altimeter

Ron Levy said:
Yes, it does so automatically. There will be an altimeter setting readout at the controller's position, and that's the setting that controller's display is using, and the one the controller will read to all aircraft in that sector. Note that this varies from sector to sector, so each controller may read you a different number. Further, with a big enough sector, there may be different settings within that same sector, so the controller may read you one setting while reading another to someone a distance away.
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Most of the time the Denver Center altimeter setting is not the same (and more than 0.1" different) than KFTG. Distance between the two is about 35 nm.
 
murphey said:
Most of the time the Denver Center altimeter setting is not the same (and more than 0.1" different) than KFTG. Distance between the two is about 35 nm.
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Denver Center is probably using a half a dozen or more different settings at any one time, depending on which sector. But that's not the issue -- we're talking about local altimeter setting at an airport and the comparison between what you see on your altimeter and the elevation of where you're parked at that airport.
 
Ron Levy said:
I'd like to know how the Universal Gas Law would explain an altimeter set to the pressure setting obtained from another altimeter at the same elevation and temperature (i.e., the one in your plane sitting on the ramp versus the ASOS/AWOS/other source on the same field) reading a different altitude from the reference altimeter other than an internal error of one of the altimeters.

I don't care how hot or cold it is at the airport -- absent a malfunction of either your altimeter or the pressure setting source on that airport, the actual ambient temperature will not perceptibly affect the comparison between your altimeter reading and the elevation of the place where you're parked. Yes, in theory, if you're parked 20 feet higher than the pressure source, temperatures of more than 40 degrees or so off standard might result in a couple of feet of difference, but nobody can read their altimeter that closely, and the thing's only calibrated to within 20 feet to start with. Temperature just is not a factor in determining altimeter accuracy based on local altimeter setting when you're parked on the airport where that setting was determined.
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So what you're saying is nonstandard temperature affects the displayed altitude, but only to a small degree?
 
Thanks all. I have learned a lot from this thread (about altimeters AND people).
 
JOhnH said:
Ok, thanks. I understand that much better now.
Another question: When I am on the ground and set my altimeter according to the reading given by ATIS, why would my altimeter read below sea level? (the airport is at 29ft above sea level). Is there an adjustment I or my A&P can make?

Some day, I hope to know enough that I won't kill myself, if I live that long.
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How flat is the airport? If there is much variation in field elevation as you taxi around, that might be the culprit. The altimeter setting for the airport is at a fixed point, and field elevation (plus or minus the allowance for error roncachamp mentions) is only correct at that point. I have been at many airports where I could not see the ramp from the takeoff end of the runway because it was lower than the ramp.

Bob Gardner
 
roncachamp said:
ARTCC airspace is divided into hundreds of Radar Sort Boxes, each RSB is assigned a specific source for the local altimeter setting.
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I'm curious, how big is an RSB typically? I'm sure it varies from airspace to airspace, but I wondered how big we're talking. Would a class D airport have it's own RSB? Larger? smaller?
 
bdlarkin said:
I'm curious, how big is an RSB typically? I'm sure it varies from airspace to airspace, but I wondered how big we're talking. Would a class D airport have it's own RSB? Larger? smaller?
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They were uniformly rectangular, I'd guess 15 miles east-west and 10 miles north-south. I don't know if there's a national standard or if it varies from center to center.
 
Ron Levy said:
Temperature just is not a factor in determining altimeter accuracy based on local altimeter setting when you're parked on the airport where that setting was determined.
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It IS a factor, just not a significant or easily measurable factor with regards to aircraft altimeters.

Given this thread and this board is the realm of nitpickers, I guess this is just keeping in that vein..
 
Doggtyred said:
It IS a factor, just not a significant or easily measurable factor with regards to aircraft altimeters.

Given this thread and this board is the realm of nitpickers, I guess this is just keeping in that vein..
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Proper picking of nits requires that you provide some basis for your bias.
 
Ron Levy said:
Nevertheless, nonstandard temperature is not why the OP was seeing the error he did.
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Can you explain why nonstandard temperature could not have contributed anything, however small it may have been, to the error the OP was seeing?
 
roncachamp said:
Can you explain why nonstandard temperature could not have contributed anything, however small it may have been, to the error the OP was seeing?
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I can, but only if the altimeter in the AC was at the same physical altitude as the ASOS/AWOS sensor (yes that's unlikely). In that case the effects of non ISA temps would be zilch.

But even if there was a 10ft difference (very plausible) it's almost equally likely that the theoretical temp based error happened to be in the opposite direction of other much more sizable errors and actually made the total error smaller (hence not only "not contributing" to it, but also reducing it.

But this doesn't prove or even suggest that non-ISA temps "could not have contributed" since scenario #1 (same alt) wasn't a given and #2 is only 50/50. Of course a better question IMO is could it be explained why (better yet how) non-ISA temp would produce an error so small as to be undetectable by the pilot and I think you already know the answer to that one.
 
Aren't you supposed to have a Pitot-Static Altimeter check every 24 months? Or is that only for IFR pilots?
 
And only the altimeter and static system are checked, not the pitot system or airspeed indicator, no matter what inaccurate name folks put on the 91.411 check.
 
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