The autopilot alititude hold won't hold current altitude if the altimeter setting is not 760mmHg

[Akiazusa]

Hi.

I recently noticed when playing su25t,the altitude hold on it won't hold current altitude if you change altimeter setting away from 760mmHg using Rshift plus +/- keys.

The autopilot will command the aircraft to climb/dive to an new altitude and hold the aircraft there,instead of just maintaining the current altitude.

The more the pressure setting is away from 760mmHg,the more the aircraft will climb/dive away from the initial altitude.

Su-25T AP alt hold .trk

 

Hope the team could take a look.

Best regards

Edited August 22, 2023 by Akiazusa

[BigBorner]

Maybe I misunderstand your description but; 

I would expect that to happen if you use "Barometric Altitude Hold". Cuz thats how it works. And, if i remember correct, "Altitude Hold" and "Barometric Altitude Hold" are the same. Only "Radar altitude hold" should not react to the Baro pressure change. 

[Akiazusa]

16 hours ago, BigBorner said:

Maybe I misunderstand your description but; 

I would expect that to happen if you use "Barometric Altitude Hold". Cuz thats how it works. And, if i remember correct, "Altitude Hold" and "Barometric Altitude Hold" are the same. Only "Radar altitude hold" should not react to the Baro pressure change. 

Yes,I understand you that on some aircraft if you change the altimeter setting after the AP already entered the alt hold mode,the ap will be affected by that and result in a climb/dive.

But on su25t if you change altimeter setting before you even engage the autopilot(ie before you take off you want to set your altimeter setting to your local QNH/QFE,and after take off for some reason you want to use altitude hold),then when you engage the autopilot,the ap will still be affected by that,which seems odd to me.

 

Edited August 24, 2023 by Akiazusa

[BigBorner]

vor 4 Minuten schrieb Akiazusa:

But on su25t if you change altimeter setting before you even engage the autopilot,then when you engage the autopilot,the ap will still be affected by that,which seems odd to me.

 

Well.. that’s really weird. I agree. 

[MAXsenna]

2 hours ago, Akiazusa said:

hold),then when you engage the autopilot,the ap will still be affected by that,which seems odd to me.

Hi there! Affected in what way? Do you mean that it won't hold the current altitude when you engage the AP, but rather use the settings before you changed it? 

Cheers! 

[Akiazusa]

On 8/24/2023 at 5:02 PM, MAXsenna said:

Hi there! Affected in what way? Do you mean that it won't hold the current altitude when you engage the AP, but rather use the settings before you changed it? 

Cheers! 

Hi there,sorry for the late replay.

It seems that when engage the alt hold,the system will remember the current altitude reading from the altimeter.

But instead of flying the aircraft to that altitude using the current altimeter setting,the AP will command the aircraft to that altitude but using 760mmHg.

 

For example your altimeter setting is 780mmHg,so your current altitude reading from altimeter should be "X" meters higher than in 760mmHg

When you engage the alt hold with an altitude reading of 2000 m and altimeter setting of 780mmHg,the AP will climb to a new altitude of (2000+"X")m under altimeter setting of 780mmHg,if you keep flying level and change the altimeter setting back to 760mmHg now,your altimeter reading should be right at 2000m.

 

TL/DR

When alt hold trying to memorize your current altitude and before it send that to AP,it will consider your altimeter setting.

But when the AP commanding the aircraft,it will always use 760mmHg.

Edited August 26, 2023 by Akiazusa

[MAXsenna]

Hi there,sorry for the late replay.
It seems that when engage the alt hold,the system will remember the current altitude reading from the altimeter.
But instead of flying the aircraft to that altitude using the current altimeter setting,the AP will command the aircraft to that altitude but using 760mmHg.
 
For example your altimeter setting is 780mmHg,so your current altitude reading from altimeter should be "X" meters higher than in 760mmHg
When you engage the alt hold with an altitude reading of 2000 m and altimeter setting of 780mmHg,the AP will climb to a new altitude of (2000+"X")m under altimeter setting of 780mmHg,if you keep flying level and change the altimeter setting back to 760mmHg now,your altimeter reading should be right at 2000m.
 
TL/DR
When alt hold trying to memorize your current altitude and before it send that to AP,it will consider your altimeter setting.
But when the AP commanding the aircraft,it will always use 760mmHg.

I see. I did try to test this the other day but got distracted.
Anyway, what I usually do any way, is to invoke the level flight AP. If I'm not satisfied, I will press disengage, then correct the AP, and maybe invoke the radar altitude.
To be honest I didn't even know one could the change the altimeter settings.
If I remember, I will try to test it next time I'm home.
Have a great Saturday!

Sent from my MAR-LX1A using Tapatalk

[Akiazusa]

1 minute ago, MAXsenna said:

I see. I did try to test this the other day but got distracted.
Anyway, what I usually do any way, is to invoke the level flight AP. If I'm not satisfied, I will press disengage, then correct the AP, and maybe invoke the radar altitude.
To be honest I didn't even know one could the change the altimeter settings.
If I remember, I will try to test it next time I'm home.
Have a great Saturday!

Sent from my MAR-LX1A using Tapatalk
 

Good idea,I'm going to try the level flight AP next time I fly it.

And you too,have a great weekend

[MAXsenna]

Good idea,I'm going to try the level flight AP next time I fly it.
And you too,have a great weekend

Oh, I forgot. Try that, and then test the baro hold. If it then changes, it might be a bug or working differently from expected!
Thanks!

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