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#1
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ATC Altimeter Settings
When entering an ATC region, we often get an altimeter setting, usually
at some fix. So for example, when entering the Chicago center ATC region, they say something like, "Midway altimeter is xx.xx." I have some questions. 1. So, do they pick some fix for the region and give everyone (those below FL 180) the altimeter of that fix? Do they break up into sub-regions and give everyone in that sub-region that same fix altimeter setting? 2. At what point after departing an airport do you change altimeter setting to the one supplied by ATC? Similarly, on approaching an airport to land, at what point do you switch over to the altimeter setting of the airport (from that given by ATC). The reason I ask is that sometimes, the altimeter settings can be quite different. Especially if there is VFR traffic around an airport, it seems that there is some risk if everyone entering the pattern (or approaching the pattern) does not share the same altimeter setting. Just curious. -Sami N2057M, Piper Turbo Arrow III |
#2
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O. Sami Saydjari wrote:
When entering an ATC region, we often get an altimeter setting, usually at some fix. So for example, when entering the Chicago center ATC region, they say something like, "Midway altimeter is xx.xx." I have some questions. 1. So, do they pick some fix for the region and give everyone (those below FL 180) the altimeter of that fix? Do they break up into sub-regions and give everyone in that sub-region that same fix altimeter setting? You should get the altimeter setting for the airport nearest your location which has weather reporting. 2. At what point after departing an airport do you change altimeter setting to the one supplied by ATC? I update my altimeter setting whenever ATC gives me a new one. Similarly, on approaching an airport to land, at what point do you switch over to the altimeter setting of the airport (from that given by ATC). If ATC has given me a setting for my destation, I'll just use that. If not, then I'll set my altimeter whenever I get the ATIS/AWOS. The reason I ask is that sometimes, the altimeter settings can be quite different. What do you consider "quite different"? Unless there's some severe weather going on, it's unusual to have to change the setting more than a couple of 1/100's at a time. Especially if there is VFR traffic around an airport, it seems that there is some risk if everyone entering the pattern (or approaching the pattern) does not share the same altimeter setting. Risk of what? If somebody's 50 feet high or low in the pattern, it doesn't make much difference. As long as it's VFR, see and avoid is the rule of the day. The real reason for having sensitive altimeters is for instrument work. |
#3
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Roy Smith wrote: O. Sami Saydjari wrote: When entering an ATC region, we often get an altimeter setting, usually at some fix. So for example, when entering the Chicago center ATC region, they say something like, "Midway altimeter is xx.xx." I have some questions. 1. So, do they pick some fix for the region and give everyone (those below FL 180) the altimeter of that fix? Do they break up into sub-regions and give everyone in that sub-region that same fix altimeter setting? You should get the altimeter setting for the airport nearest your location which has weather reporting. So, if ATC gives you a setting on initial contact out of the airport you just took off from, you use the airport's setting, not ATC's? When do you cut over to ATC's? Or do you just listen to every AWOS/ASOS enroute and change to the closest one? 2. At what point after departing an airport do you change altimeter setting to the one supplied by ATC? I update my altimeter setting whenever ATC gives me a new one. See above. What I am asking is when to change over? Similarly, on approaching an airport to land, at what point do you switch over to the altimeter setting of the airport (from that given by ATC). If ATC has given me a setting for my destation, I'll just use that. If not, then I'll set my altimeter whenever I get the ATIS/AWOS. So, ATC expects you to change your altimeter whenever you first hear the ATIS at your destination? That could be 50 miles out. Seems a little odd. The reason I ask is that sometimes, the altimeter settings can be quite different. What do you consider "quite different"? Unless there's some severe weather going on, it's unusual to have to change the setting more than a couple of 1/100's at a time. I have seen differences of 0.5 inches....that is alot! Especially if there is VFR traffic around an airport, it seems that there is some risk if everyone entering the pattern (or approaching the pattern) does not share the same altimeter setting. Risk of what? If somebody's 50 feet high or low in the pattern, it doesn't make much difference. As long as it's VFR, see and avoid is the rule of the day. The real reason for having sensitive altimeters is for instrument work. Well, good point, except that if you are descending through a cloud deck into VFR conditions. Also, if you change your altimeter 50 miles out and I change mine 5 miles out, it seems like their is some distance from the airport where we are using different settings, and that seems like a bad thing to me. |
#4
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O. Sami Saydjari wrote:
You should get the altimeter setting for the airport nearest your location which has weather reporting. So, if ATC gives you a setting on initial contact out of the airport you just took off from, you use the airport's setting, not ATC's? I think I may have confused you with some sloppy wording. What I meant was, "The altimeter setting ATC gives you will mostly likely be for the airport nearest your current location". The bottom line is anytime ATC gives me an altimeter setting, I reset my altimeter to whatever the controller gives me. Or do you just listen to every AWOS/ASOS enroute and change to the closest one? Well, I do that too. Maybe not *every* one, but on a long flight, I will certainly tune in the occassional ATIS as I go by airports. It's also a good way to stay aware of weather trends; if the altimeter settings keep going down, you know you're flying into worsening weather. What do you consider "quite different"? Unless there's some severe weather going on, it's unusual to have to change the setting more than a couple of 1/100's at a time. I have seen differences of 0.5 inches....that is alot! 0.5 inches is a HUGE altimter setting change. Are you sure about that? |
#5
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On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote:
0.5 inches is a HUGE altimter setting change. Are you sure about that? I have to agree with Roy here. I just finished coming back from Ohio to Mississippi, and I experienced a very large altimeter setting adjustments in my journey southbound north of Bowling Green KY, but I sure did not experience .50 inches adjustments en route. I did end up .25 inch less over the whole trip though. If I remember correctly on one ATC hand off, I went from 29.96 to 29.88 which lowered my altitude by 80 feet. This really alarmed me as I sure don't like deviating more then 20 feet from assigned altitude. Needless to say, I made a quick climb to my assigned altitude! There was a rather steep pressure gradient even in my trip I just did this week, but as Roy says, .5 inch altimeter change in a flight range distance in planes I fly is probably indicative of weather I wouldn't / shouldn't be flying in anyway. Allen |
#6
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Well, I thought it was 0.5 inches, but I am not absolutley certain...it
was a while back. But, I do not think this changes my point substantially. Even a 0.25 inch change is substantial, and if there is not a standard of when one changes settings, it seems like a risk, albeit small. My experience is that ATC does not give me settings of nearby airports. For example, I recall entering Chicgo Center airspace from the south. I was just north of Lafayette, Indiana. The controller gave me the altimeter setting for Chicago Midway. That is a long way away. Lafayette airport was at least 0.1 inches different. I used ATC's setting, because I figure they must have some standard reference point in a region, or sub-region, so that all the planes are at least on the same relative setting. -Sami N2057M, Piper Turbo Arrow III A Lieberman wrote: On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote: 0.5 inches is a HUGE altimter setting change. Are you sure about that? I have to agree with Roy here. I just finished coming back from Ohio to Mississippi, and I experienced a very large altimeter setting adjustments in my journey southbound north of Bowling Green KY, but I sure did not experience .50 inches adjustments en route. I did end up .25 inch less over the whole trip though. If I remember correctly on one ATC hand off, I went from 29.96 to 29.88 which lowered my altitude by 80 feet. This really alarmed me as I sure don't like deviating more then 20 feet from assigned altitude. Needless to say, I made a quick climb to my assigned altitude! There was a rather steep pressure gradient even in my trip I just did this week, but as Roy says, .5 inch altimeter change in a flight range distance in planes I fly is probably indicative of weather I wouldn't / shouldn't be flying in anyway. Allen |
#7
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On Fri, 01 Apr 2005 14:19:56 -0600, O. Sami Saydjari wrote:
Sami Well, I thought it was 0.5 inches, but I am not absolutley certain...it was a while back. But, I do not think this changes my point substantially. Even a 0.25 inch change is substantial, and if there is not a standard of when one changes settings, it seems like a risk, albeit small. The .25 inch change was over 684 NM. Unless you are crossing a cold front or a rather intense low pressure, you are not too likely to experience a significant pressure change to the point that you describe. My experience is that ATC does not give me settings of nearby airports. For example, I recall entering Chicgo Center airspace from the south. I was just north of Lafayette, Indiana. The controller gave me the altimeter setting for Chicago Midway. That is a long way away. Lafayette airport was at least 0.1 inches different. What day were you flying? If you go to http://www.hpc.ncep.noaa.gov/dailywxmap/frame.html, you can retrieve historical weather maps to verify what you say. Was it windy that day? Was there a low pressure passing through. If so, then yes, it is possible to experience a .10 inch change over a short range. I used ATC's setting, because I figure they must have some standard reference point in a region, or sub-region, so that all the planes are at least on the same relative setting. I always use ATC's settings. I figure their scope reads my transponder, and my transponder readout is dependent on what I put in for my altimeter setting. I sure don't want an altitude deviation based on something different then what ATC tells me. Like Roy said, if you are arriving at an airport VFR, the altimeter is not quite as important as when you are arriving at an airport under instrument conditions. It's see and avoid for VFR, and if your altimeter is only .01 off, that only puts you approximately 10 feet off pattern altitude. .10 would put you 100 feet off pattern altitude, and again, see and avoid would be the ruling factor. It becomes more critical on an instrument approach, as you want to be as close as you can for an accurate altimeter setting for missed approaches purposes at MDA or DH. Allen |
#8
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"A Lieberman" wrote in message ... On Fri, 01 Apr 2005 14:19:56 -0600, O. Sami Saydjari wrote: Sami Well, I thought it was 0.5 inches, but I am not absolutley certain...it was a while back. But, I do not think this changes my point substantially. Even a 0.25 inch change is substantial, and if there is not a standard of when one changes settings, it seems like a risk, albeit small. The .25 inch change was over 684 NM. Unless you are crossing a cold front or a rather intense low pressure, you are not too likely to experience a significant pressure change to the point that you describe. The only time I had seen a significant change in altimeter setting was when crossing over from the central valley in California (Bakersfield sector) to the LA basin (Socal). I remember my altitude was off by 100-150ft which really bothered me and I could do nothing about it since I switched to the altimeter setting provided when given to me. One of them was certainly off for sure. Also, the radio comm was weak in the transition area. The Socal controller was not too happy. |
#9
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I always use ATC's settings. I figure their scope reads my transponder, and my transponder readout is dependent on what I put in for my altimeter setting. IIRC the Mode C is pressure altitude, usually from a separate transducer. |
#10
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AIM 7-2-3 says your altimeter can be in error by 74 feet without a trip to
the shop, so at least that much slop is built into the system. The ATC separates IFR from IFR by miles, not feet (OK, feet vertically, but opposite direction traffic is separated by at least 1000 feet vertically). If my preflight weather analysis showed a pressure difference of .5 inches from one area to another but within my range, I would seriously consider going somewhere else. One-tenth of an inch is 100 feet...I would not get ulcers over it. Bob Gardner "O. Sami Saydjari" wrote in message ... Well, I thought it was 0.5 inches, but I am not absolutley certain...it was a while back. But, I do not think this changes my point substantially. Even a 0.25 inch change is substantial, and if there is not a standard of when one changes settings, it seems like a risk, albeit small. My experience is that ATC does not give me settings of nearby airports. For example, I recall entering Chicgo Center airspace from the south. I was just north of Lafayette, Indiana. The controller gave me the altimeter setting for Chicago Midway. That is a long way away. Lafayette airport was at least 0.1 inches different. I used ATC's setting, because I figure they must have some standard reference point in a region, or sub-region, so that all the planes are at least on the same relative setting. -Sami N2057M, Piper Turbo Arrow III A Lieberman wrote: On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote: 0.5 inches is a HUGE altimter setting change. Are you sure about that? I have to agree with Roy here. I just finished coming back from Ohio to Mississippi, and I experienced a very large altimeter setting adjustments in my journey southbound north of Bowling Green KY, but I sure did not experience .50 inches adjustments en route. I did end up .25 inch less over the whole trip though. If I remember correctly on one ATC hand off, I went from 29.96 to 29.88 which lowered my altitude by 80 feet. This really alarmed me as I sure don't like deviating more then 20 feet from assigned altitude. Needless to say, I made a quick climb to my assigned altitude! There was a rather steep pressure gradient even in my trip I just did this week, but as Roy says, .5 inch altimeter change in a flight range distance in planes I fly is probably indicative of weather I wouldn't / shouldn't be flying in anyway. Allen |
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