Altimeter Setting

JDE · February 11th, 2007, 22:42

I've noticed that most of the time the altimeter setting ATC gives is usually .01 or .02 off of what the ATIS/ASOS is advertising. Why is that?

Fox Xray · February 11th, 2007, 23:12

Is this at a particular station or everywhere you fly ??? In the center environment we have the altimeter displayed on our screen along with the time which usually coincides with when that stations ATIS updates. It is done automatically though we can update it manually if need be. I would say 95% of the time we match what the ATIS showing...

JDE · February 11th, 2007, 23:25

No, not a particular place, but it seems like its almost everywhere. It doesn't happen all the time, but most of the time. And not just Center controllers either.

Stone Cold · February 12th, 2007, 16:32

If it's at towered airports, I believe they will give you what they are actually reporting, not what was on the ATIS, up to 1 hour prior. That may be where you are seeing the discrepancies. Also, remember, an ASOS, along with the ATIS, only sends out automated reports every hour, for the most part, so when you listen to the ASOS, it's giving you a reading for right then, not when the report was sent.

I could be wrong, but that's my theory. Also, a .01 or .02 change will give you how much change in altitude? 10 or 20 feet, right, so it's well within tolerances. Now, if you're off by 1", then you need to worry.

JDE · February 12th, 2007, 18:55

Quote:

Originally Posted by Stone Cold

I could be wrong, but that's my theory. Also, a .01 or .02 change will give you how much change in altitude? 10 or 20 feet, right, so it's well within tolerances. Now, if you're off by 1", then you need to worry.

Yeah, not something I'm worried about, rather, something I've been curious about for some time now.

Mr.Maximus · February 14th, 2007, 20:09

Stone Cold is correct. As an ATIS could be almost an hour old, the Controllers are giving you a current reading as they have the equipment (read out) right in front of them. While this is not a big deal on most days, it can be a huge deal on days shooting and ILS to mins. 29.92 vs. 29.96 is 40 feet - and can make the difference between landing and going around. Thats why when wx is low, the good controllers will say something like: "XYZ440 turn left 120, maintain 3,000' until established, cleared for ILS 9 approach, current altimeter 29.96 contact Twr on 118.3".

February 11th, 2007, 22:42	#1
JDE Senior Member Join Date: Mar 2002 Location: Chattanooga, TN Posts: 1,465	Altimeter Setting I've noticed that most of the time the altimeter setting ATC gives is usually .01 or .02 off of what the ATIS/ASOS is advertising. Why is that? __________________ Jason Pics from the road - updated 01-24-08

February 11th, 2007, 23:12	#2
Fox Xray ATC Join Date: Mar 2006 Posts: 402	Re: Altimeter Setting Is this at a particular station or everywhere you fly ??? In the center environment we have the altimeter displayed on our screen along with the time which usually coincides with when that stations ATIS updates. It is done automatically though we can update it manually if need be. I would say 95% of the time we match what the ATIS showing...

February 11th, 2007, 23:25	#3
JDE Senior Member Join Date: Mar 2002 Location: Chattanooga, TN Posts: 1,465	Re: Altimeter Setting No, not a particular place, but it seems like its almost everywhere. It doesn't happen all the time, but most of the time. And not just Center controllers either. __________________ Jason Pics from the road - updated 01-24-08

February 12th, 2007, 16:32	#4
Stone Cold Old Skool Join Date: Oct 2003 Location: SE US Posts: 2,942	Re: Altimeter Setting If it's at towered airports, I believe they will give you what they are actually reporting, not what was on the ATIS, up to 1 hour prior. That may be where you are seeing the discrepancies. Also, remember, an ASOS, along with the ATIS, only sends out automated reports every hour, for the most part, so when you listen to the ASOS, it's giving you a reading for right then, not when the report was sent. I could be wrong, but that's my theory. Also, a .01 or .02 change will give you how much change in altitude? 10 or 20 feet, right, so it's well within tolerances. Now, if you're off by 1", then you need to worry.

February 14th, 2007, 20:09	#6
Mr.Maximus Junior Member Join Date: Feb 2007 Location: This One Posts: 289	Re: Altimeter Setting Stone Cold is correct. As an ATIS could be almost an hour old, the Controllers are giving you a current reading as they have the equipment (read out) right in front of them. While this is not a big deal on most days, it can be a huge deal on days shooting and ILS to mins. 29.92 vs. 29.96 is 40 feet - and can make the difference between landing and going around. Thats why when wx is low, the good controllers will say something like: "XYZ440 turn left 120, maintain 3,000' until established, cleared for ILS 9 approach, current altimeter 29.96 contact Twr on 118.3".

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