Question for the controllers: Altimeter setting

Pilawt

Pilawt

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Pilawt
While on VFR flight following, if ATC issues an altimeter setting, is the pilot expected to use it, even if the altimeter setting at a much closer location (per METAR, ASOS or ATIS) is considerably different?

Situation: Today while on VFR flight following from KSOW to KGYR at 12,500', I was handed off from ZAB to PHX APC. On initial contact the PHX controller said, "Phoenix altimeter 29.84." At that moment KPHX was 60 nm away from me; and I was directly over KPAN, which was reporting 30.14. It's not unusual for the pressure in the Valley to be lower than it is up in the high country.

FAR 91.121(a)(1)(i) says the pilot "shall maintain the cruising altitude ... by reference to an altimeter that is set ... to ... [t]he current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft."

So the question is, does the issuance of an altimeter setting by ATC imply that they want me to use that one, even though it would put my altimeter at odds by 300 feet with traffic in my immediate area?
 
Not a controller, but from what I understand the computer system uses the altimeter setting the controller gives you to correct the altitude report received from the encoder. I can't explain it well, but my primary instructor was a controller and did a much better job explaining it to me.

TL;DR: If on FF or on an instrument flight plan and in the system, use the altimeter setting given to you by the controller.
 
Bill Jennings said:
Not a controller, but from what I understand the computer system uses the altimeter setting the controller gives you to correct the altitude report received from the encoder. I can't explain it well, but my primary instructor was a controller and did a much better job explaining it to me.

TL;DR: If on FF or on an instrument flight plan and in the system, use the altimeter setting given to you by the controller.
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Anyone the controller is separating is going to be on the PHX altimeter setting. If terrain separation is a concern, use the local setting and advise the controller why. It's generally not enough of an issue to mention, but you may want to let the controller know since there's a 200 difference between the two settings.
 
Never ran across that but then I was an approach controller and our airspace usually went out about 35 miles more or less. Sounds like center issued you that, whose airspace spans several states in some places, so you could always ask which setting he wanted you to use.
 
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91.121 Altimeter settings.

a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating—

(1) Below 18,000 feet MSL, to—

(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;

91.123 Compliance with ATC clearances and instructions.
b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.

The controller instructed you to set your altimeter to a 29.84. By doing so you are in compliance with both regs.

When the controller’s displayed altitude varies 300 feet from your assigned altitude, you are going to here from the controller.
 
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Clip4 said:
91.123 Compliance with ATC clearances and instructions.
b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.

The controller instructed you to set your altimeter to a 29.84 and when the controllers displayed altitude varies by 300 feet of your assigned altitude, you are going to here from the controller.
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But is it an instruction or just information? They say "Farmington altimeter 29.92" not "set your altimeter to 29.92."

If I'm on an IFR flight plan then I always use what I've given by ATC, but if I'm on VFR flight following, I view it as information and I use my discretion as to what to use unless I'm inside of D, C or B airspace.

On flight following I'm VFR and responsible for my own separation.

I'm not saying this is how it is, rather how I do it. I'll be interested to see what the controllers say.
 
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timwinters said:
But is it an instruction or just information? They say "Farmington altimeter 29.92" not "set your altimeter to 29.92."

If I'm on an IFR flight plan then I always use what I've given by ATC, but if I'm on VFR flight following, I view it as information and I use my discretion as to what to use unless I'm inside of D, C or B airspace.

On flight following I'm VFR and responsible for my own separation.

I'm not saying this is how it is, rather how I do it. I'll be interested to see what the controllers say.
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So under IFR it is required and under VFR its advisory? What would you do VFR on a Bravo clearance?
 
2-7-2

c. Issue the altimeter setting:
1. To en route aircraft at least one time while operating in your area of jurisdiction. Issue the setting for the nearest reporting station along the aircraft’s route of flight:

NOTE−
14 CFR Section 91.121(1) requires that the pilot set his/her altimeter to the setting of a station along his/her route of flight within 100 miles of the aircraft if one is available. However, issuance of the setting of an adjacent station during periods that a steep gradient exists will serve to inform the pilot of the difference between the setting he/she is using and the pressure in the local area and better enable him/her to choose a more advantageous setting within the limitations of 14 CFR Section 91.121.
 
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Velocity173 said:
2-7-2

c. Issue the altimeter setting:
1. To en route aircraft at least one time while operating in your area of jurisdiction.

NOTE−
14 CFR Section 91.121(1) requires that the pilot set his/her altimeter to the setting of a station along his/her route of flight within 100 miles of the aircraft if one is available. However, issuance of the setting of an adjacent station during periods that a steep gradient exists will serve to inform the pilot of the difference between the setting he/she is using and the pressure in the local area and better enable him/her to choose a more advantageous setting within the limitations of 14 CFR Section 91.121.
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And therein lies the conundrum. It was appropriate for the controller to issue the KPHX altimeter (29.84) since it was within 20 miles of my route, and it's good to know that I was going from an area of high pressure to low ("high to low, look out below!"). But the question is, did she expect me to immediately set my altimeter to the PHX setting, when I was still directly over KPAN, which was reporting thirty ticks higher; or wait until I got closer to the lower-pressure area?

Here's the track of my flight, with the reported altimeter settings at that moment:

Screen Shot 2018-06-04 at 6.28.18 AM.png
 
Pilawt said:
And therein lies the conundrum.
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I really don't see the conundrum. Velocity's post, along with reading "14 CFR Section 91.121" would seem to define everything. It indicates that ATC's issuance of an altimeter setting is advisory only.

Again, I'd like to hear what the atc guys think. Calling @Radar Contact
 
timwinters said:
I really don't see the conundrum. Velocity's post, along with reading "14 CFR Section 91.121" would seem to define everything. It indicates that ATC's issuance of an altimeter setting is advisory only.

Again, I'd like to hear what the atc guys think. Calling @Radar Contact
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I agree with you Tim. @Velocity173 pretty much nailed it.
 
This thread is funny considering a lot of posts on this site have indicated a belief that controllers give you altimeter settings as a subtle way to tell you that you aren't on altitude.
 
Salty said:
This thread is funny considering a lot of posts on this site have indicated a belief that controllers give you altimeter settings as a subtle way to tell you that you aren't on altitude.
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That's when the same controller gives you the altimeter setting "out of the blue." You always get the setting when checking in with a different controller.
 
Salty said:
This thread is funny considering a lot of posts on this site have indicated a belief that controllers give you altimeter settings as a subtle way to tell you that you aren't on altitude.
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No not really. Controllers are required to issue the altimeter setting.
 
Bill Jennings said:
But, it's true that what you see on your screen is what is reported by the aircraft encoder and then adjusted by the altimeter setting, correct?
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Yup. The encoder is set to 29.92. The local altimeter is entered into Radar, it does the math and puts an altitude on the scope. If you are in a piece of 30.14 air and you set your altimeter to 29.84, like in @Pilawts example, there should be a 140 foot discrepancy. The 'radar' is mentally moving things 80 feet in one direction (the difference between 29.92 and 29.84) and your airplane is moving things, literally, in the other. The airplane is taking the permanently set at 29.92 encoder up and down with it. I think i got this right, my brain hurts, I need a beer. Anyway, the encoder is set at 29.92. Your altimeter cause you to move the airplane up or down, and take the encoder with it. The ATC Radar computer corrects for the difference between 29.92 and 'local' pressure. If you are in a piece of air that differs from 'local pressure' something has to give.
 
If on flight following and Tracon has given you an altimeter setting 30 miles out, when do you change your altimeter to the final destination airport? The airport altimeter setting is given is often different than the final destination airport.

I change my altimeter setting to the final destination airport as soon as I can hear or pick up the final airports weather broadcast, then call and inform ATC that I have the weather at K_ _ _ airport. Then change my altimeter to that airport. This should comply with 91.121.

What do others do?

Do you keep the same altimeter setting until you change frequencies or do you change to the final destination airport altimeter setting as soon as you can pick up the frequency?
 
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ahypnoz said:
If on flight following and Tracon has given you an altimeter setting 30 miles out, when do you change your altimeter to the final destination airport? The airport altimeter setting is given is often different than the final destination airport.

I change my altimeter setting to the final destination airport as soon as I can hear or pick up the final airports weather broadcast, then call and inform ATC that I have the weather at K_ _ _ airport. Then change my altimeter to that airport. This should comply with 91.121.

What do others do?

Do you keep the same altimeter setting until you change frequencies or do you change to the final destination airport altimeter setting as soon as you can pick up the frequency?
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I put it in when I get it. Either from the Controller or the ATIS/ASOS/AWOS via Radio. For what it’s worth, just adding another tidbit on altimetry, distance between ‘reporting’ stations is one of the big factors in the 2000 foot thing in Designated Mountiness Terrain.
 
If I get it from the controller, I put it in immediately. If I get it from AWOS or ATIS, then I put it in when the controller cuts me loose (if on flight following) or clears me for the approach (if IFR).

That said, I've never encountered a situation where the ATC-assigned altimeter setting and the one on the ATIS/AWOS were different enough to cause a separation issue. It would have to be 0.1" Hg to even make a difference of 100 feet, and the most difference I've ever seen is about 0.05" or so. Usually much less.
 
azure said:
If I get it from the controller, I put it in immediately. If I get it from AWOS or ATIS, then I put it in when the controller cuts me loose (if on flight following) or clears me for the approach (if IFR).

That said, I've never encountered a situation where the ATC-assigned altimeter setting and the one on the ATIS/AWOS were different enough to cause a separation issue. It would have to be 0.1" Hg to even make a difference of 100 feet, and the most difference I've ever seen is about 0.05" or so. Usually much less.
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Ditto.
 
azure said:
That said, I've never encountered a situation where the ATC-assigned altimeter setting and the one on the ATIS/AWOS were different enough to cause a separation issue. It would have to be 0.1" Hg to even make a difference of 100 feet, and the most difference I've ever seen is about 0.05" or so. Usually much less.
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I get it, and that's kinda what brought the question up in the first place. In the situation I posed initially there was a 0.30" (300 foot) difference between the ATC-issued* setting, for a station 60 nm away that was not my destination, and the ASOS at the airport directly below me. That's not all that unusual in this area.

(*The way I read the regs, ATC Handbook and AIM, ATC issues altimeter settings -- does not "assign" them. FAR 91.121(a)(1)(i).)
 
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Pilawt said:
I get it, and that's kinda what brought the question up in the first place. In the situation I posed initially there was a 0.30" (300 foot) difference between the ATC-issued* setting, for a station 60 nm away that was not my destination, and the ASOS at the airport directly below me. That's not all that unusual in this area.

(*The way I read the regs, ATC Handbook and AIM, ATC issues altimeter settings -- does not "assign" them. FAR 91.121(a)(1)(i).)
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Just wondering. You had been with Albuquerque Center. What was the altimeter they had given you and what was the source? Was it KPAN? Did you dial up the KPAN ASOS and get it yourself?
 
Pilawt said:
I get it, and that's kinda what brought the question up in the first place. In the situation I posed initially there was a 0.30" (300 foot) difference between the ATC-issued* setting, for a station 60 nm away that was not my destination, and the ASOS at the airport directly below me. That's not all that unusual in this area.
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Interesting. I guess it varies a lot depending on the part of the country you're in. I don't think I've ever been given (okay, ISSUED ;)) an altimeter setting from a station more than 20 or 30 nm away, except possibly when overflying Canada between Port Huron and Buffalo. Certainly never in Michigan, and never here in New England that I can recall either.

And a lot depends on the pressure gradient as well. A 0.30" difference over 60 nm is a pretty tight pressure gradient. If the gradient is tight, conditions are likely to be quite windy, and I tend not to be out flying on days like that.
 
luvflyin said:
Just wondering. You had been with Albuquerque Center. What was the altimeter they had given you and what was the source? Was it KPAN? Did you dial up the KPAN ASOS and get it yourself?
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Everything in the higher elevations from KPAN to the north and east was about the same. ZAB issued the KSOW altimeter when I called for flight following on departure. Just listening to the chatter with other traffic I knew there were big pressure differences at various stations, so I checked the KPAN ASOS on the #2 radio as we neared that area.
 
azure said:
And a lot depends on the pressure gradient as well. A 0.30" difference over 60 nm is a pretty tight pressure gradient. If the gradient is tight, conditions are likely to be quite windy, and I tend not to be out flying on days like that.
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This is pretty normal in this area. Right now, as I'm typing this, KPHX is 29.87, and KPAN, KPRC and KSEZ are all 30.15 or 30.16. KSOW is 30.25. And surface winds at all of them, and winds aloft at 12,000', are all less than 10 knots. Go figure.

KPHX is around 1,000' MSL; KPAN, KPRC and KSEZ all 5,000' MSL give or take a few; and KSOW is 6,400' MSL.
 
Pilawt said:
This is pretty normal in this area. Right now, as I'm typing this, KPHX is 29.87, and KPAN, KPRC and KSEZ are all 30.15 or 30.16. KSOW is 30.25. And surface winds at all of them, and winds aloft at 12,000', are all less than 10 knots. Go figure.
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Looks like it has more to do with pressure vs altitude than a pressure gradient. Possibly due to temperature being above standard, therefore pressure levels more spread out than in standard atmosphere, change in pressure with altitude less than than that assumed by altimeter, therefore need a higher altimeter setting to read the correct station elevation.
 
Hank S said:
That's when the same controller gives you the altimeter setting "out of the blue." You always get the setting when checking in with a different controller.
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Not even with a new controller. Just on new frequency. Twice this weekend I got the "switch to my frequency" and when reporting on the new frequency (yes, same controller) he gave me the altimeter setting I was previously issued. Same setting and same location.
 
azure said:
Interesting. I guess it varies a lot depending on the part of the country you're in. I don't think I've ever been given (okay, ISSUED ;)) an altimeter setting from a station more than 20 or 30 nm away, except possibly when overflying Canada between Port Huron and Buffalo. Certainly never in Michigan, and never here in New England that I can recall either.

And a lot depends on the pressure gradient as well. A 0.30" difference over 60 nm is a pretty tight pressure gradient. If the gradient is tight, conditions are likely to be quite windy, and I tend not to be out flying on days like that.
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Iron Mountain altimeter is given a LONG LONG ways away, but that's because there's no other good reporting stations.
 
EdFred said:
Iron Mountain altimeter is given a LONG LONG ways away, but that's because there's no other good reporting stations.
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Could well be. Truth be told I've never flown in the UP, much as I wanted to make it to 6Y9. So yeah, "Michigan" was my LP chauvinism talking... :(
 
EdFred said:
Not even with a new controller. Just on new frequency. Twice this weekend I got the "switch to my frequency" and when reporting on the new frequency (yes, same controller) he gave me the altimeter setting I was previously issued. Same setting and same location.
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Lol. Blame Pavlov. Ring bell, dog drools. Check in, controller gives altimeter.
 
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A lot of controllers I work with are prior radar controllers. When the police helicopter or one of the three medical companies' helicopters checks in (all 500' AGL) they always give them the altimeter. When I asked why, they just gave me the deer in the headlights look and shrug.
 
ahypnoz said:
If on flight following and Tracon has given you an altimeter setting 30 miles out, when do you change your altimeter to the final destination airport? The airport altimeter setting is given is often different than the final destination airport.

I change my altimeter setting to the final destination airport as soon as I can hear or pick up the final airports weather broadcast, then call and inform ATC that I have the weather at K_ _ _ airport. Then change my altimeter to that airport. This should comply with 91.121.

What do others do?

Do you keep the same altimeter setting until you change frequencies or do you change to the final destination airport altimeter setting as soon as you can pick up the frequency?
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If you are on FF and close enough to pick up ATIS, the altimeter setting isn't likely to be different. Of course, my home airport is 3 nm in the air from the location of the TRACON, so by the time I'm getting that last altimeter setting, it really isn't going to be different.
 
N1120A said:
If you are on FF and close enough to pick up ATIS, the altimeter setting isn't likely to be different. Of course, my home airport is 3 nm in the air from the location of the TRACON, so by the time I'm getting that last altimeter setting, it really isn't going to be different.
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Location of the TRACON got nothing to do with it. If you were flying into VNY the TRACON is 105 miles away.
 
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