AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Altimeter accuracy Goto page 1, 2  Next          AirTalk.org Forum Index -> Owning Airplanes View previous topic :: View next topic   Author Message Jim Stewart Guest Posted: Fri May 04, 2007 5:17 am    Post subject: Altimeter accuracy How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. Back to top karl gruber Guest Posted: Fri May 04, 2007 5:37 am    Post subject: Re: Altimeter accuracy 75' "Jim Stewart" <jstewart (AT) jkmicro (DOT) com> wrote in message news:vq-dnVmIvpMU5qfbnZ2dnUVZ_jmdnZ2d (AT) omsoft (DOT) com... Quote: How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. Back to top Ron Rosenfeld Guest Posted: Fri May 04, 2007 6:12 am    Post subject: Re: Altimeter accuracy On Fri, 04 May 2007 01:42:07 GMT, Rip <rquinby (AT) snet (DOT) net> wrote: Quote: Jim Stewart wrote: How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. +/- 75 feet for IFR Rip It depends on your field elevation. You should be able to check and see what the error was at the last altimeter certification for IFR. The tolerances, at the lower altitudes, are: (Appendix E 14 CFR 43) 0 ±20' 500 ±20' 1000 ±20' 1500 ±25' 2000 ±30' 3000 ±30 4000 ±35 6000 ±40 8000 ±60 For a VFR only a/c, I do not believe there is any particular requirement. --ron Back to top Rip Guest Posted: Fri May 04, 2007 6:12 am    Post subject: Re: Altimeter accuracy Jim Stewart wrote: Quote: How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. +/- 75 feet for IFR Rip Back to top Capt. Geoffrey Thorpe Guest Posted: Sat May 05, 2007 10:37 pm    Post subject: Re: Altimeter accuracy "Jim Stewart" <jstewart (AT) jkmicro (DOT) com> wrote in message news:vq-dnVmIvpMU5qfbnZ2dnUVZ_jmdnZ2d (AT) omsoft (DOT) com... Quote: How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. What's the accuracy of the AWOS? -- Geoff The Sea Hawk at Wow Way d0t Com remove spaces and make the obvious substitutions to reply by mail When immigration is outlawed, only outlaws will immigrate. Back to top Jim Stewart Guest Posted: Tue May 08, 2007 12:53 am    Post subject: Re: Altimeter accuracy Capt. Geoffrey Thorpe wrote: Quote: "Jim Stewart" <jstewart (AT) jkmicro (DOT) com> wrote in message news:vq-dnVmIvpMU5qfbnZ2dnUVZ_jmdnZ2d (AT) omsoft (DOT) com... How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. What's the accuracy of the AWOS? Glad you asked. I called some friends at Allweather (which made the AWOS at my field) and they said the units are factory calibrated to +/- 5 feet and that FAA specifications are +/- 20 feet. Quote: -- Geoff The Sea Hawk at Wow Way d0t Com remove spaces and make the obvious substitutions to reply by mail When immigration is outlawed, only outlaws will immigrate. Back to top Jim Carter Guest Posted: Tue May 08, 2007 1:41 am    Post subject: Re: Altimeter accuracy I think what you've cited is for bench testing during certification. Once installed in the aircraft then comparison against the pressure as recorded by the official weather observation is +/- 75'. An interesting discussion point came up recently that exposed the difference in teaching over the past 30 years. 30 years ago we were taught to set the altimeter to the known field elevation and record the difference between the official pressure and the indicated pressure, then apply that difference to every setting you received along your route of flight. Today they teach to set your altimeter to the official pressure and that's it. So my question becomes, when executing a precision approach to a minimum DH of 200' and then executing the missed procedure, the aircraft is allowed to descend slightly below the DH as things spool up. If you are already 75' lower than you think because of altimeter error, and you descend only 20' more (one gradient on the altimeter) aren't you really only 105' off the deck? -- Jim Carter Rogers, Arkansas "Ron Rosenfeld" <ronrosenfeld (AT) nospam (DOT) org> wrote in message news:eq6l3393qa34o2u3tgjudc0ebhe3i8bn2m (AT) 4ax (DOT) com... Quote: On Fri, 04 May 2007 01:42:07 GMT, Rip <rquinby (AT) snet (DOT) net> wrote: Jim Stewart wrote: How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. +/- 75 feet for IFR Rip It depends on your field elevation. You should be able to check and see what the error was at the last altimeter certification for IFR. The tolerances, at the lower altitudes, are: (Appendix E 14 CFR 43) 0 ±20' 500 ±20' 1000 ±20' 1500 ±25' 2000 ±30' 3000 ±30 4000 ±35 6000 ±40 8000 ±60 For a VFR only a/c, I do not believe there is any particular requirement. --ron Back to top Ron Rosenfeld Guest Posted: Tue May 08, 2007 3:45 am    Post subject: Re: Altimeter accuracy On Mon, 07 May 2007 20:41:13 GMT, "Jim Carter" <jim.carter (AT) swbell (DOT) net> wrote: Quote: I think what you've cited is for bench testing during certification. Once installed in the aircraft then comparison against the pressure as recorded by the official weather observation is +/- 75'. An interesting discussion point came up recently that exposed the difference in teaching over the past 30 years. 30 years ago we were taught to set the altimeter to the known field elevation and record the difference between the official pressure and the indicated pressure, then apply that difference to every setting you received along your route of flight. Today they teach to set your altimeter to the official pressure and that's it. So my question becomes, when executing a precision approach to a minimum DH of 200' and then executing the missed procedure, the aircraft is allowed to descend slightly below the DH as things spool up. If you are already 75' lower than you think because of altimeter error, and you descend only 20' more (one gradient on the altimeter) aren't you really only 105' off the deck? I did cite the bench testing numbers, as they are the ones of which I am aware. I've heard of the 75' "allowance" for a field measurement, but I've not seen a regulatory justification for that value. As someone who flies instrument approaches to minimums, I, personally, would not be happy with a 75' error, and would have the system rechecked. So far as you being only 105' off the deck in your hypothetical instance, that is obviously the case. By the way, for Category II operations, which may have a DH as low as 100'AGL, it is a requirement to have readily available the altimeter calibration information from the last certification, (and to apply it appropriately). --ron Back to top Jim Carter Guest Posted: Tue May 08, 2007 6:12 am    Post subject: Re: Altimeter accuracy Isn't a radar altimeter required equipment for CAT II and CAT III work? We were trying to get an aircraft and aircrew certified for CAT II in Seattle many years ago and I thought that was the reason they had the RA installed. -- Jim Carter Rogers, Arkansas "Ron Rosenfeld" <ronrosenfeld (AT) nospam (DOT) org> wrote in message news:3nav33ttjf4ipbpv0lpuamd1am3n45gn4s (AT) 4ax (DOT) com... Quote: On Mon, 07 May 2007 20:41:13 GMT, "Jim Carter" <jim.carter (AT) swbell (DOT) net wrote: I think what you've cited is for bench testing during certification. Once installed in the aircraft then comparison against the pressure as recorded by the official weather observation is +/- 75'. An interesting discussion point came up recently that exposed the difference in teaching over the past 30 years. 30 years ago we were taught to set the altimeter to the known field elevation and record the difference between the official pressure and the indicated pressure, then apply that difference to every setting you received along your route of flight. Today they teach to set your altimeter to the official pressure and that's it. So my question becomes, when executing a precision approach to a minimum DH of 200' and then executing the missed procedure, the aircraft is allowed to descend slightly below the DH as things spool up. If you are already 75' lower than you think because of altimeter error, and you descend only 20' more (one gradient on the altimeter) aren't you really only 105' off the deck? I did cite the bench testing numbers, as they are the ones of which I am aware. I've heard of the 75' "allowance" for a field measurement, but I've not seen a regulatory justification for that value. As someone who flies instrument approaches to minimums, I, personally, would not be happy with a 75' error, and would have the system rechecked. So far as you being only 105' off the deck in your hypothetical instance, that is obviously the case. By the way, for Category II operations, which may have a DH as low as 100'AGL, it is a requirement to have readily available the altimeter calibration information from the last certification, (and to apply it appropriately). --ron Back to top dave Guest Posted: Tue May 08, 2007 4:01 pm    Post subject: Re: Altimeter accuracy I read somewhere on the web that your A&P can adjust your altimeter by removing a pin or a screw that allows the barometric pressure to be set independently from the altitude. Once it's set correctly, the pin or screw is replaced. Dave Jim Stewart wrote: Quote: How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. Back to top Ron Rosenfeld Guest Posted: Tue May 08, 2007 5:18 pm    Post subject: Re: Altimeter accuracy On Tue, 08 May 2007 03:38:30 GMT, "Jim Carter" <jim.carter (AT) swbell (DOT) net> wrote: Quote: Isn't a radar altimeter required equipment for CAT II and CAT III work? We were trying to get an aircraft and aircrew certified for CAT II in Seattle many years ago and I thought that was the reason they had the RA installed. It is definitely not required for Category A aircraft flying under Part 91 for CAT II. I don't know about other categories. There was a period of time when I and my Mooney were CAT II qualified and certified. The inner marker can substitute for a radio altimeter for 100' DH. The RA is not necessary at all for the 150 DH. The altimeter and static system has to have had the IFR check within the past 12 months; and altimeter correction data must be available to the pilot, including both the scale error and, wheel height correction if the wheel to instrument height is greater than 10 feet. There are a very few CAT II approaches that require a RA for use of the 100' DH, because of absent or siting problems with the IM. I don't know of any guidance for Category A Part 91 a/c under CAT III, or if any authorizations have ever been issued for that. --ron Back to top Al G Guest Posted: Tue May 08, 2007 9:03 pm    Post subject: Re: Altimeter accuracy "Ron Rosenfeld" <ronrosenfeld (AT) nospam (DOT) org> wrote in message news:13q043hmhlrsv33km3s3lm21v8bcjsli1p (AT) 4ax (DOT) com... Quote: On Tue, 08 May 2007 03:38:30 GMT, "Jim Carter" <jim.carter (AT) swbell (DOT) net wrote: Isn't a radar altimeter required equipment for CAT II and CAT III work? We were trying to get an aircraft and aircrew certified for CAT II in Seattle many years ago and I thought that was the reason they had the RA installed. It is definitely not required for Category A aircraft flying under Part 91 for CAT II. I don't know about other categories. There was a period of time when I and my Mooney were CAT II qualified and certified. The inner marker can substitute for a radio altimeter for 100' DH. The RA is not necessary at all for the 150 DH. The altimeter and static system has to have had the IFR check within the past 12 months; and altimeter correction data must be available to the pilot, including both the scale error and, wheel height correction if the wheel to instrument height is greater than 10 feet. There are a very few CAT II approaches that require a RA for use of the 100' DH, because of absent or siting problems with the IM. I don't know of any guidance for Category A Part 91 a/c under CAT III, or if any authorizations have ever been issued for that. --ron Ron is correct. We once had a 182 and pilot approved for the 150' DH for fog seeding. Al G Back to top Jim Stewart Guest Posted: Tue May 08, 2007 10:43 pm    Post subject: Re: Altimeter accuracy dave wrote: Quote: I read somewhere on the web that your A&P can adjust your altimeter by removing a pin or a screw that allows the barometric pressure to be set independently from the altitude. Once it's set correctly, the pin or It's under warranty and the dealer says his A&P can't break the seal. Quote: screw is replaced. Dave Jim Stewart wrote: How close should an altimeter be to field elevation when set to the pressure indicated by the field's AWOS? I'm seeing a 50' error. Back to top Guest Posted: Tue May 08, 2007 11:11 pm    Post subject: Re: Altimeter accuracy On May 7, 2:41 pm, "Jim Carter" <jim.car...@swbell.net> wrote: Quote: I think what you've cited is for bench testing during certification. Once installed in the aircraft then comparison against the pressure as recorded by the official weather observation is +/- 75'. The installed altimeter should give the same reading error on the ground as it did on the bench unless the static system is way out of whack, in which case the airplane shouldn't be flown. The Canadian specs for scale error are here: http://www.tc.gc.ca/civilaviation/RegServ/Affairs/cars/Part5/Standards/t571s02.htm Dan Back to top Ron Rosenfeld Guest Posted: Wed May 09, 2007 6:12 am    Post subject: Re: Altimeter accuracy On Tue, 8 May 2007 09:03:07 -0700, "Al G" <agerhart2 (AT) charter (DOT) net> wrote: Quote: We once had a 182 and pilot approved for the 150' DH for fog seeding. That wouldn't happen to have been in Medford, OR? --ron Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AirTalk.org Forum Index -> Owning Airplanes All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum Aviation----------------alt.aviationAviation safety EnhancementPlanes, gliders and flying in AustraliaAviation et pilotage (Français)rec.aviationAviation (Aerobatics)Hot air, gas balloons and airshipsHang-gliding, paragliding, foot-launched flightAircraft Design & BuildingInstrument Flight RulesAviation (Classifieds)Military AircraftNaval AviationAviation (Miscellaneous Topics)Owning AirplanesAviators (General Discussions)Aviation related productsAircraft RestorationHelicopters and rotary wing aircraftsFlight SimulationSailplanesAviation (Students)Light aircraftFlying related activities in the UK Airports----------------alt.airportsEdinburgh Airport Glasgow Airport Humberside Airport Luton Airport Aberdeen Airport Glasgow Prestwick Airport  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.