Altimeter Calibration Height

"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

"Neil Gould" wrote in message
. net...

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.


Why not?

Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message
. net...

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.


Why not?

After puzzling the difficulties that several of us have had in trying to
explain the differences between "indicated" (e.g. what the pilot sees) --
and "senses" (how the altimeter is calibrated), I can only guess that for
you, sitting in the tower, there is no practical difference because you
are always at the same altitude. ;-) You can review some of the
excellent explanations that Jose and others have provided to answer your
question.

The original question asked whether the altimeter is set to indicate the
altitude at the wheels or "at the level...",
and if that question has been definitively answered, I missed it.

Neil

The original question asked whether the altimeter is set to indicate the
altitude at the wheels or "at the level...",
and if that question has been definitively answered, I missed it.

Somebody posted a link to the altimeter calibration procedure, and that
procedure calibrates the altimeter to the altitude of the instrument.
There is no provision in that document for adjusting the indicated
altitude to account for the relative position of the instrument in the
aircraft.

There may be another document that does so - an altimeter is not
certified for IFR just by itself, the =installation= has to be certified
too, and the FAA could easily require such an adjustment as not.

OTOH, there may =not= be another document which does so. When an
altimeter =installation= is certified, what procedure is followed?

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

"Neil Gould" wrote:
The original question asked whether the altimeter is set to indicate
the altitude at the wheels or "at the level...",
and if that question has been definitively answered, I missed it.

Assuming an aircraft of large vertical dimension...

If the point of an altimeter is to directly display to the pilot how far
the lowest point on the aircraft is above surface obstructions (or wheels
above a runway), then I would presume the altimeter would be calibrated so
that when the aircraft rests on the ground it reads the same altitude as
that ground level.

If the point of an altimeter is to directly display to the pilot the
altitude to fly so that the aircraft is as far as possible from other
aircraft also obeying 91.159, then I would presume the altimeter would be
calibrated so that when the aircraft rests on the ground it reads the same
altitude as the ground level plus half the height of the aircraft.

And so on for other uses (e.g. keeping the distance of the top of aircraft
safely below a cloud ceiling of known altitude).

"Neil Gould" wrote in message
t...

After puzzling the difficulties that several of us have had in trying to
explain the differences between "indicated" (e.g. what the pilot sees) --
and "senses" (how the altimeter is calibrated), I can only guess that for
you, sitting in the tower, there is no practical difference because you
are always at the same altitude. ;-) You can review some of the
excellent explanations that Jose and others have provided to answer your
question.


What made you think I needed the difference between "indicated" and "senses"
explained to me?



The original question asked whether the altimeter is set to indicate the
altitude at the wheels or "at the level...",
and if that question has been definitively answered, I missed it.



Let's take a different tack, maybe this will help.

Let's say you've got an altimeter with a 100' flexible static line sitting
at the base of a 100' tower. You note the altitude indicated on the
altimeter and set off with it to the top of the tower, letting the static
line play out. Don't look down. When you get to the top, would you expect
to see a change in the indicated altitude, or would you expect it to be the
same as noted previously?

Now pull the 100' flexible line to the top of the tower. Would you expect
to see a change in the indicated altitude, or would you expect it to be the
same as that indicated when the open end of the static line was alone on the
surface?

Now fasten the 100' flexible line to the top of the tower and descend the
tower with the altimeter. When you return to the surface would you expect
to see a change in the indicated altitude, or would you expect it to be the
same as that indicated when the altimeter and static line were at the top of
the tower?

Now go back up the tower and unfasten your static line.