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| |-+  Pilot/Controller Forum (Moderators: dave, RonR)
| | |-+  TRACON Altimeter
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Author Topic: TRACON Altimeter  (Read 9981 times)
mielsonwheals
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« on: July 22, 2013, 02:53:22 PM »

Where do approach controllers that cover a wide area get their altimiter setting, like Boston approach? Is it from the hub airport?

I used an altimiter setting from an approach controller on approach to an uncontrolled field, and turned out the local setting was very different, which made for an interesting/dangerous approach.  (I absolutely realize my mistake, forgetting to use the setting from the AWOS). It was Portland Approach and I was flying into Sanford, ME.

Thanks!
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StuSEL
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« Reply #1 on: July 22, 2013, 11:45:01 PM »

Per the AIM, you should be able to receive the local AWOS/ASOS observation within 25 miles of the airport below 10,000 feet. At that time, you should set your altimeter to that local setting as opposed to the one issued by the controller. Most approach controls will have their radar display set up to advise them of the altimeter setting of the primary airport.

Mode C transponders broadcast your pressure altitude, or the altitude you'd be at if your altimeter were set to 29.92." The radar system takes that pressure altitude, applies the local altimeter setting to it, and provides the controller with an accurate depiction of the altitude you see on your altimeter. If you were to switch to a local altimeter that was vastly different than that of the primary airport, your altitude will change on the controller's radar display, but simply informing the controller that you have received the "one-minute" weather at your destination should alleviate his or her confusion, if there is any at all.
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sykocus
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« Reply #2 on: July 25, 2013, 12:11:53 AM »

StuSEL suggestion is good, but I would advise being a little more direct. E.g. "approach I have the XXXX weather and going to use their local altimeter setting." Also I would say this as soon as possible. If you're IFR or even VFR on a code that is setup for MSAW processing and near the MVA the, MSAW alarm may start going off. If the controller isn't prepared he could be unnecessary distracted. At the very least it will save a few unnecessary transmissions. The 1st step for the controller when noticing you're mode C being off is to give you their closest altimeter they have.
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Yesterday I couldn't spell air traffic controller. Today I R one.
StuSEL
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« Reply #3 on: July 31, 2013, 01:53:26 AM »

StuSEL suggestion is good, but I would advise being a little more direct. E.g. "approach I have the XXXX weather and going to use their local altimeter setting." Also I would say this as soon as possible. If you're IFR or even VFR on a code that is setup for MSAW processing and near the MVA the, MSAW alarm may start going off. If the controller isn't prepared he could be unnecessary distracted. At the very least it will save a few unnecessary transmissions. The 1st step for the controller when noticing you're mode C being off is to give you their closest altimeter they have.
This is indeed a good idea. I haven't had an experience where my destination altimeter is that far off of the approach controller's primary airport to make any noticeable difference in my reported altitude, but if I were to ever encounter such a situation, I'd definitely let them know I was switching settings.
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jmcmanna
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« Reply #4 on: August 05, 2013, 07:55:52 PM »

When we are issuing an altimeter setting (we're supposed to tell you where it is from), we're doing it because we want to be able to use altitude separation between you and other traffic that might be flying around in the area.  If everyone is using the ORD altimeter, for example, and the UGN altimeter (35 miles north of ORD) is significantly different, and you are using the UGN altimeter setting, 3000 feet for everyone else might be 3100 feet for you.

What does that mean?  Maybe someone flying overhead is only separated by 800 feet instead of 1000.  Or maybe we assigned a VFR 3500' and they are only 400 feet above you instead of 500 feet.  It's probably not a big deal, but consider that ATC might be issuing the altimeter setting based on a common point for all of their traffic in that area for a reason.

Now, I'm not suggesting you not use the local altimeter setting, but I would suggest not using the local altimeter setting 30 or 40 miles away from the airport and wait until you are approaching the airport (maybe cleared for the approach?) before making the adjustment.
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