If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Altimeter inaccurate
My Altimeter is accurate when on the ground or at lower alt. When I climb up
to 7k and above and check it against GPS and approach controls read out it is about 300+ feet off. Is it the altimeter I'm having problems with? Steve |
#2
|
|||
|
|||
"smf" wrote: Is it the altimeter I'm having problems with? Probably not. Your altimeter has no means to correct for non-standard temperature; GPS altitude is unaffected by temperature. -- Dan C172RG at BFM |
#3
|
|||
|
|||
"smf" wrote in message = news:jpblc.12364$bS1.6242@okepread02... My Altimeter is accurate when on the ground or at lower alt. When I = climb up to 7k and above and check it against GPS and approach controls read = out it is about 300+ feet off. =20 Is it the altimeter I'm having problems with? =20 Steve =20 It's possible you are not having any problems at all. Barometric altimetry simply does not measure geometric height. And most GPSs ignore barometric settings completely. I'm unsure what you mean by "approach controls read out". The controller can see only what your Mode C transponder sends. Usually that reading is to the nearest 100 feet, and it derives from a barometric sensor in your airplane, permanently set to 29.92 in.hg. With a correct barometric setting, your altimeter should show the = relevant airport's elevation at its true height, but normally *not* any other = elevation. If your altimeter passes its 24-month 91.411 check, it's surely OK. If it fails its check, though, you do have an altimeter problem. ---JRC--- |
#4
|
|||
|
|||
AIM 1-1-20 (a)(8) tells us not to rely upon GPS to determine aircraft
altitude. Bob Gardner "smf" wrote in message news:jpblc.12364$bS1.6242@okepread02... My Altimeter is accurate when on the ground or at lower alt. When I climb up to 7k and above and check it against GPS and approach controls read out it is about 300+ feet off. Is it the altimeter I'm having problems with? Steve |
#5
|
|||
|
|||
It begs an interesting question, though...
If an IAP is based on object clearance (in some cases, as low as 100' clearance, right?), but does not account for temperature variations and does not actually measure geometric altitude above the ground, is it not possible that one would find oneself at an altitude that from a barometric standpoint is legal and correct, but from a geometric altitude is within the bounds of that tower that would have been 100' lower if it also changed altitude with the temperature? Just a thought... "John R. Copeland" wrote in : "smf" wrote in message news:jpblc.12364$bS1.6242@okepread02... My Altimeter is accurate when on the ground or at lower alt. When I climb up to 7k and above and check it against GPS and approach controls read out it is about 300+ feet off. Is it the altimeter I'm having problems with? Steve It's possible you are not having any problems at all. Barometric altimetry simply does not measure geometric height. And most GPSs ignore barometric settings completely. I'm unsure what you mean by "approach controls read out". The controller can see only what your Mode C transponder sends. Usually that reading is to the nearest 100 feet, and it derives from a barometric sensor in your airplane, permanently set to 29.92 in.hg. With a correct barometric setting, your altimeter should show the relevant airport's elevation at its true height, but normally *not* any other elevation. If your altimeter passes its 24-month 91.411 check, it's surely OK. If it fails its check, though, you do have an altimeter problem. ---JRC--- |
#6
|
|||
|
|||
It begs an interesting question, though... If an IAP is based on object clearance (in some cases, as low as 100' clearance, right?), but does not account for temperature variations and does not actually measure geometric altitude above the ground, is it not possible that one would find oneself at an altitude that from a barometric standpoint is legal and correct, but from a geometric altitude is within the bounds of that tower that would have been 100' lower if it also changed altitude with the temperature? IN theory yes, the altitude would be different from the indicated altitude. However, you'd only be 100 feet away from the place where the altimiter setting was derived, so the amount of error would be slight. Jose -- (for Email, make the obvious changes in my address) |
#7
|
|||
|
|||
Temperature variations will cause an error but it is very small. The AIM
has a table that lists these errors. At 200' AGL (ILS minimum), you will get a 20' error if the temperature is -20 C. It becomes 60' when the temperature is -50C (not a common temperature in the US except may be Alaska). The error is larger at higher altitudes (as much as 300' at 1000 AGL when the temperature is -50C). Judah wrote in : It begs an interesting question, though... If an IAP is based on object clearance (in some cases, as low as 100' clearance, right?), but does not account for temperature variations and does not actually measure geometric altitude above the ground, is it not possible that one would find oneself at an altitude that from a barometric standpoint is legal and correct, but from a geometric altitude is within the bounds of that tower that would have been 100' lower if it also changed altitude with the temperature? Just a thought... "John R. Copeland" wrote in : "smf" wrote in message news:jpblc.12364$bS1.6242@okepread02... My Altimeter is accurate when on the ground or at lower alt. When I climb up to 7k and above and check it against GPS and approach controls read out it is about 300+ feet off. Is it the altimeter I'm having problems with? Steve It's possible you are not having any problems at all. Barometric altimetry simply does not measure geometric height. And most GPSs ignore barometric settings completely. I'm unsure what you mean by "approach controls read out". The controller can see only what your Mode C transponder sends. Usually that reading is to the nearest 100 feet, and it derives from a barometric sensor in your airplane, permanently set to 29.92 in.hg. With a correct barometric setting, your altimeter should show the relevant airport's elevation at its true height, but normally *not* any other elevation. If your altimeter passes its 24-month 91.411 check, it's surely OK. If it fails its check, though, you do have an altimeter problem. ---JRC--- |
#8
|
|||
|
|||
If an IAP is based on object clearance (in some cases, as low as 100'
clearance, right?), but does not account for temperature variations and does not actually measure geometric altitude above the ground, is it not possible that one would find oneself at an altitude that from a barometric standpoint is legal and correct, but from a geometric altitude is within the bounds of that tower that would have been 100' lower if it also changed altitude with the temperature? Just a thought... And a good thought it is. Try this: http://www.aircraftbuyer.com/learn/train06.htm John Bell www.cockpitgps.com |
#9
|
|||
|
|||
On Mon, 03 May 2004 02:40:18 GMT, Judah wrote:
It begs an interesting question, though... If an IAP is based on object clearance (in some cases, as low as 100' clearance, right?), but does not account for temperature variations and does not actually measure geometric altitude above the ground, is it not possible that one would find oneself at an altitude that from a barometric standpoint is legal and correct, but from a geometric altitude is within the bounds of that tower that would have been 100' lower if it also changed altitude with the temperature? It is my understanding that in Canada there is a requirement to alter minimums for certain approaches in very cold weather. No such requirement exists (Part 91, at least) in the US. I don't believe there have been any accidents in the US due to this phenomenon. Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA) |
#10
|
|||
|
|||
"Ron Rosenfeld" wrote in message = ... On Mon, 03 May 2004 02:40:18 GMT, Judah wrote: =20 It begs an interesting question, though... If an IAP is based on object clearance (in some cases, as low as 100' = clearance, right?), but does not account for temperature variations = and=20 does not actually measure geometric altitude above the ground, is it = not=20 possible that one would find oneself at an altitude that from a=20 barometric standpoint is legal and correct, but from a geometric = altitude=20 is within the bounds of that tower that would have been 100' lower if = it=20 also changed altitude with the temperature? =20 It is my understanding that in Canada there is a requirement to alter minimums for certain approaches in very cold weather. No such = requirement exists (Part 91, at least) in the US. =20 I don't believe there have been any accidents in the US due to this phenomenon. =20 Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA) Along that line, perhaps you've noticed that the new VNAV approaches typically carry a note to the effect that Baro-VNAV is not authorized at temperatures below some limiting number. ---JRC--- |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
encoding altimeter pinout? | Bill Chernoff | Home Built | 3 | July 15th 04 04:34 PM |
Inaccurate airspeed indicator | Wyatt Emmerich | Instrument Flight Rules | 20 | April 20th 04 12:08 AM |
Adjustment of Altimeter | Ron | Home Built | 5 | April 5th 04 03:12 AM |
GPS Altitude with WAAS | Phil Verghese | Instrument Flight Rules | 42 | October 5th 03 12:39 AM |
Recommendation for Radio, transponder and Altimeter | Ron Natalie | Home Built | 0 | July 8th 03 03:40 PM |