Altimeter Calibration Height

Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message
Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message

Perhaps you misunderstood Jose's example, but I believe he's
correct; while an altimeter senses the pressure at the level of the
insturment, it *indicates* the calibration set by either the shop
(e.g. compensating for the altitude of the installation) or the
pilot via the Kollsman window.


I understood it, I didn't say he was wrong.

Perhaps I misunderstood your response.

Jose stated:
[...] An altimeter indicates whatever it is set to indicate.
[...]

and:
If you don't get an altimeter setting, you will (likely) set the
altimeter so that the hands indicate the airport elevation as
indicated on your charts, even though you and the instrument are a
hundred feet higher.

If you do get an altimeter setting, you'll set it for that. Then
the question becomes (since the altimeter doesn't know that it's
in a tall airplane) whether, at calibration in the shop, it was
set to indicate actual instrument altitude or to indicate
something else (like instrument altitude minus a hundred feet).
[...]

To which you replied:

In other words, an altimeter indicates altitude at the level of the
instrument itself.

How do you reconcile your comment against Jose's statements? Both
statements will only be true under a very limited set of
circumstances that I would think excludes "In other words...".


You snipped part of Jose's message:

"Also, even if the static port and the instrument themselves are
separated in altitude, it would be the instrument altitude's pressure
that is sensed, since the "column of air" is connected, and ends at
the instrument."

That part was snipped because it didn't alter or directly address what an
altimeter indicates.

In other words, an altimeter indicates altitude at the level of the
instrument itself.

I think the issue is with your use of the term "indicates altitude", which
should be "senses pressure". The altimeter "indicates" via the display
(dial or digital), and the display is adjustable both during
installation/calibration and by the pilot to adjust for atmospheric
pressure. Consider that when you tell a pilot that the "altimeter is
30.12", the pilot adjusts the _indicated altitude_ by setting the Kollsman
window to that _pressure setting_. We don't watch the Kollsman (the only
"indication" of "altitude" consistent with the controller's information)
when trying to land. ;-)

Neil

"Neil Gould" wrote in message
et...

I think the issue is with your use of the term "indicates altitude", which
should be "senses pressure".


It's not an issue with me. Altimeters indicate altitude by sensing
pressure.



The altimeter "indicates" via the display
(dial or digital), and the display is adjustable both during
installation/calibration and by the pilot to adjust for atmospheric
pressure. Consider that when you tell a pilot that the "altimeter is
30.12", the pilot adjusts the _indicated altitude_ by setting the Kollsman
window to that _pressure setting_.


When you tell a pilot "altimeter 3012" the pilot simply adjusts the Kollsman
setting to 3012.



We don't watch the Kollsman (the only
"indication" of "altitude" consistent with the controller's information)
when trying to land. ;-)


Correct.

Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message
et...

I think the issue is with your use of the term "indicates altitude",
which should be "senses pressure".


It's not an issue with me. Altimeters indicate altitude by sensing
pressure.

Correct, however the "indication" (display) is not the *method* (sensing
pressure). So, your assertion: "In other words, an altimeter indicates
altitude at the level of the instrument itself." is only valid under a few
specific circumstances.

[...] Consider that when you tell a pilot that the "altimeter is
30.12", the pilot adjusts the _indicated altitude_ by setting the
Kollsman window to that _pressure setting_.


When you tell a pilot "altimeter 3012" the pilot simply adjusts the
Kollsman setting to 3012.

Do you see a difference between my statement and yours, other than the
decimal point?

Neil

"Neil Gould" wrote in message
t...

Correct, however the "indication" (display) is not the *method* (sensing
pressure).


Who said it was?



So, your assertion: "In other words, an altimeter indicates
altitude at the level of the instrument itself." is only valid under a few
specific circumstances.


What would those few specific circumstances be?



Do you see a difference between my statement and yours, other than the
decimal point?


Yes. You say the pilot adjusts the indicated altitude, I say he adjusts the
altimeter setting.

Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message
t...

Correct, however the "indication" (display) is not the *method*
(sensing pressure).


Who said it was?

You do when you imply that there is a necessary agreement between what the
altimeter senses and what it displays.

So, your assertion: "In other words, an altimeter indicates
altitude at the level of the instrument itself." is only valid under
a few specific circumstances.


What would those few specific circumstances be?

Jose listed them in response to an earlier post of yours.

Do you see a difference between my statement and yours, other than
the decimal point?


Yes. You say the pilot adjusts the indicated altitude, I say he
adjusts the altimeter setting.

You actually said, "When you tell a pilot "altimeter 3012" the pilot
simply adjusts the *Kollsman
setting* to 3012." (emphasis mine). As we described the same action, this
distinction is without a functional difference.

Neil

"Neil Gould" wrote in message
t...

You do when you imply that there is a necessary agreement between what the
altimeter senses and what it displays.


What implied that?



Jose listed them in response to an earlier post of yours.


I must have missed that one.



You actually said, "When you tell a pilot "altimeter 3012" the pilot
simply adjusts the *Kollsman setting* to 3012." (emphasis mine).
As we described the same action, this distinction is without a functional
difference.


I think there's a significant functional difference between adjusting the
altimeter setting and adjusting the indicated altitude.

Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message
t...

You do when you imply that there is a necessary agreement between
what the altimeter senses and what it displays.


What implied that?

On 4/2/07, you wrote:
"An altimeter indicates altitude at the level of the instrument itself. "

and, again:
"In other words, an altimeter indicates altitude at the level of the
instrument itself. "

And so forth. That is not necessarily so.

You actually said, "When you tell a pilot "altimeter 3012" the pilot
simply adjusts the *Kollsman setting* to 3012." (emphasis mine).
As we described the same action, this distinction is without a
functional difference.


I think there's a significant functional difference between adjusting
the altimeter setting and adjusting the indicated altitude.

Well, I wrote:
"Consider that when you tell a pilot that the "altimeter is 30.12", the
pilot adjusts the indicated altitude by setting the Kollsman window to
that pressure setting."

The "adjustment" in both cases is to the Kollsman setting. The result is a
change in the displayed altitude. So, what is different is the structure
of the sentence, not the action or intent, unless you think your omission
of the resulting displayed altitude is significant. If so, why do you
think so?

Neil

It's not an issue with me. Altimeters indicate altitude by sensing
pressure.

Yes, but there's a very important step in between, and that step is the
one that answers the original question.

Jose
--
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