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