Altimeter Calibration Height

"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.

On Apr 3, 12:11 am, TheSmokingGnu
wrote:
The imaginary aircraft is sitting at Imaginary Airfield (KIMG), which
sits at a billiards-table-flat 50 MSL. The aircraft's static port is
mounted at the centerline of the fuselage, which is 10 feet from the
ground. The instrument in question is mounted 4 feet above the centerline.

What will the gauge read when set to the proper barometric scale? 50,
60, or 64 feet?

Any of the above ;-) To repeat what I posted back on March 9th...

The actual altimeter test is described in FAR 43 CFR Appendix E, and
allowable error varies from 20' at sea level, to much more at higher
altitudes.

http://www.flightsimaviation.com/dat...t_43-appE.html

In theory, the instrument is calibrated on the bench to read at its
own height, but can later be adjusted in the aircraft to read wheel
height. Whether it should be or not, doesn't seem to be addressed in
the regulations that I could find.

There's an old saying, that "hell for scientists" is defined as a
place where all the conditions are perfect, but none of the
instruments are.

Kev

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.

if you mount an altimeter at the 100' level of a 200' tower,
equiped it with a static tube 100' long,
that the altimeter will read the same regardless if end of the tube,
is stationed at the top or bottom of the tower.

That is a mostly true statement. Conditions inside the tube could
differ from ambient conditions, this would engender a teeny (but real)
difference in readings.

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.

"Jose" wrote in message
t...
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.


Maybe I'm missing something here, but would it simplify to say:

That if you mount an altimeter at the 100' level of a 200' tower,
equiped it with a static tube 100' long,
that the altimeter will read the same regardless if end of the tube,
is stationed at the top or bottom of the tower.

?????

That's not logical.

AFAIC, logic itself describes the process of reasoning and deducing, not the
axioms under which it operates. To take the falling-object-example: In a
universe where gravitational force and inertia are both equally
proportional to the object's mass, it follows that ceteris paribus,
increased mass will not increase the object's acceleration. In a universe
where inertia increases faster with mass than gravitational force, a
heavier object will ceteris paribus have a lower acceleration. Both
sentences are entirely logical, that is, the final statement is inferred
from the axioms in a manner conforming to the rules of logic. Whether it
is applicable to the world we live in depends on whether the axioms apply
to the real world.

To get back to the discussion at hand, it thus makes no sense to say
that "heavy objects fall faster is logical" or "heavy objects fall faster
is not logical". The process of arriving at that statement is what logic is
all about, a single statement thus cannot be either logical or not. You
always need the axioims that you start out with, and the statement one that
you get when combining the axioms. Only then can you judge whether the
statement logically follows from the axioms.

The statement "heavy objects fall faster" may seem more or less in line with
a person's intuition, but it is in and of itself neither logical nor
unlogical.

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

"Jose" wrote in message
t...

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.


Correct.