Altimeter Correction Height - Some Answer(s)

[Kev]

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/altimetersetting.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/questions%20and%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

[Jose]

> 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.

[Kev]

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

[Neil Gould]

Recently, Kev > posted:

> I did some more research and found interesting tidbits about altimeter
> calibrations. References at end.
>
(rest snipped for brevity)

Kev, thanks for digging up this information. It is quite interesting, and
logical!

Neil

[Al G[_1_]]

"Neil Gould" > wrote in message
t...
> Recently, Kev > posted:
>
>> I did some more research and found interesting tidbits about altimeter
>> calibrations. References at end.
>>
> (rest snipped for brevity)
>
> Kev, thanks for digging up this information. It is quite interesting, and
> logical!
>
> Neil
>
>
>

Very Good, Thanks.

Al G

[Kev]

On Apr 6, 7:21 am, "Neil Gould" > wrote:
> Kev, thanks for digging up this information. It is quite interesting, and
> logical!

Thank you for your kind words, Neil, Jose, Al. That 10' offset in
the pressure formula has been bugging me for years, and it took much
more searching than I expected, to find out it's still being used by
the NOAA, and even in the FARs!

Makes a great trivia question, too ;-) "You're in a C172, with a
brand new, impossibly accurate altimeter in the instrument panel
located five feet above level ground. You put in the local Weather
Service altimeter setting. What altitude relative to the ramp level
does the altimeter show? Answer: minus five feet! "

Unless, that is, if your mechanic took the trouble to recalibrate it
in the panel, like this:

http://www.mrkent.com/flying/altimeter/index.htm

Regards, Kev