PHX787
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Newbie ATC Question

Sat Feb 02, 2013 4:01 am

I'm listening to LiveATC right now and when they mention PHX and the arrivals, they say something along the lines of "expect rwy 25L Altimiter 2900" something along those lines. What does the 2900 altimiter mean? PHX is at an altitude of around 1100FT ASL.
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N353SK
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RE: Newbie ATC Question

Sat Feb 02, 2013 4:15 am

29.92 inches of mercury would be a standard altimeter setting, with inches of mercury being a barometric measurement.

29.00" would be abnormally low. But if the controller said "two niner niner zero," it would mean that if you dug a hole in Phoenix down to sea level, the barometer would read 29.90 inches of mercury. The altimeter setting varies as pressure rises and falls.
 
ak907
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RE: Newbie ATC Question

Sat Feb 02, 2013 4:15 am

It is the barometric setting for the altimeter. Standard pressure is 29.92 at 15 degrees Celsius (59F), so you have a bit of low pressure. It is done to assure the altimeter reads the correct altitude. They already have this information long before they land, but they tower repeats it to insure they have it set correctly in case it changed.
 
PHX787
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RE: Newbie ATC Question

Sat Feb 02, 2013 4:39 am

Ok I wasn't reporting the exact barometric pressure, and now they're saying something like 3000 something (along those lines)

Anyway, what is the importance of this to the readings, you're saying they adjust this to the ground altimeter?
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AmericanAirFan
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RE: Newbie ATC Question

Sat Feb 02, 2013 5:57 am

Quoting PHX787 (Reply 3):
Ok I wasn't reporting the exact barometric pressure, and now they're saying something like 3000 something (along those lines)

Anyway, what is the importance of this to the readings, you're saying they adjust this to the ground altimeter?

Altimeters in the cockpit sense the EXACT pressure of the air pressure at the altitude they are out. However, the air is not perfect, and there are high and low pressure systems that an aircraft flies through. With a high pressure system if an aircraft is at 5,000' MSL then the aircraft would indicate an altitude lower due to the denser air. In order to correct for this, they have a "pressure sensitive altimeter" This means an altimeter that can be adjusted for a non-standard pressure. So if it was a high pressure system you might hear something like 30.14" Hg which is above the 29.92" Hg standard. So when the pilot then sets 30.14 into his altimeter he will read the correct altitude on his altimeter, corresponding to the actual altitude MSL for the aircraft.

Lets say that we are on the ground at an airport that is at zero feet MSL. Right at sea level, and the altimeter setting is 30.14. The pilot might have 29.92 set into his altimeter, and he would then indicate -220'. He would then put 30.14 into the altimeter and it would then read 0'. If there was any small deviation from that indicated altitude once the altimeter is set, there is some error. +/- 75' is the legal error allowed I believe. A good rule of thumb for figuring out pressure altitude is 1" = 1,000'.

So here's a basic pressure altitude problem.

29.92
-30.14
______
-00.22

-0.22 X 1000' = -220'

0' (Field Elevation) + (-220') (Deviation from standard pressure) = -220' (Pressure altitude)

So Basically the current most up to date and local altimeter setting is important to the pilot in order to have an accurate altitude indication when coming in for a descent to land. This is why ATC gives the altimeter. This also ensures proper altitude separation for all of the aircraft in that air traffic controller's sector.

Let me know if you have any more questions about it.

AAF

[Edited 2013-02-01 22:00:56]

[Edited 2013-02-01 22:01:49]
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Starlionblue
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RE: Newbie ATC Question

Sat Feb 02, 2013 6:24 am

Quoting PHX787 (Reply 3):
Anyway, what is the importance of this to the readings, you're saying they adjust this to the ground altimeter?

AmericanAirFan explains it very well. To summarize operational usage.

ATC will tell you the altimeter setting when you switch to a new controller. The importance of the altimeter setting is twofold.
1. Ensure that when you indicate a certain altitude you actually are at this altitude. This is important especially in low visibility and during approaches.
2. Ensure that all the aircraft in the area are "on the same page" for traffic avoidance.

Note that in Class A airspace (18000-60000 feet MSL in the US) the altimeter is set to 29.92 for all aircraft. 18000 feet also marks the altitude where you start talking about flight levels instead of feet.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
jetblueguy22
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RE: Newbie ATC Question

Sat Feb 02, 2013 7:15 am

Quoting PHX787 (Reply 3):
Anyway, what is the importance of this to the readings, you're saying they adjust this to the ground altimeter?

It can be very important depending on where you are. If you are doing some weekend flying around a county it may not be a huge deal, but if you've flown from JFK-PHX the pressure can be completely different. AmericanAirFan explained it best. That is what you're taught in ground school. One little riddle I learned is Low to high, look up in the sky. High to low, look out below. For high to low it just means if you fly from high pressure to low pressure you are going to have a higher indicated altitude than your actual altitude. It can be a little tricky to get used to. I'm still pretty fresh flying wise so I still have trouble remembering. But it is always pretty crazy if we've had a system move through the night before and you hop in the plane and adjust the altimeter and see a huge pressure change. It can change your altitude a couple hundred feet depending on how strong the system was.
Blue
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Geezer
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RE: Newbie ATC Question

Sat Feb 02, 2013 8:28 am

Quoting PHX787 (Thread starter):
I'm listening to LiveATC right now and

PHX787; Your question made me remember something having to do with altimeters and airliners many years ago;

I'm sure this happened before your time, but I'm betting Starlionblue will either remember it, or at least will have heard about it; as I know you lived in Cincinnati, I'm sure you know where the old Anderson Ferry is, (or was), on River Road, just west of downtown Cincinnati.

Back in the 1960s, I worked at the Texaco Bulk Terminal, on the river, right at Anderson Ferry; at the time, I operated the loading rack, which loads the fuel into the tank trucks; I forget the exact year , but it was in the mid 60s; at maybe 7 or 8 PM, while I was loading 3 or 4 tank trucks, we heard this tremendous explosion, which seemed to come "up river" from the west; within several minutes, there were dozens of police cars racing down River Road, heading west, then came ambulances; within the next half hour or so, (and I don't recall now how we found out), but we did find out that the huge explosion we had heard was a B 727, on final into CVG, had impacted the hillside, app. 300 ft from the hill top. The airport is maybe 1/4 mile beyond the top of that hill top. When the plane impacted the hill, it broke into a lot of pieces, and continued sliding toward the top of the hill side; when I saw it the next day, the only recognizable part was vertical tail assembly and the No.2 engine; miraculously, one of the female F/As and one pax survived.I don't recall the number of "souls on board" anymore. After months and months of investigation, the accident was finally attributed to "pilot error", having to do with improper setting of the altimeter.

I've read several stories about that accident through the years, but it's not "fresh enough" in my memory just now to intelligently talk about it, but it did have to do with the altimeter.

I'm not sure, but I don't think it was quite dark yet, but there may have been fog involved. I'm not even sure of the carrier any more, but I'm thinking it may have been AA. Anyway, it's a very sad example of how important proper altimeter setting is.

When I was going into work the next day, I drove on down River Road about a mile and a half; the impact site was about an 1/8 of a mile across the river, and about 3/4 of the way up the hill. I read later that the flight crew thought they were about 500 feet higher than they actually were.

Charley
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Starlionblue
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RE: Newbie ATC Question

Sat Feb 02, 2013 8:47 am

I don't remember it. I wasn't born yet!  But I have read about the accident. This was AA383. If memory serves the altimeter may indeed have played a part.

As explained by AmericanAirFan, it is easy to make mistakes (which is why airline crews cross-check) and even a small mistake of 0.1 inches means 100 feet. In a round gauge aircraft an adjustment of 0.1 inches has you twisting the little altimeter knob only a tiny amount. Let alone that it is hard to see the exact notches from most angles, and if you're in serious turbulence, well good luck to you... On an precision approach 100 feet is not to be trifled with.

This is also one of the reasons larger aircraft have radar altimeters. You simply can't get enough precision out of a barometric altimeter.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
sprout5199
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RE: Newbie ATC Question

Sat Feb 02, 2013 3:47 pm

Quoting Geezer (Reply 7):
Anyway, it's a very sad example of how important proper altimeter setting is.

Thats so true. There is also this:
http://www.youtube.com/watch?v=dTCwGLjSf2A

Dan in Jupiter
 
flight152
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RE: Newbie ATC Question

Sun Feb 03, 2013 2:58 am

Quoting jetblueguy22 (Reply 6):
but if you've flown from JFK-PHX the pressure can be completely different.

While completely true, this is a misleading example. Since you would be at standard pressure almost the entire flight, the pressure in New York will be long forgotten by the time you descend below 18,000 on the way to Phoenix.
 
jetblueguy22
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RE: Newbie ATC Question

Sun Feb 03, 2013 6:45 am

Quoting flight152 (Reply 10):

Good point, that slipped my mind. I've never flown that high as a pilot so I don't really know how that works. When center descends you below 18,000 do they tell you the altimeter nearest to your location or do you have to tune to ATIS to get it?
Blue
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flight152
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RE: Newbie ATC Question

Sun Feb 03, 2013 1:30 pm

The center will give you the altimeter setting when they give you a descent below FL180.
 
NBGSkyGod
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RE: Newbie ATC Question

Mon Feb 04, 2013 12:50 pm

Most approach controllers will give you the altimeter setting at your destination, or if you are just transiting, the nearest airport.
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Woodreau
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RE: Newbie ATC Question

Mon Feb 04, 2013 1:13 pm

In the AA 383 accident and the AA1572 accident 20 years later, American set their altimeters using QFE (where the altimeter would read 0 on the ground) instead of QNH where it reads field elevation on the ground. So even when ATC would issue an altimeter setting to the flight, the pilots would not set that into their altimeter they'd get their own setting to put into the altimeter.
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