Altimeter Calibration Height

[TheSmokingGnu]

Here's one that may seem silly: to what height, precisely, is an
altimeter calibrated to?

The bottom of the landing gear? The height of the static port? the chord
line of the fuselage? In a Cessna, the difference between these is
minimal, but on an airliner, you're talking about a possible +/- 10-15
feet (even discounting the radar altimeter).

What is the answer, o Swammi of Salami?

TheSmokingGnu

[Jim Macklin]

the wheels.


"TheSmokingGnu" > wrote
in message ...
| Here's one that may seem silly: to what height, precisely,
is an
| altimeter calibrated to?
|
| The bottom of the landing gear? The height of the static
port? the chord
| line of the fuselage? In a Cessna, the difference between
these is
| minimal, but on an airliner, you're talking about a
possible +/- 10-15
| feet (even discounting the radar altimeter).
|
| What is the answer, o Swammi of Salami?
|
| TheSmokingGnu

[Tony]

When you dial in the altimeter setting, don't you check the altimeter
readout against the field elevation? That's what it's set to, whatever
is the datum for the field elevation. I do think the only fields that
are flat to within 10 feet might be places like the seaplane airport
in San Diego. Or maybe airports in Florida -- there just aren't many
hills there.


On Mar 8, 2:36 am, "Jim Macklin"
> wrote:
> the wheels.
>
> "TheSmokingGnu" > wrote
> in ...
> | Here's one that may seem silly: to what height, precisely,
> is an
> | altimeter calibrated to?
> |
> | The bottom of the landing gear? The height of the static
> port? the chord
> | line of the fuselage? In a Cessna, the difference between
> these is
> | minimal, but on an airliner, you're talking about a
> possible +/- 10-15
> | feet (even discounting the radar altimeter).
> |
> | What is the answer, o Swammi of Salami?
> |
> | TheSmokingGnu

[Michelle P]

TheSmokingGnu wrote:
> Here's one that may seem silly: to what height, precisely, is an
> altimeter calibrated to?
>
> The bottom of the landing gear? The height of the static port? the chord
> line of the fuselage? In a Cessna, the difference between these is
> minimal, but on an airliner, you're talking about a possible +/- 10-15
> feet (even discounting the radar altimeter).
>
> What is the answer, o Swammi of Salami?
>
> TheSmokingGnu
When doing the bi-annual? the calbration is at the height of static port.

Michelle

[[email protected]]

On Mar 8, 6:56 am, Michelle P
> wrote:
> TheSmokingGnu wrote:
> > Here's one that may seem silly: to what height, precisely, is an
> > altimeter calibrated to?
>
> > The bottom of the landing gear? The height of the static port? the chord
> > line of the fuselage? In a Cessna, the difference between these is
> > minimal, but on an airliner, you're talking about a possible +/- 10-15
> > feet (even discounting the radar altimeter).
>
> > What is the answer, o Swammi of Salami?
>
> > TheSmokingGnu
>
> When doing the bi-annual? the calbration is at the height of static port.
>
> Michelle

But a static port at a lower level won't result in a higher
static pressure at the altimeter. There's a pressure drop with
altitude inside the line, too, just as there is in a water-filled
tube. I would imagine the pilot of a machine with its altimeter 20
feet off the ground would note a 20-foot discrepancy with the field
elevation once the altimeter has been set to the controller's numbers.

Dan

[Capt. Geoffrey Thorpe]

"TheSmokingGnu" > wrote in message
...
> Here's one that may seem silly: to what height, precisely, is an altimeter
> calibrated to?
>
> The bottom of the landing gear? The height of the static port? the chord
> line of the fuselage? In a Cessna, the difference between these is
> minimal, but on an airliner, you're talking about a possible +/- 10-15
> feet (even discounting the radar altimeter).
>
> What is the answer, o Swammi of Salami?
>
> TheSmokingGnu

The altitude of the bench in the avionics shop.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Michelle P]

wrote:
> On Mar 8, 6:56 am, Michelle P
> > wrote:
>
>>TheSmokingGnu wrote:
>>
>>>Here's one that may seem silly: to what height, precisely, is an
>>>altimeter calibrated to?
>>
>>>The bottom of the landing gear? The height of the static port? the chord
>>>line of the fuselage? In a Cessna, the difference between these is
>>>minimal, but on an airliner, you're talking about a possible +/- 10-15
>>>feet (even discounting the radar altimeter).
>>
>>>What is the answer, o Swammi of Salami?
>>
>>>TheSmokingGnu
>>
>>When doing the bi-annual? the calbration is at the height of static port.
>>
>>Michelle
>
>
> But a static port at a lower level won't result in a higher
> static pressure at the altimeter. There's a pressure drop with
> altitude inside the line, too, just as there is in a water-filled
> tube. I would imagine the pilot of a machine with its altimeter 20
> feet off the ground would note a 20-foot discrepancy with the field
> elevation once the altimeter has been set to the controller's numbers.
>
> Dan
>
that is why they allow a 75ft deviation.
Michelle

[george]

On Mar 9, 12:24 pm, Michelle P
> wrote:
> wrote:
> > On Mar 8, 6:56 am, Michelle P
> > > wrote:
>
> >>TheSmokingGnu wrote:
>
> >>>Here's one that may seem silly: to what height, precisely, is an
> >>>altimeter calibrated to?
>
> >>>The bottom of the landing gear? The height of the static port? the chord
> >>>line of the fuselage? In a Cessna, the difference between these is
> >>>minimal, but on an airliner, you're talking about a possible +/- 10-15
> >>>feet (even discounting the radar altimeter).
>
> >>>What is the answer, o Swammi of Salami?
>
> >>>TheSmokingGnu
>
> >>When doing the bi-annual? the calbration is at the height of static port.
>
> >>Michelle
>
> > But a static port at a lower level won't result in a higher
> > static pressure at the altimeter. There's a pressure drop with
> > altitude inside the line, too, just as there is in a water-filled
> > tube. I would imagine the pilot of a machine with its altimeter 20
> > feet off the ground would note a 20-foot discrepancy with the field
> > elevation once the altimeter has been set to the controller's numbers.
>
> > Dan
>
> that is why they allow a 75ft deviation.
> Michelle


Thats what we need deviant altimeters :-)

[Jose]

> Thats what we need deviant altimeters :-)

They go with deviant pilots, no?

Jose
--
Humans are pack animals. Above all things, they have a deep need to
follow something, be it a leader, a creed, or a mob. Whosoever fully
understands this holds the world in his hands.
for Email, make the obvious change in the address.

[Kev]

>Here's one that may seem silly: to what height, precisely, is an
>altimeter calibrated to?
>The bottom of the landing gear? The height of the static port? the chord
>line of the fuselage? In a Cessna, the difference between these is
>minimal, but on an airliner, you're talking about a possible +/-
>10-15 feet (even discounting the radar altimeter).

Good question. In practice, it would depend on the mechanic, and
whether he cares enough to fine-tune the altimeter that much.

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/data/FARS/part_43-appE.html

The oft-quoted "75" feet error check is just a rule of thumb from the
FAA Instrument Flying Handbook, Chapter 3, page 3-4:

http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/

Now for a bit of weird history. I don't have the reference handy, but
one common atmospheric formula used for converting between indicated
altitude and outside pressure, includes a .01" Hg correction between
the altimeter and the ground, or about 10 feet. This was apparently
the most common instrument position some 70 years ago when it was
created (!!)

Cheers, Kev

[Dallas]

On Wed, 07 Mar 2007 21:37:53 -0800, TheSmokingGnu wrote:

> The bottom of the landing gear? The height of the static port?

If you want to complicate your question ask about the other end of the
measurement, MSL.

From where do they measure MSL? Sea surface topography is constantly
changing. Do you measure from the tops of the waves? Bottoms? High tide?
Average tide? Average tide in the Pacific or average tide in the Atlantic?
How do you measure MSL in Panama? How do you get an average when currents,
air pressure variations, temperature and salinity variations are constantly
in flux? What about the melting and freezing of snow and glaciers? How
much time do you need to create this average?


--
Dallas

[[email protected]]

On Mar 9, 10:37 pm, Dallas > wrote:
> On Wed, 07 Mar 2007 21:37:53 -0800, TheSmokingGnu wrote:
> > The bottom of the landing gear? The height of the static port?
>
> If you want to complicate your question ask about the other end of the
> measurement, MSL.
>
> From where do they measure MSL? Sea surface topography is constantly
> changing. Do you measure from the tops of the waves? Bottoms? High tide?
> Average tide? Average tide in the Pacific or average tide in the Atlantic?
> How do you measure MSL in Panama? How do you get an average when currents,
> air pressure variations, temperature and salinity variations are constantly
> in flux? What about the melting and freezing of snow and glaciers? How
> much time do you need to create this average?
>
> --
> Dallas

I know Wikipedia has something of a reputation, but they
put it in simple terms. Read http://en.wikipedia.org/wiki/Sea_level

They say MSL is 20' higher at the Atlantic end of Panama than at
the Pacific end. In the canal, of course, it will be much higher
between locks. That's not sea level anymore, it's manmade lake, about
80' MSL.

Dan

[george]

On Mar 11, 6:10 am, wrote:
> On Mar 9, 10:37 pm, Dallas > wrote:
>
>
>
> > On Wed, 07 Mar 2007 21:37:53 -0800, TheSmokingGnu wrote:
> > > The bottom of the landing gear? The height of the static port?
>
> > If you want to complicate your question ask about the other end of the
> > measurement, MSL.
>
> > From where do they measure MSL? Sea surface topography is constantly
> > changing. Do you measure from the tops of the waves? Bottoms? High tide?
> > Average tide? Average tide in the Pacific or average tide in the Atlantic?
> > How do you measure MSL in Panama? How do you get an average when currents,
> > air pressure variations, temperature and salinity variations are constantly
> > in flux? What about the melting and freezing of snow and glaciers? How
> > much time do you need to create this average?
>
> > --
> > Dallas
>
> I know Wikipedia has something of a reputation, but they
> put it in simple terms. Readhttp://en.wikipedia.org/wiki/Sea_level
>
> They say MSL is 20' higher at the Atlantic end of Panama than at
> the Pacific end. In the canal, of course, it will be much higher
> between locks. That's not sea level anymore, it's manmade lake, about
> 80' MSL.


Which is where QFE comes in :-)

[Steven P. McNicoll]

"TheSmokingGnu" > wrote in message
...
>
> Here's one that may seem silly: to what height, precisely, is an altimeter
> calibrated to?
>
> The bottom of the landing gear? The height of the static port? the chord
> line of the fuselage? In a Cessna, the difference between these is
> minimal, but on an airliner, you're talking about a possible +/- 10-15
> feet (even discounting the radar altimeter).
>
> What is the answer, o Swammi of Salami?
>

An altimeter indicates altitude at the level of the instrument itself.

[Stefan]

Steven P. McNicoll schrieb:

> An altimeter indicates altitude at the level of the instrument itself.

Source?

[Steven P. McNicoll]

"Stefan" > wrote in message
...
>
> Source?
>

Logic. Where is the sensing mechanism located?

[Jose]

>>> An altimeter indicates altitude at the level of the instrument itself.
>>Source?
> Logic. Where is the sensing mechanism located?

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.

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

[Steven P. McNicoll]

"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.

[Stefan]

Steven P. McNicoll schrieb:

>> Source?

> Logic. Where is the sensing mechanism located?

You are aware that the altimeter displays the true altitude only in very
untypical, special cirumstances, are you?

[Steven P. McNicoll]

"Stefan" > wrote in message
...
>
> You are aware that the altimeter displays the true altitude only in very
> untypical, special cirumstances, are you?
>

Yes. I said nothing about that.

[Stefan]

Steven P. McNicoll schrieb:

>> You are aware that the altimeter displays the true altitude only in very
>> untypical, special cirumstances, are you?

> Yes. I said nothing about that.

Message-ID: et>:
An altimeter indicates altitude at the level of the instrument itself.

Message-ID: k.net>:
In other words, an altimeter indicates altitude at the level of the
instrument itself.


Did you write this or was I just hearing voices?

[Steven P. McNicoll]

"Stefan" > wrote in message
...
>
> Message-ID: et>:
> An altimeter indicates altitude at the level of the instrument itself.
>
> Message-ID: k.net>:
> In other words, an altimeter indicates altitude at the level of the
> instrument itself.
>
>
> Did you write this or was I just hearing voices?
>

Yes, I wrote that. What is your point?

[John Godwin]

Stefan > wrote in
:

> Did you write this or was I just hearing voices?
>
Hearing voices. Nothing was said about the accuracy of the altitude
shown.


--

[Stefan]

Steven P. McNicoll schrieb:

> What is your point?

You:
>> An altimeter indicates altitude at the level of the instrument
>> itself.

Me:
>> Source?

You:
> Logic. Where is the sensing mechanism located?


Now if the altimeter doesn't display the true altitude, your logic is
unfounded.

[Stefan]

John Godwin schrieb:

> Nothing was said about the accuracy of the altitude shown.

The question was whether the altimeter shows the altitude of the cockpit
or that of the wheels, and you say that accuracy wasn't a topic? :-))))

[Steven P. McNicoll]

"Stefan" > wrote in message
. ..
>
> Now if the altimeter doesn't display the true altitude, your logic is
> unfounded.
>

What are the voices saying to you? I said nothing about true altitude.

Think of a 200' tower situated at sea level in a standard atmosphere.
There are three identical, perfectly accurate altimeters mounted on
the tower at the 100' level, all set to 29.92. One of the altimeters is
vented directly to the atmosphere, one has a static line running to the
top of the tower, and one has a static line running to the base.

Question: What altitudes are indicated on the three altimeters?















Answer: They all indicate 100'.

The static lines are vented to the atmosphere, so the change in pressure
with altitude takes place in the static lines just as it does in the
atmosphere.
The pressure in the three altimeter cases is the same, so the indicated
altitude is the same.

Hope this helps.

[Stefan]

Steven P. McNicoll schrieb:

> The static lines are vented to the atmosphere, so the change in pressure
> with altitude takes place in the static lines just as it does in the
> atmosphere.
> The pressure in the three altimeter cases is the same, so the indicated
> altitude is the same.

Ever heard of Kollman?

[Steven P. McNicoll]

"Stefan" > wrote in message
. ..
>
> Ever heard of Kollman?
>

No. Is there any reason I should have?

[Stefan]

Steven P. McNicoll schrieb:

>> Ever heard of Kollman?

> No. Is there any reason I should have?

No, not with that typo. In the kollsman window the pilot can choose what
altitude the altimeter should display. This choice may or may not be
related to the actual altitude of the airplane.

But I start to understand your reasoning: *If* the airplane sits on the
ground, and *if* the QNH tuned into the kollmans window is accurate,
*then* the altitude displayed is the altitude of the instrument itself.

But the original poster didn't specifiy any of these circumstances, and
so you didn't, either.

[Jose]

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

No. The altimeter senses pressure at the level of the instrument
itself. It indicates altitude depending on its calibration and kollsman
setting.

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

[Steven P. McNicoll]

"Jose" > wrote in message
. ..
>
> No. The altimeter senses pressure at the level of the instrument itself.
> It indicates altitude depending on its calibration and kollsman setting.
>

You're contradicting yourself.

[Jose]

> The static lines are vented to the atmosphere, so the change in pressure
> with altitude takes place in the static lines just as it does in the
> atmosphere.

That's not logic, that's physics. Many things that are logical (heavy
objects fall faster) are not supported by the laws of physics. That
degree of physics knowledge is not necessary to piloting an aircraft
(though it is helpful), thus I expect not all pilots know this.

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

[Steven P. McNicoll]

"Jose" > wrote in message
...
>
> Many things that are logical (heavy objects fall faster) are not supported
> by the laws of physics.

That's not logical.

[Jose]

>>No. The altimeter senses pressure at the level of the instrument itself.
>> It indicates altitude depending on its calibration and kollsman setting.
> You're contradicting yourself.

Only if pressure and altitude are the same.

And only if sensing and indicating are the same.

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

[Jose]

> That's not logical.

Of course it's logical. The heavier they are, the more force is exerted
on them. You can test this by dropping a stone and a feather.

As it turns out, in our universe, increased inertia exactly
counterbalances the increased gravitational force, but there is no
(logical) reason for the universe to operate that way. In fact, I am
not convinced that it does operate that way to the last decimal point.

If logic were sufficient, there would be no experimental physics.

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

[Jose]

>>Huh? Drag force isn't logical.
> It is to me.

No, it is merely consistant with your experience. That is empirical,
not theoretical. Drag force can be derived in theory, but that requires
knowledge of atoms and molecules, which themselves are not logical. The
universe could be made up of wiggly strings, for all we know. That it
is made up of atoms is another =experimental= result.

That they are governed by quantum mechanics is yet =another=
experimental result, which is most =definately= not logical.

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

[Neil Gould]

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

[Steven P. McNicoll]

"Jose" > wrote in message
t...
>
> Of course it's logical. The heavier they are, the more force is exerted
> on them. You can test this by dropping a stone and a feather.
>

I'd have to do it in a vacuum to eliminate the drag force.

[Steven P. McNicoll]

"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.

[Travis Marlatte]

I agree that it measures pressure and that the indicated altitude is based
on calibration. I agree that it could be calibrated for the bottom, mid, or
top of the plane (for whatever it would matter). Personally, my altimeter
was calibrated sitting on a bench with no regard for the difference in
height between the landing gear, the static port, or the position of the
altimeter.

From a purely theoretical view, I don't think you can say that an altimeter
measures pressure at the location of the instrument. It's a closed system to
the static port. While the pressure inside the tubing will respond to
elevation changes similar to the open atmosphere, there is a difference.
Admitedly, the difference over the height of an aircraft is insignificant.
But stretch that tubing to a low pressure system in the next state and your
altimeter will defininetly not be reading the local pressure.

The question remains, are big airplane altimeters calibrated to account for
the distance between the landing gear and the instrument. Someone else asked
about when setting the altimeter based on field elevation rather than by
Kollsman setting. Another way to ask the question is, "When setting the
altimeter to field elevation in a tall airliner, how close is the Kollsman
value to the local barametric pressure?"

--
-------------------------------
Travis
Lake N3094P
PWK
"Jose" > wrote in message
. ..
>> In other words, an altimeter indicates altitude at the level of the
>> instrument itself.
>
> No. The altimeter senses pressure at the level of the instrument itself.
> It indicates altitude depending on its calibration and kollsman setting.
>
> Jose
> --
> Get high on gasoline: fly an airplane.
> for Email, make the obvious change in the address.

[Don Tuite]

Tangential question for Steven:

I think you work in a control tower. If you work in a center in a
building at ground level, pretend you work in a tower.

You have a barometer there that tells you the sea-level pressure at
the airport. (That is, what a pressure gauge would read if you dug a
hole down to sea level and read the pressure down there.) That's the
pressure you announce on ATIS.

When the barometer was calibrated, did the calibration take into
account the height of the control tower?

Don

[Steven P. McNicoll]

"Don Tuite" > wrote in message
...
>
> Tangential question for Steven:
>
> I think you work in a control tower. If you work in a center in a
> building at ground level, pretend you work in a tower.
>

I presently work in a tower, I used to work in a center.


>
> You have a barometer there that tells you the sea-level pressure at
> the airport. (That is, what a pressure gauge would read if you dug a
> hole down to sea level and read the pressure down there.) That's the
> pressure you announce on ATIS.
>
> When the barometer was calibrated, did the calibration take into
> account the height of the control tower?
>

We don't have a barometer in the tower. We have an ASOS on the field,
that's the source of the altimeter setting we put on the ATIS.

[Newps]

Don Tuite wrote:


>
> You have a barometer there that tells you the sea-level pressure at
> the airport.

Yes, every tower has one.


(That is, what a pressure gauge would read if you dug a
> hole down to sea level and read the pressure down there.) That's the
> pressure you announce on ATIS.

Some facilities use their ASOS some use their instrument in the tower.

[Jose]

> I'd have to do it in a vacuum to eliminate the drag force.

Huh? Drag force isn't logical. It is an empirical observation.

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

[Steven P. McNicoll]

"Jose" > wrote in message
. net...
>
> Huh? Drag force isn't logical.

It is to me.

[Steven P. McNicoll]

"Jose" > wrote in message
. ..
>
> No, it is merely consistant with your experience. That is empirical, not
> theoretical. Drag force can be derived in theory, but that requires
> knowledge of atoms and molecules, which themselves are not logical. The
> universe could be made up of wiggly strings, for all we know. That it is
> made up of atoms is another =experimental= result.
>
> That they are governed by quantum mechanics is yet =another= experimental
> result, which is most =definately= not logical.
>

Nope, it's logical to me.

[Jose]

>> ... quantum mechanics [...] is most =definately= not logical.
> Nope, it's logical to me.

That explains a lot. :)

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

[Steven P. McNicoll]

"Jose" > wrote in message
et...
>
> That explains a lot. :)
>

Glad I could help!

[Matt Whiting]

Steven P. McNicoll wrote:
> "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.

No, it indicates a pressure differential at the level of the instrument.

Matt

[Matt Whiting]

Steven P. McNicoll wrote:
> "Stefan" > wrote in message
> ...
>> Message-ID: et>:
>> An altimeter indicates altitude at the level of the instrument itself.
>>
>> Message-ID: k.net>:
>> In other words, an altimeter indicates altitude at the level of the
>> instrument itself.
>>
>>
>> Did you write this or was I just hearing voices?
>>
>
> Yes, I wrote that. What is your point?
>
>

I think his point is that if you turn the knob and change the setting in
the Kollsman window, you can indicate any altitude you want. Since the
altitude of the instrument hasn't changed, but the altitude indicated by
the instrument has changed; it isn't possible then that the altimeter is
indicating the altitude at the level of the instrument. Is it
indicating whatever altitude you tell it to indicate.

Matt

[Matt Whiting]

Stefan wrote:
> Steven P. McNicoll schrieb:
>
>> The static lines are vented to the atmosphere, so the change in pressure
>> with altitude takes place in the static lines just as it does in the
>> atmosphere.
>> The pressure in the three altimeter cases is the same, so the indicated
>> altitude is the same.
>
> Ever heard of Kollman?

No, is he any relation to Kollsman?

Matt

[Matt Whiting]

Jose wrote:
>> I'd have to do it in a vacuum to eliminate the drag force.
>
> Huh? Drag force isn't logical. It is an empirical observation.

Sure it is. You are moving one thing through another thing. It is
quite logical that the thing you are moving through is going to be
unhappy about that and resist your motion! :-)

Matt

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> No, it indicates a pressure differential at the level of the instrument.
>

Show me an altimeter that indicates in units of pressure.

[Matt Whiting]

Steven P. McNicoll wrote:
> "Matt Whiting" > wrote in message
> ...
>> No, it indicates a pressure differential at the level of the instrument.
>>
>
> Show me an altimeter that indicates in units of pressure.

Show me one that indicates in units of "altitude at the level of the
instrument."

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> Show me one that indicates in units of "altitude at the level of the
> instrument."
>

http://www.auf.asn.au/groundschool/altimeter1.jpg

[Matt Whiting]

Steven P. McNicoll wrote:
> "Matt Whiting" > wrote in message
> ...
>> Show me one that indicates in units of "altitude at the level of the
>> instrument."
>>
>
> http://www.auf.asn.au/groundschool/altimeter1.jpg
>
>

Try again. It is adjustable so I can make it read any altitude I like.

[TheSmokingGnu]

Honestly, I don't know how you guys got Newton involved. :D

How about a (better) hypothetical?

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?

TheSmokingGnu

[[email protected]]

TheSmokingGnu > wrote:
> Honestly, I don't know how you guys got Newton involved. :D

> How about a (better) hypothetical?

> 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?

Not enough potential aswers, you for got:

Under IFR

Under VFR

Pass Go and collect $200

No one is going to tell me how to set my altimeter

--
Jim Pennino

Remove .spam.sux to reply.

[Jose]

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

That depends on what the people in the calibration shop did to it. My
understanding is that the =installation= is certified. It would be
logical for the certified installation to be calibrated to the wheel
height. It would be equally logical for it to be certified to the
instrument height.

It's an FAA certification. 'nuf said. :)

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

[Neil Gould]

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...".

Neil

[Steven P. McNicoll]

"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.

[Kev]

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/data/FARS/part_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

[Jose]

> 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.

[Jose]

> 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.

[Steven P. McNicoll]

"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.

[Jose]

> 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.

[Maxwell]

"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.

?????

[Anno v. Heimburg]

> 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.

[Neil Gould]

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

[Steven P. McNicoll]

"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.

[Steven P. McNicoll]

"Maxwell" > wrote in message
...
>
> 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.
>
> ?????
>

Why didn't I think of that?

[Steven P. McNicoll]

"Neil Gould" > wrote in message
et...
>
> I think the issue is with your use of the term "indicates altitude", which
> should be "senses pressure".
>

It's not an issue with me. Altimeters indicate altitude by sensing
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_.
>

When you tell a pilot "altimeter 3012" the pilot simply adjusts the Kollsman
setting to 3012.


>
> We don't watch the Kollsman (the only
> "indication" of "altitude" consistent with the controller's information)
> when trying to land. ;-)
>

Correct.

[Neil Gould]

Recently, Steven P. McNicoll > posted:

> "Neil Gould" > wrote in message
> et...
>>
>> I think the issue is with your use of the term "indicates altitude",
>> which should be "senses pressure".
>>
>
> It's not an issue with me. Altimeters indicate altitude by sensing
> pressure.
>
Correct, however the "indication" (display) is not the *method* (sensing
pressure). 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.

>> [...] 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_.
>>
>
> When you tell a pilot "altimeter 3012" the pilot simply adjusts the
> Kollsman setting to 3012.
>
Do you see a difference between my statement and yours, other than the
decimal point?

Neil

[Maxwell]

"Jose" > wrote in message
. ..
>> 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

You mean like pressure loss of the long length or something?

[Steven P. McNicoll]

"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.

[Jose]

> It's not an issue with me. Altimeters indicate altitude by sensing
> pressure.

Yes, but there's a very important step in between, and that step is the
one that answers the original question.

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

[Capt. Geoffrey Thorpe]

"Matt Whiting" > wrote in message
...
> Steven P. McNicoll wrote:
>> "Matt Whiting" > wrote in message
>> ...
>>> Show me one that indicates in units of "altitude at the level of the
>>> instrument."
>>>
>>
>> http://www.auf.asn.au/groundschool/altimeter1.jpg
>
> Try again. It is adjustable so I can make it read any altitude I like.

Is that a good way to avoid busting Class Bravo?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Neil Gould]

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

[Matt Whiting]

Jose wrote:
>> 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.

Right and the altitude indicated has little to do with the altitude of
the instrument.

Matt

[Matt Whiting]

Maxwell wrote:
> "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.

Yes, that is correct. There might be a slight transient as you moved
the tube, but at steady state the location of the tube would not matter.

Matt

[Peter Dohm]

> >> 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
>
> You mean like pressure loss of the long length or something?
>
>
Humidity or moisture content comes most readily to my mind.

Peter

[Jose]

> You mean like pressure loss of the long length or something?

The conditions inside the tube are different from the conditions outside
the tube. I expect they'd be very similar, but given a very long, very
insulated tube, which starts out with (say) dry air, followed by the
passage of a moist front, there would be a difference.

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

[Jose]

>
> Right and the altitude indicated has little to do with the altitude of the instrument.

Well, that's a little extreme. They are related like integrals to the
same function: +C

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

[Maxwell]

"Jose" > wrote in message
t...
>> You mean like pressure loss of the long length or something?
>
> The conditions inside the tube are different from the conditions outside
> the tube. I expect they'd be very similar, but given a very long, very
> insulated tube, which starts out with (say) dry air, followed by the
> passage of a moist front, there would be a difference.
>

Agreed, and even moisture would be a factor as Peter suggested.

By the tower example I was just trying to verify that the altimeter measured
presssure based on it's location, not the location of the static port, with
the exception of errors caused a longer or shorter length of hose to the
port.

[Steven P. McNicoll]

"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.

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> 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.

[Neil Gould]

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

[Steven P. McNicoll]

"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?

[Neil Gould]

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

[Jose]

> 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.

[Jim Logajan]

"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).

[Jose]

> 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.

[Matt Whiting]

Steven P. McNicoll wrote:
> "Matt Whiting" > wrote in message
> ...
>> 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

[Jose]

> 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.

[Travis Marlatte]

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).

[Steven P. McNicoll]

"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.

[Steven P. McNicoll]

"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.

[Steven P. McNicoll]

"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.

[Jose]

> 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.

[Jim Logajan]

"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?

[Jose]

>> 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?

To answer that question, one must understand that an altimeter does not
sense altitude, it senses pressure. The pressure would be some sort of
average over the entire height of the sensing surface, whose exact value
would depend on the construction of the device. That pressure would
likely be equal to the ambient pressure at some altitude within that range.

By painting appropriate numbers next to the pointer, one could get the
altimeter to indicate altitude. It is the choice of numbers that is the
issue, as that is arbitrary, and not subject to the laws of physics,
only to the laws of the FAA.

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

[Neil Gould]

Recently, Steven P. McNicoll > posted:

> "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?
>
Your usage of the two terms.

> Let's take a different tack, maybe this will help.
>
BTDT, and understand the mechanics involved. I do appreciate your
challenges that make me think about this more thoroughly, though. Also,
consider the distinction between calibration and adjustment, where
calibration is how the altimeter is installed, and adjustment is made via
the Kollsman window/knob. Both of these factors can affect the indicated
altitude regardless of the actual altitude of the altimeter.

Neil

[Matt Whiting]

Steven P. McNicoll wrote:

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

Yes, great idea.

Take an altimeter and set it on a table. Don't move it!

Turn the knob that adjusts the pressure scale to one extreme. Record
the indication on the face of the altimeter. Now turn the knob to the
other extreme of travel. Record the indication on the face of the
instrument. Now the big question: which value recorded is the altitude
at the level of the instrument?

Matt

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
> Steven P. McNicoll wrote:
>> "Matt Whiting" > wrote in message
>> ...
>>>
>>> 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.
>

Do you have a webcam? I'd like to see you typing with your foot in your
mouth.

[Steven P. McNicoll]

"Jose" > wrote in message
t...
>
> 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.
>

Eureka!


>
> The physics of the instrument is not at issue. Rather, the calibration
> rules are, and
> those are made by the FAA.
>

I've said nothing about calibration rules.

[Steven P. McNicoll]

"Jim Logajan" > wrote in message
.. .
>
> That seems reasonable.
>
> But what if the instrument itself is 100 ft tall?
>

Then it wouldn't fit in the airplane.

[Steven P. McNicoll]

"Neil Gould" > wrote in message
...
>
> Your usage of the two terms.
>

I used them correctly. Perhaps your understanding of them is incorrect.


>
> BTDT, and understand the mechanics involved. I do appreciate your
> challenges that make me think about this more thoroughly, though. Also,
> consider the distinction between calibration and adjustment, where
> calibration is how the altimeter is installed, and adjustment is made via
> the Kollsman window/knob. Both of these factors can affect the indicated
> altitude regardless of the actual altitude of the altimeter.
>

One step at a time. Let's see where you are in your understanding of
altimetry before we move on. Answer the questions.

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> Yes, great idea.
>

I thought so.


>
> Take an altimeter and set it on a table. Don't move it!
>
> Turn the knob that adjusts the pressure scale to one extreme. Record the
> indication on the face of the altimeter. Now turn the knob to the other
> extreme of travel. Record the indication on the face of the instrument.
> Now the big question: which value recorded is the altitude at the level of
> the instrument?
>

One step at a time. Let's see where you are in your understanding of
altimetry before we move on. Answer the questions I posed to Neil Gould.

[Matt Whiting]

Steven P. McNicoll wrote:
> "Matt Whiting" > wrote in message
> ...
>> Yes, great idea.
>>
>
> I thought so.
>
>
>> Take an altimeter and set it on a table. Don't move it!
>>
>> Turn the knob that adjusts the pressure scale to one extreme. Record the
>> indication on the face of the altimeter. Now turn the knob to the other
>> extreme of travel. Record the indication on the face of the instrument.
>> Now the big question: which value recorded is the altitude at the level of
>> the instrument?
>>
>
> One step at a time. Let's see where you are in your understanding of
> altimetry before we move on. Answer the questions I posed to Neil Gould.

Yes, one step at a time. Tell me how an instrument that indicates its
own altitude can indicate two vastly different values without changing
its altitude? It is a very simple question.

Matt

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> Yes, one step at a time. Tell me how an instrument that indicates its own
> altitude can indicate two vastly different values without changing its
> altitude? It is a very simple question.
>

Let's answer the questions in the order they were asked.

[Mike Young]

"Matt Whiting" > wrote in message
...
> Steven P. McNicoll wrote:
>> "Matt Whiting" > wrote in message
>> ...
>>> Yes, great idea.
>>>
>>
>> I thought so.
>>
>>
>>> Take an altimeter and set it on a table. Don't move it!
>>>
>>> Turn the knob that adjusts the pressure scale to one extreme. Record
>>> the
>>> indication on the face of the altimeter. Now turn the knob to the other
>>> extreme of travel. Record the indication on the face of the instrument.
>>> Now the big question: which value recorded is the altitude at the level
>>> of
>>> the instrument?
>>>
>>
>> One step at a time. Let's see where you are in your understanding of
>> altimetry before we move on. Answer the questions I posed to Neil Gould.
>
> Yes, one step at a time. Tell me how an instrument that indicates its own
> altitude can indicate two vastly different values without changing its
> altitude? It is a very simple question.

Which altitude changed? Indicated altitude? Pressure altitude? Density
altitude? True altitude? (Does one presumes WGS84 when speaking of true
altitude?)

[Matt Whiting]

Steven P. McNicoll wrote:
> "Matt Whiting" > wrote in message
> ...
>> Yes, one step at a time. Tell me how an instrument that indicates its own
>> altitude can indicate two vastly different values without changing its
>> altitude? It is a very simple question.
>>
>
> Let's answer the questions in the order they were asked.

Ok, I understand that you don't have an answer.

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> Ok, I understand that you don't have an answer.
>

Then you understand incorrectly. If I'm to help you achieve a proper
understanding of altimetry you'll have to let me run the class.

[Steven P. McNicoll]

"Neil Gould" > wrote in message
. ..
>
> Your opinon of this is not shared by at least a few here, including me.
>

It's not a matter of opinion.


>
> Considering the definitive answer to the original question provided by
> Kev, and the fact that the presented information agrees with my
> understanding and usage of the terms, I think it puts the question to
> rest.
>

I'm glad you're content.

[Matt Whiting]

Steven P. McNicoll wrote:
> "Matt Whiting" > wrote in message
> ...
>> Ok, I understand that you don't have an answer.
>>
>
> Then you understand incorrectly. If I'm to help you achieve a proper
> understanding of altimetry you'll have to let me run the class.

We don't let students teach classes here.

[Neil Gould]

Recently, Steven P. McNicoll > posted:

> "Neil Gould" > wrote in message
> ...
>>
>> Your usage of the two terms.
>>
>
> I used them correctly.
>
Your opinon of this is not shared by at least a few here, including me.

> Perhaps your understanding of them is
> incorrect.
>
Considering the definitive answer to the original question provided by
Kev, and the fact that the presented information agrees with my
understanding and usage of the terms, I think it puts the question to
rest.

Neil

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> We don't let students teach classes here.
>

Good. Now, answer the questions and we can get back on track with your
education.

[Jim Logajan]

"Steven P. McNicoll" > wrote:
> "Jim Logajan" > wrote in message
> .. .
>>
>> That seems reasonable.
>>
>> But what if the instrument itself is 100 ft tall?
>>
>
> Then it wouldn't fit in the airplane.

It could fit in a balloon.

;-)