MSFS AP Hold Ref with Multiple Baros

[Geoff_D]

What determines the Baro Index when you have multiple Baros in the plane, and what determined what baro the AP hold will use ?

I believe there is a BUG / issue with the Altitude Hold in the C172 Steam AP.

It gets complicated : --- **But here Goes**. (Posted on MSFS Forum, but don't hold out hope for a answer from causal users )
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Some have reported that when not flying at Altimeter setting STD, (ie typically real weather), the plane sometimes does not hold at the correct altitude.

Actually, it does not hold at the correct altitude at any non Std Pressure, unless you "Cheat" and hit the B key.

If the pressure changes and you Manually set both the Altitude gauge, and the AP Baro setting to the correct value, the plane does NOT level off at the expected altitude as set by the AP. -- Until you press the B key .. then it will.

The C172 has 3 Baro sensors
Index #1 for the Altitude Gauge (Manually adjustable)
Index #2 for the AP (Manually adjustable
Index #3 for the Transponder (SHOULD BE FIXED @ Std Pressure Calibration)
but it is NOT -- it gets adjusted when you press the B key

The problem is, the Altitude hold reference is being got from the Transponder Baro sensor, and not the AP Baro sensor.

You can easily confirm this by
(a) Flying in Preset Clear weather with Altimeter setting standard.

All is well, and the plane will hold at the AP selected Altitude.

Now, adjust the AP Baro .. NOTHING CHANGES
(because the AP's altitude Hold is being referenced Incorrectly from the Transponder !!)
Now change the Weather Pressure, and adjust the Altimeter and the AP Baro correctly

The AP will not hold the correct Altitude, because it is referenced incorrectly from the Transponder, that is still at STD Pressure

Until you hit B, when the Transponder Baro is set by the B button, and now the planes Altitude is being got correctly, and the AP will hold the correct altitude.

But it will only hold correct altitude, if the Transponder Baro is set by the B key ..
Note: YOU cannot manually set it , and it is NOT meant to change in any case.

so, bottom line
The AP should be getting its Altitude reference from the AP Baro, not the Transponder Baro
The Transponder Baro should remain at Std Pressure calibration, and not be altered by the B key.

So two things wrong, that semi cancel each other out, but ONLY if you use the B key to set all 3 Baro.

This may or may not be happening with other planes, but its a definite BUG with the C172 Steam gauge, which has the 3 Baros.

Q: What sets which Baro the AP uses, as it is currently Incorrectly not using it's own Baro.

@@@@@@@

So,
**#1** Need some confirmation that this is really happening, from the Technical Memebers here.


and if so

**#2** Needs to be flagged as a significant bug, as at least the C172 classic cannot hold correct altitude, if flying in anything but Std Pressure,

(*Told you it was complicated !!!*)

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Bear in mind, different planes has different quantities of Baros.

Simple C150 without Transponder has 1
C150 with Transponder has 2
C172 with additional AP has 3

Bigger planes, may have duplicate Baros, for Pilot & Co-Pilot so > 3 baros

If the Gauge itself contains the Index, and the gauge is shared between planes with different quantities of Baros, how does that work ??

How did it work in FSX (if it did), as it certainly is not working correctly in MSFS>

 

[Geoff_D]

Basic issue is
The C172 Classic, is using the incorrect BARO for its AP.
And I wonder how many other Aircraft, especially the more complex ones, with multiple APs, and multiple Baro setting for those AP, are doing, and if underneath, they are also suffering form similar issues.


Should be of Interest to anyone working on Garmin AP systems …


For the Asobo C172 Steam gauge, this is specifically what is happening


The Plane has 3 Baros :
(a) For the needle panel gauge, adjustable by a rotary control.
(b) For the KAP140 AP, adjustable by the KAP140 controls (Buttons & rotary knob)
© For the Transponder, that has no adjustment, and should be set calibrated to STD Pressure.

The problem is, the KAP140 AP is using the Transponder Baro, for it altitude reference,

so
Adjusting the KAP140 Baro has no effect. to correct for atmospheric pressure change.


This may go UN-noticed, if the Baros are set with the “B” key, as the B key sets all Baros, even the one in the Transponder that is meant to stay at Std Pressure , but does not-- so it tend to cover up the issue.


What is not clear to me is WHY the AP is using the Transponder Baro ??


In more complex aircraft, with dual AP systems, and Pilot & C0-Pilot Baro adjustments, I suspect its an even bigger mess !!

 

[WarpD]

I see 4 baros defined in the cockpit.cfg file... not three. The 'B' key has always set all baros, even in FSX.

 

[ronh]

If you look at the AP code/xml you will see the baro assigned/associated with the AP is set to 2. In the DA40 this is the standby altimeter. so I always use the b key to set things, just easier. Setting the G1000 baro makes the aircraft fly at the incorrect altitude, unless you reset the standby alt also.

 

[Geoff_D]

I see 4 baros defined in the cockpit.cfg file... not three. The 'B' key has always set all baros, even in FSX.

yes, so if you do not want your Transponder baro to be set by "b" key, you need to keep resetting it back to Std Alt ..

 

[Geoff_D]

If you look at the AP code/xml you will see the baro assigned/associated with the AP is set to 2. In the DA40 this is the standby altimeter. so I always use the b key to set things, just easier. Setting the G1000 baro makes the aircraft fly at the incorrect altitude, unless you reset the standby alt also.

I'm still not getting it .. and this isi after spending days trying to figure it out.

As far as I can see, the KAP140 AP is always refrencing baro 2 in the code, and can sucessfully alter that baro.
So then, what is causing the MSFS AP PID (?) to be trying to set the altitude, based on the Baro #3

Its almost as if the Kernel, is hard coded to use the last defined baro,, or baro #3 as the ALT Hold reference ??

If so, then The Kap140 should be baro #3, and the Tranponder baro #2, implying, that specific #baros have to be used for specific function.

is Both the Kap140 AND the KT76c need to have their js code modified, to swap over the baros ??

or is there somewhere when the AP code inthe kernel can be told which Baro to use

 

[Geoff_D]

Good start to a new year .. got it fixed ...
Needed to change the Asobo code in both the AP and the Transponder, so the Transponder baro 3 -->2, and the AP baro from 2---3>
Then , have the Transponder's # 2 baro, always be checking and being set to Std, so when "b" is pressed, while it jumps to the altimeter setting, it quickly returns to Std Pressure using a custom PID, , so it always reads FL (as it should)

The AP is now on Baro #3, and this seems to be the one hard Coded into the kerel to be the one that the sims kernel AP code uses, to achieve correct altitude hold.
(who know what will happen when we syart getting multiple AP systems in complex planes ...!! )


SO its working correctly now .....

Still do not know what determines the AP Alt Hold baro #, ie always #3, or the Last one,
I have to assume it is FIXED at Baro #3, although I don;t see anything in the SDK that addresses this ...

Thanks ronh , made me think outside the "Transponder" box !!