Altimeter Calibration Height

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

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

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 :-)

"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.

Steven P. McNicoll schrieb:

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

Source?

"Stefan" wrote in message
...

Source?


Logic. Where is the sensing mechanism located?

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.

"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.

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?

"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.