Altimeter Calibration Height

"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).

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 there's much of a flare, the pilot might be surprised.

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

Steven P. McNicoll wrote:
"Matt Whiting" wrote in message
news
Right and the altitude indicated has little to do with the altitude of the
instrument.


Right. That's why changing the altitude of the instrument, as is done by
climbing or descending, has so little effect on the indicated altitude.



I can just as easily change the Kollsman setting and change the
indicated altitude without changing the altitude of the instrument at
all. Or I can wait for the ambient pressure to change and watch the
indicated altitude change without moving the instrument. Thus the
instrument doesn't indicate the altitude of itself, it indicates the
pressure it is experiencing.

If the altimeter truly indicated the altitude of the instrument, then it
would only change when the altitude of the instrument changed. Since
that isn't the case, it follows logically that the altimeter isn't
indicating its own altitude, but rather something else ... pressure.

You can write whatever on the dial you want to, that doesn't change what
is being indicated. I could write 10 oranges, 20 oranges, 30 oranges,
etc, around my speedometer, but that doesn't make my speedometer
indicate oranges.


Matt

Thus the instrument doesn't indicate the altitude of itself, it indicates the pressure it is experiencing.

No, it indicates the altitude (in feet, for example). It just does so
incorrectly.

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

The accuracy of the displayed altitude is such that neither of these can be
depended on. In other words, it doesn't really matter because we are not
juding flare by indicated altitude nor are we using it to vertically
separate planes by merely a few hundred feet.

But, I would love to hear from a pilot of a big plane on what occurs when
they set the altimeter based on field elevation rather than by barametric
pressure. From the lack of better information, I am presuming that there is
a noticable difference in the indicated altitude.

--
-------------------------------
Travis
Lake N3094P
PWK
"Jim Logajan" wrote in message
.. .
"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.

"Jose" wrote in message
...

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.


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

"Jim Logajan" wrote in message
.. .

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 that was the aim the altimeter would have to be calibrated to read the
same altitude as ground level when the aircraft was in a pre-touchdown nose
high attitude.

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

Your continued and insistant misuse of the words, despite your
hair-splitting nature.

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?

This tack has been taken, and shows our understanding of the issue.
Yes, the indicated altitude will change when the altimeter is moved up
to the top of the tower, despite the static port being at the base. The
physics of the instrument is not at issue. Rather, the calibration
rules are, and those are made by the FAA.

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

"Steven P. McNicoll" wrote:
In other words, an altimeter indicates altitude at the level of the
instrument itself.

That seems reasonable.

But what if the instrument itself is 100 ft tall? Or maybe just 30 inches
tall? Which point on the altimeter should one pick and why?