Mode C Altitude Q - no Kalisman Correction? (Barometric Pressuredeviation correction?)

[GEG]

Being a student pilot, I'm eating up all kinds of info.
So I go through various instrument info, and then get
to the "Mode C Transponder" that sends altitude info.

Is that alt info built in?

If so, is there no correction for barometric pressure changes?

I don't know that it would be severe, but it seems like the
window could be +/- 100 feet from day to day that the controllers
get from the transponder.

Anyone provide some more info on this issue?

Thanks!

Gary

[Steven P. McNicoll]

"GEG" > wrote in message
...
>
> Being a student pilot, I'm eating up all kinds of info.
> So I go through various instrument info, and then get
> to the "Mode C Transponder" that sends altitude info.
>
> Is that alt info built in?
>
> If so, is there no correction for barometric pressure changes?
>
> I don't know that it would be severe, but it seems like the
> window could be +/- 100 feet from day to day that the controllers
> get from the transponder.
>
> Anyone provide some more info on this issue?
>
> Thanks!
>

There is no barometric adjustment for altitude encoders, they are set to
29.92. The correction for nonstandard pressure is made at the ATC end.

It's easy to understand why it's done this way. If the adjustment was made
by the pilot he could easily enter the wrong value in his encoder and his
altimeter. He'd then be at the wrong altitude and his encoder would provide
the same wrong altitude to ATC. By having the adjustment made at the ATC
end the radar displays the correct altitude and ATC can advise the pilot of
the error.

[Steven P. McNicoll]

"GEG" > wrote in message
...
>
> Being a student pilot, I'm eating up all kinds of info.
> So I go through various instrument info, and then get
> to the "Mode C Transponder" that sends altitude info.
>
> Is that alt info built in?
>
> If so, is there no correction for barometric pressure changes?
>

By the way, it's Kollsman, not Kalisman.

[George Patterson]

GEG wrote:
>
> Is that alt info built in?

Yes.

> If so, is there no correction for barometric pressure changes?

No.

> I don't know that it would be severe, but it seems like the
> window could be +/- 100 feet from day to day that the controllers
> get from the transponder.

True. The equipment at the controllers' end corrects for barometric pressure
fluctuations, so the info the controller sees is pretty accurate.

George Patterson
There's plenty of room for all of God's creatures. Right next to the
mashed potatoes.

[BTIZ]

> True. The equipment at the controllers' end corrects for barometric
> pressure fluctuations, so the info the controller sees is pretty accurate.
>
> George Patterson
> There's plenty of room for all of God's creatures. Right next to the
> mashed potatoes.

and when the altimeter settings get loaded into the ATC computer
incorrectly.. he sees that everyone is off altitude by the same error
factor..

BT

[Don Byrer]

>and when the altimeter settings get loaded into the ATC computer
>incorrectly.. he sees that everyone is off altitude by the same error
>factor..
>BT


hehehe
Good one! Although the separation would at least be correct....

At CLE, they do indeed change it every time the baro setting
changes...it is noted in the system every time it is changed.

--Don



Don Byrer
Instrument Pilot Commercial/CFI Student
Electronics Technician, RADAR/Data/Comm @ CLE
Amateur Radio KJ5KB

"I have slipped the surly bonds of earth; now if I can just land without bending the gear..."

[Joe Johnson]

"Don Byrer" > wrote in message
...
>
....snip...

> hehehe
> Good one! Although the separation would at least be correct....
>
....snip...

separation from other aircraft would be correct, but not necessarily from
terrain...

[George Patterson]

Joe Johnson wrote:
>
> separation from other aircraft would be correct, but not necessarily from
> terrain...

Neither would be correct for long. As soon as the controller tells one of the
pilots to fly a particular incorrect altitude and the pilot complies, things
will go to hell in a hurry.

George Patterson
There's plenty of room for all of God's creatures. Right next to the
mashed potatoes.

[Steven P. McNicoll]

"George Patterson" > wrote in message
news:TuT6e.6556$ff4.2610@trndny08...
>
> Neither would be correct for long. As soon as the controller tells one of
> the pilots to fly a particular incorrect altitude and the pilot complies,
> things will go to hell in a hurry.
>

Vertical separation between aircraft would be correct as long as the
affected aircraft are on the same altimeter setting.

[Cockpit Colin]

> True. The equipment at the controllers' end corrects for barometric
> pressure fluctuations, so the info the controller sees is pretty accurate.

In our neck of the woods (NZ) they only apply a correction below the
transition altitude - no need for any correction for flight levels.

Assume it's the same in the states

[Steven P. McNicoll]

"Cockpit Colin" > wrote in message
...
>
> In our neck of the woods (NZ) they only apply a correction below the
> transition altitude - no need for any correction for flight levels.
>
> Assume it's the same in the states
>

It's the same everywhere. It can be no other way.

[Stefan]

Steven P. McNicoll wrote:

> It's the same everywhere. It can be no other way.

Of course it could. Whether it would make sense is a different question.

Stefan

[Steven P. McNicoll]

"Stefan" > wrote in message
...
>
> Of course it could. Whether it would make sense is a different question.
>

How could it be any other way?

[Stefan]

Steven P. McNicoll wrote:

> How could it be any other way?

Just a few examples: It could be QNH everywhere (requires defined QNH
regions). It could be GPS altitude (would even simplify things a lot,
but requires all aircraft to be expensively equipped). Heck, it could
even be radar altitude everywhere, of course compensated for ground
elevation. As I said, whether it would make sense or even be practical
is a different question, but there are a lot of solutions which are
thinkable.

Stefan

[Steven P. McNicoll]

"Stefan" > wrote in message
...
>
> Just a few examples: It could be QNH everywhere (requires defined QNH
> regions).
>

If it's QNH everywhere there's no transition altitude.


>
> It could be GPS altitude (would even simplify things a lot, but requires
> all aircraft to be expensively equipped). Heck, it could even be radar
> altitude everywhere, of course compensated for ground elevation. As I
> said, whether it would make sense or even be practical is a different
> question, but there are a lot of solutions which are thinkable.
>

How does one set the barometric pressure for GPS altitude or radar altitude?

[Stefan]

Steven P. McNicoll wrote:

> If it's QNH everywhere there's no transition altitude.
....

Right.

As I understood, you said: This is the only way to set the altimeter. I
answered: No, there are a myriade of ways to do it, from which one was
chosen, probably for good reasons.

Maybe I misunderstood your first statement.

Stefan

[Steven P. McNicoll]

"Stefan" > wrote in message
...
>
> Right.
>
> As I understood, you said: This is the only way to set the altimeter. I
> answered: No, there are a myriade of ways to do it, from which one was
> chosen, probably for good reasons.
>
> Maybe I misunderstood your first statement.
>

I was responding to Cockpit Colin's message on how things are done in New
Zealand. He said a correction for barometric pressure is applied only below
the transition altitude, and that there's no need for any correction for
flight levels. He assumed it's the same way in the US. I said it's the
same way everywhere, as the definitions of "transition altitude" and "flight
level" require it to be that way.

[Chris]

"Steven P. McNicoll" > wrote in message
ink.net...
>
> "Stefan" > wrote in message
> ...
>>
>> Right.
>>
>> As I understood, you said: This is the only way to set the altimeter. I
>> answered: No, there are a myriade of ways to do it, from which one was
>> chosen, probably for good reasons.
>>
>> Maybe I misunderstood your first statement.
>>
>
> I was responding to Cockpit Colin's message on how things are done in New
> Zealand. He said a correction for barometric pressure is applied only
> below the transition altitude, and that there's no need for any correction
> for flight levels. He assumed it's the same way in the US. I said it's
> the same way everywhere, as the definitions of "transition altitude" and
> "flight level" require it to be that way.

For example in the UK, transition level tends to be 3000 ft except around
the major airports where it can be as high as 6000ft. This means that the
lowest FL is usually FL035.

The normal practice is to take off on the airfieldQNH, and then if on a
cross country trip, readjust the setting to the Regional QNH. This is the
lowest forecast QNH for the respective region.

On arrival at an airfield we set QFE which gives zero elevation when
landed.

Only IFR traffic is obliged to use flight levels above 3000ft or the
transition altitude (in fact one of the definitions of IFR is the use of
flight levels). VFR traffic can use then if they want but then should follow
Instrument flight Rules.

cb