Altimeter Correction Height - Some Answer(s)

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

As mentioned before, the altimeter is tested for calibration,
friction, case leak, and hysteresis as per F.A.R. 43 appendix E
paragraph (b). The static system is tested for leaks as per F.A.R. 43
appendix E paragraph (a).

At sea level, the altimeter must be accurate to within 20 feet. The
transponder encoder must match altimeter within 125 feet. (F.A.R.
91.217)

Okay, now to the meat of the question about to what height is it
_supposed_ to be calibrated to? The answer can be taken as: the
height of its installation... as referenced to a standard datum of 10'
above the wheels.

AC43-6B, Recommended Test Procedures Chapter 8, Section C. (pg 6),
says that once in the aircraft, you can do a field elevation check by
holding a certified altimeter next to the one you're checking.
Doesn't mean much.

But F.A.R. Part 91, Appendix 9 speaks of Category II certification.
This requires: "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." Installation manuals go on to quote
further:

"These altimeters will be acceptable under that section if: (a) the
altimeters and their static systems meet the requirements of § 91.411
within the past 12 months; and(b) altimeter correction data, which
considers both scale error and main landing gear wheel height of the
airplane, is available to the PIC. Scale error is determined by an
altimeter test and inspection under 14 CFR part 43, appendix E, and
the wheel height correction is necessary if the wheel-to-instrument
height is in excess of 10 feet presently allowed for in U.S. Weather
Bureau altimeter settings provided for aircraft. For instance, a large
aircraft which has a 19-foot wheel-to-instrument height would require
a nine-foot correction under this rule. Barometric altimeters meeting
the requirements above are acceptable for CAT II operations to
establish DHs down to 150 feet."

Now... remember me mentioning the above 10' government correction
formula, dating back to DC-3 times? Apparently it's still in use.
You can easily see it when using official Weather Bureau calculators.
Put in 0' altitude, and 29.92 pressure, and the altimeter setting is
29.91, which allows for the altimeter being installed 10' (-.01" Hg)
above the wheels (field elevation).

http://www.srh.noaa.gov/elp/wxcalc/a...rsetting.shtml

So that question is answered for some aircraft, anyway. I then got
curious about radio altimeters, and about RVSM over the ocean. What
standards do they follow?

For Cat III operations. "(c) Radio altimeter. (1) It must display to
the flight crew clearly and positively the wheel height of the main
landing gear above the terrain. (2) It must display wheel height above
the terrain to an accuracy of plus or minus 5 feet or 5 percent,
[...]" So radar is to the wheel height, not 10' above.

For RVSM, "The static system of each aircraft is installed in a manner
and position that is the same as those of the other aircraft in the
group. The same static source error correction is incorporated in each
aircraft of the group." So the height isn't specified, just that
each model of aircraft should all have the same height difference.

Hope this was interesting. Cheers, Kev

http://www.avionicswest.com/myviewpoint/faroutfars.htm
http://bryanwristonaviation.com/ques...20answers.html
http://www.risingup.com/fars/info/part91-A-APPX.shtml
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/ef966dc4550da8af86256f6200600e22/$FILE/AC43-6B.pdf