Altimeter Correction Height - Some Answer(s)

I did some more research and found interesting tidbits about altimeter
calibrations. References at end.

Thanks Kev. Very interseting.

The answer can be taken as: the
height of its installation... as referenced to a
standard datum of 10' above the wheels.

I'm not sure what this means. No surprise, it was probably written by a
government lawyer. However, at least wheel height is being taken into
consideration. Somehow. Maybe it means the altimeter is supposed to
read ten feet low, to account for the wheels.

[cat II] "Two sensitive altimeters adjustable for barometric
pressure, having markings at 20-foot intervals and each having a
placarded correction for altimeter scale error and for the wheel
height of the aircraft."

From this I glean the altimeter should read its own height, and the
altimeter's height above the wheels (on flare? on the ground?) must be
available to the pilot. Further:

For instance, a large aircraft which has a
19-foot wheel-to-instrument height would require
a nine-foot correction under this rule.

I infer from this that the first ten feet require no correction, and
thus I infer that the altimeter indeed should =indicate= ten feet lower
than its own height, even as it =senses= the =pressure= =at= it's own
height.

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

On Apr 6, 12:03 am, Jose wrote:
For instance, a large aircraft which has a
19-foot wheel-to-instrument height would require
a nine-foot correction under this rule.

I infer from this that the first ten feet require no correction, and
thus I infer that the altimeter indeed should =indicate= ten feet lower
than its own height, even as it =senses= the =pressure= =at= its own
height.

Yes, using the current official altimeter setting. Which assumes the
altimeter will be 10' above the wheels and compensates for that error
by subtracting .01"Hg before you input it in the Kollsman window. Thus
making it indicate 10' lower, or supposedly the elevation at the
bottom of the wheels. My head hurts now :-)

Of course, since altimeters need only be accurate within 20' at sea
level, the 10' doesn't matter that much. As someone else rightfully
pointed out, you don't manually land by watching the altimeter. And
autolanding is done only by radio altimeter set to wheel height.

OTOH, we can use this excuse to explain why we often flare five feet
too high in a GA plane... it's the difference in actual installation
height vs official assumptions ;-)

Regards, Kev