Altimeter Calibration Height

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 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

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

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

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

"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.

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

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 :-)

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.

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/dat...t_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/a...ying_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