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