Of Transponder mode and Altimeter setting

[lordofwings]

As reference I am citing the FSUIPC Offset Status document I have from the latest (4.756) release and I am using FSX. Here are my two questions:

Altimeter Setting

As we all know there is the local altimeter setting and the standard altimeter setting. In FS one can set the current altimeter setting by pressing the B key on the keyboard even though in FS all transition altitudes are 18,000 (which is wrong).

My question is, how can my application know what is the current *local* altimeter setting so that I can compare it with what I find at offset 0x0330 ?

Transponder Setting

I see that there is an offset (0x0354) with the actual setting of the transponder (squawk) code; however, I do not see any reference to whether the transponder radio is on/off or its current mode (say Mode Charlie). All the aircraft I have flown in FS (except for the old ones which I never fly) have a transponder.

For that matter, what I want to do is to be able to detect (a) when the aircraft is at the runway ready to roll and (B) that the user has activated the transponder.

If knowing the activation/mode of the transponder is not possible, can anybody suggest an alternative?

[Pete Dowson]

My question is, how can my application know what is the current *local* altimeter setting so that I can compare it with what I find at offset 0x0330 ?

I assume you mean the local QNH (pressure at sea level)? That's at offset 0EC6.

If, instead, you mean the Transition Altitude (the altitude above which you set 1013 hPA or 29.92") or the Transition Level (the Flight Level below which you set QNH), then that's going to vary according to the local ATC control, and there's nothing in FS which will tell you. You need charts.

I see that there is an offset (0x0354) with the actual setting of the transponder (squawk) code; however, I do not see any reference to whether the transponder radio is on/off or its current mode (say Mode Charlie). All the aircraft I have flown in FS (except for the old ones which I never fly) have a transponder.

FS doesn't actually simulate the transponder mode switch at all. That's why there's no offset. If you are flying on-line you use the facilities to communicate that mode to the on-line flight add-on.

For that matter, what I want to do is to be able to detect (a) when the aircraft is at the runway ready to roll and ( B) that the user has activated the transponder.

There's no easy way to test whether the aircraft is on the runway -- you'd need to compare the Lat/Lon coordinates with a database of runways (such as that created by my free MakeRunways program).

If his transponder hardware switch is assignable in FSUIPC, assign it to a user offset (one of 66C0-66FF) and test that.

Regards

Pete

[lordofwings]

Thanks, the first answer is what I needed, the QNH somehow it escaped me. What is not totally clear to me is that this offset says

"Pressure (QNH) as millibars (hectoPascals) *16."

So, does that means that the value I read from there I should multiply by 16 to get millibars/hectopascals or that the value there is 16 times the millibars setting? I am inclined that the 1st is correct (multipy to get millibars) but want to make sure I am correct.

I also see that there is a 0x0330 "Kohllsman's Window" which I assume is the one that is actually set on the altimeter gauge.

[Pete Dowson]

So, does that means that the value I read from there I should multiply by 16 to get millibars/hectopascals or that the value there is 16 times the millibars setting? I am inclined that the 1st is correct (multipy to get millibars)

No, obviously not, because you'd only then get multiples of 10 hPAs! Throughout the offsets lists the values are described as what they ARE, not how to convert them to something else. So x*16 means stored in x's units times 16. This method is consistent throughout. I've always thought it much more sesnible to state how things are stored in a storage list not how to handle them when removed. Thae latter depends what you want to do.

I also see that there is a 0x0330 "Kohllsman's Window" which I assume is the one that is actually set on the altimeter gauge.

I assumed you knew that one already because you quoted it in your question. And of course it is in the same units.

Regards

Pete

[lordofwings]

And while at that I supposed that adding the QNH offset value (0EC6) to the Adventure Weather barometric drift (04BC) would then give me the QFE if I were at the airfield. I assume these Adventure Weather entries are for FSX as well. I will have to test that I guess too, because the drift offset is still listed at ?Intl (may work).

[Pete Dowson]

And while at that I supposed that adding the QNH offset value (0EC6) to the Adventure Weather barometric drift (04BC) would then give me the QFE if I were at the airfield. I assume these Adventure Weather entries are for FSX as well. I will have to test that I guess too, because the drift offset is still listed at ?Intl (may work).

"Drift" is just an inaccuracy in the pressure reading. I've never seen a value set for it, but it is defined as part of the weather structure. There are better places than those old "adventure" offsets in any case.

QFE is the pressure setting for the altimeter (written to 0330) which will make the altimeter read zero at the airfield you set it at -- the "FE" means "field elevation". There's no way you can calculate it without reference to the elevation of the field you are flying from/to. QFE is generally only used for training and practice flights at a single airfield. It is pretty useless for normal operations.

Regards

Pete