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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 |
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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. |
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Altimeter Correction Height - Some Answer(s)
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 |
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Altimeter Correction Height - Some Answer(s)
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 |
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Altimeter Correction Height - Some Answer(s)
"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 |
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Altimeter Correction Height - Some Answer(s)
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 |
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