending on whether you move from high to low pressure or vice versa.

The Standard pressure is used above the transition altitude.

In the UK, the transition altitude is 6000ft. Why? because the highest terrain we have in the UK is 5500ft approx.

Once you are safely above 6000ft, you can use standard pressure, which is 1013Hpa or mb.

In the US, and Canada I believe, the transition altitude is much higher at 18000ft, due to their terrain being much higher than ours.

 

Probably a verbose response to your simple and clear question but hope it's useful to you.

 

Stu

 

Actually my question wasnt really clear either, as i already knew the answer. It was more like "why can your AP adhere to the qnh but not the std pressure, if the std=QNH set at 1013?" are you saying the AP captures altitute with every QNH possible except when at 1013?

How does VATSIM determine altitude? Do the controllers see the plane's real altitude, or is it reported from the plane's instruments? In other words, if you set your baro to the wrong value, will VATSIM see you at a false altitude reading?

I don't use online ATC, mainly to avoid irritating my wife whose computer is in the same room as mine. I have not noticed any particular issues with altitude hold on the autopilots unless you count the >FL300 violent pitching stuff, which I don't for this discussion. That not noticing issues includes clicking the STD button rather than manually setting altitude to 29.92. It also includes using ATC provided values to manually dial in the baro pressure, and using the B key when I'm tired of ATC's other faults.

 

3 minutes ago, leprechaunlive said:

Actually my question wasnt really clear either, as i already knew the answer. It was more like "why can your AP adhere to the qnh but not the std pressure, if the std=QNH set at 1013?" are you saying the AP captures altitute with every QNH possible except when at 1013?

The problem I have is this.

Aircraft B58 or G36, (so I'm assuming the issue is with all/many aircraft in the sim)

1) On ground/apron, altitude select set to, say 7000ft. Press B to get qnh, altimeter then shows airfield elevation above sea level. All good so far.

2) Climbing through 6000ft, set baro to 1013, altimeter shows adjustment and altitude numbers adjust as expected.

3) approaching 7000ft (FL70) autopilot  levels off at slightly different altitude, either higher or lower than FL70. The difference is always equivalent to the QNH/std pressure difference multiplied by 30. (30ft per millibar)

So if the QNH was say 1020 and the std is 1013, the difference is 7 mb or 210 ft. That 210ft  difference would be where the autopilot settled either higher or lower than selected alt, depending on whether the QNH is higher or lower than 1013 mb

 

Hope that makes sense

 

Stu

3 minutes ago, Bunchy said:

The problem I have is this.

Aircraft B58 or G36, (so I'm assuming the issue is with all/many aircraft in the sim)

1) On ground/apron, altitude select set to, say 7000ft. Press B to get qnh, altimeter then shows airfield elevation above sea level. All good so far.

2) Climbing through 6000ft, set baro to 1013, altimeter shows adjustment and altitude numbers adjust as expected.

3) approaching 7000ft (FL70) autopilot  levels off at slightly different altitude, either higher or lower than FL70. The difference is always equivalent to the QNH/std pressure difference multiplied by 30. (30ft per millibar)

So if the QNH was say 1020 and the std is 1013, the difference is 7 mb or 210 ft. That 210ft  difference would be where the autopilot settled either higher or lower than selected alt, depending on whether the QNH is higher or lower than 1013 mb

 

Hope that makes sense

 

Stu

Ok now i finally understand. So basically its the changing of the QNH on the fly, before hiting the target altitude that the AP doesnt handle well. During my testing, i always rolled back the AP to pitch mode, before setting a new QNH, and then going back to VS and Armed Target mode. 

 

4 minutes ago, leprechaunlive said:

Ok now i finally understand. So basically its the changing of the QNH on the fly, before hiting the target altitude that the AP doesnt handle well. During my testing, i always rolled back the AP to pitch mode, before setting a new QNH, and then going back to VS and Armed Target mode. 

 

Exactly right.

I just set QNH with B now and leave it at that.

Like I said, I reported this behaviour to Zendesk, so I think it will be eventually rectified.

 

Stu

1 hour ago, Bunchy said:

Exactly right.

I just set QNH with B now and leave it at that.

Like I said, I reported this behaviour to Zendesk, so I think it will be eventually rectified.

 

Stu

Geez thats taken me 6 pages to understand what the problem could be, it was very confusing. Im gonna test that tonight to see if i get the same results. 

On 10/8/2020 at 1:41 PM, zihmer001 said:

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.

I'm a long time Vatsimmer and there has always been a potential disconnect between ATC's QNH and the one your a/c is experiencing depending on weather sources and update rates. Vatsim doesn't work like real world in this regard and if your wx and ATC's don't match then dialling in ATC's QNH will put you at the wrong altitude on their screen and vs other aircraft. Pressing B will always put you at the correct altitude on your controllers screen.

MSFS is no different to all previous sims in this regard that if ATCs wx and yours don't exactly match then B is your get out of jail free card.

21 hours ago, Bunchy said:

The problem I have is this.

Aircraft B58 or G36, (so I'm assuming the issue is with all/many aircraft in the sim)

1) On ground/apron, altitude select set to, say 7000ft. Press B to get qnh, altimeter then shows airfield elevation above sea level. All good so far.

2) Climbing through 6000ft, set baro to 1013, altimeter shows adjustment and altitude numbers adjust as expected.

3) approaching 7000ft (FL70) autopilot  levels off at slightly different altitude, either higher or lower than FL70. The difference is always equivalent to the QNH/std pressure difference multiplied by 30. (30ft per millibar)

So if the QNH was say 1020 and the std is 1013, the difference is 7 mb or 210 ft. That 210ft  difference would be where the autopilot settled either higher or lower than selected alt, depending on whether the QNH is higher or lower than 1013 mb

 

Hope that makes sense

 

Stu

Finally did some testing. Can confirm, under those conditions, the AP gets confused if you change the QNH (to std or to anyother value) before it has captured its target altitude. 

I have found yesterday that, at least in the C172, the AP acts on the barometric pressure set on the stby altimeter rather than the main. That is a bug indeed.