Altimeter Calibration Height

Recently, Steven P. McNicoll posted:

"Jose" wrote in message
t...

Poor logic. An altimeter indicates whatever it is set to indicate.
It senses pressure, and moves hands on a dial. Take the case where
you are in a one hundred foot tall aircraft with the altimeter right
at the top of the cockpit with you, a hundred feet in the air.
(We'll neglect the tail for now).

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). I bet there are
standards for that. I don't know what they are, but they may well
incorporate the hundred feet deviation, since it is the
=installation= that is certified.

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.


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

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.

Neil

"Neil Gould" wrote in message
et...

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.

Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message
et...

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...".

Neil

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

"Neil Gould" wrote in message
et...

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."

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

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."

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

No. Correctly it would be "In other words, an altimeter =senses=
=pressure= at the level of the instrument itself."

Sensing pressure and indicating altitude are two different things.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Jose" wrote in message
t...

No. Correctly it would be "In other words, an altimeter =senses=
=pressure= at the level of the instrument itself."

Sensing pressure and indicating altitude are two different things.


An altimeter senses pressure in order to indicate altitude.

An altimeter senses pressure in order to indicate altitude.

Now we're getting somewhere. The pressure is sensed based on the laws
of physics. The altitude is indicated based on calibration and Kollsman
setting.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

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