AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Altimeter Calibration Height          AirTalk.org Forum Index -> Aviators (General Discussions) View previous topic :: View next topic   Author Message TheSmokingGnu Guest Posted: Thu Mar 08, 2007 7:12 am    Post subject: 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 Back to top Kev Guest Posted: Fri Mar 09, 2007 7:12 am    Post subject: Re: Altimeter Calibration Height Quote: 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/data/FARS/part_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/aviation/instrument_flying_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 Back to top Dallas Guest Posted: Sat Mar 10, 2007 7:12 am    Post subject: Re: Altimeter Calibration Height On Wed, 07 Mar 2007 21:37:53 -0800, TheSmokingGnu wrote: Quote: The bottom of the landing gear? The height of the static port? If you want to complicate your question ask about the other end of the measurement, MSL. From where do they measure MSL? Sea surface topography is constantly changing. Do you measure from the tops of the waves? Bottoms? High tide? Average tide? Average tide in the Pacific or average tide in the Atlantic? How do you measure MSL in Panama? How do you get an average when currents, air pressure variations, temperature and salinity variations are constantly in flux? What about the melting and freezing of snow and glaciers? How much time do you need to create this average? -- Dallas Back to top TheSmokingGnu Guest Posted: Tue Apr 03, 2007 6:12 am    Post subject: Re: Altimeter Calibration Height Honestly, I don't know how you guys got Newton involved. :D How about a (better) hypothetical? The imaginary aircraft is sitting at Imaginary Airfield (KIMG), which sits at a billiards-table-flat 50 MSL. The aircraft's static port is mounted at the centerline of the fuselage, which is 10 feet from the ground. The instrument in question is mounted 4 feet above the centerline. What will the gauge read when set to the proper barometric scale? 50, 60, or 64 feet? TheSmokingGnu Back to top Guest Posted: Tue Apr 03, 2007 6:12 am    Post subject: Re: Altimeter Calibration Height TheSmokingGnu <anonymityisavirtue (AT) 1111011010011 (DOT) com> wrote: Quote: Honestly, I don't know how you guys got Newton involved. :D How about a (better) hypothetical? The imaginary aircraft is sitting at Imaginary Airfield (KIMG), which sits at a billiards-table-flat 50 MSL. The aircraft's static port is mounted at the centerline of the fuselage, which is 10 feet from the ground. The instrument in question is mounted 4 feet above the centerline. What will the gauge read when set to the proper barometric scale? 50, 60, or 64 feet? Not enough potential aswers, you for got: Under IFR Under VFR Pass Go and collect $200 No one is going to tell me how to set my altimeter -- Jim Pennino Remove .spam.sux to reply. Back to top Jose Guest Posted: Tue Apr 03, 2007 6:12 am    Post subject: Re: Altimeter Calibration Height Quote: What will the gauge read when set to the proper barometric scale? 50, 60, or 64 feet? That depends on what the people in the calibration shop did to it. My understanding is that the =installation= is certified. It would be logical for the certified installation to be calibrated to the wheel height. It would be equally logical for it to be certified to the instrument height. It's an FAA certification. 'nuf said. :) Jose -- Get high on gasoline: fly an airplane. for Email, make the obvious change in the address. 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