Simvar for Alt Reporting and ATC

What’s the simvar in the A320 for Alt Reporting and ATC on the Transponder panel?

Thanks :wink:


(For future reference, please do not edit your forum post to change the question. If needed, you can remove the post and/or create a new one)

The APU MASTER does not have a single control variable, as it is plane dependant.
If your goal is to control only the a320, you can do the following:


To monitor ther value use the following code, replacing #ID# by the ID of the Pump (which can be found in flight_model.cfg)


To control it, use:


The value reported when talking to the ATC is stored in SIMVAR_INDICATED_ALTITUDE.
If you refer to the state of the transponder, it is stored in SIMVAR_TRANSPONDER_STATE (enum with the following values: 0 = off, 1 = standby, 2 = test, 3 = on, 4 = alt report, 5 = ground).


Sol / Asobo

Hi Sol,
I should report this (I might have, I forget). Transponders in real life only report the Standard pressure altitude, so, unless it’s a Standard Pressure day, the transponder altitude and indicated altitude from a properly set Altimeter will not agree. Also, in Class A airspace, pilots are supposed to change their altitude setting on the altimeter to Standard Pressure Altitude.

The transponders in MSFS are linked to the Indicated altitude, 3rd register typically. The MSFS Altimeters are linked to the Indicated altitude, typically Register 1. If the user changes the Kollsman value, then the Indicated Altitude in Register 1 updates appropriately to the Altitude corresponding to the Kollsmann setting, and the Indicated Altitude simvar is updated by that user input. The value of Register 3 will not be affected by this. If it already contains the Indicated altitude, however, then the Transponder is already set to the wrong altitude. But if the flight started at 29.92 inHg, then it will still be correct to what a normal transponder would indicate for altitude.

But, if the user hits the B key, then all three registers are set to Indicated altitude. So the Transponder will display Indicated altitude, not the correct Standard Pressure altitude it should be displaying. Where this becomes a problem is when the aircraft passes into Class A airspace. Now the pilot changes the Kollsman on the Altimeter back to 29.92. But the there is no way to adjust Register 3, so, now the Transponder and the altimeter don’t agree, and as far as ATC knows, the plane is at the wrong altitude. At least that’s how it would work in real life, I’m not sure what MSFS ATC reads.

Point is, the Transponder should read from the Pressure Altitude simvar, not Indicated Altitude simvar. Then it will always display the altitude it should be displaying. This is on purpose. ATC knows what the current pressure is everywhere. So, when they get the Pressure altitude from the Transponder, they can convert it and they know the actual altitude of the plane, in case the pilot has not set the correct indicated altitude.

The same problem exists for the Autopilot. But I need to study more to find the consequences of using the B key with the Autopilot. I do know, most KAP140’s are not linked to the altimeter, they have their own setting function for outside pressure which the pilot controls. Anyway… I mostly wanted to say that pilots have to watch out for using SIMVAR_TRANSPONDER_STATE, it may give back a result that is inconsistent with their logic.

I created my own mod for the three transponders that changes the source to Pressure Altitude. It’s not a huge deal, but, it is supposed to be a sim…

Yes, but that is another beast altogether for various autopilot systems. For example some reference what is known as geometric altitude encoded from the transponder, as well as the barometic altitude, against each other to ensure precise altitude. (this is also reported from ADS-B “out” modules and a requirement now in the USA). As you say, this is a sim, and I would hope when the Garmin updates roll out we’ll see an improvement in both of the areas mentioned.

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