Barometric pressure

[bgets]

Just wondering. In a modern jet airliner, does the barometric pressure setting constantly change with the actual outside pressure? Then your altitude would always be correct for the real pressure. I assume it doesn't because ATC will alway give the pressure. If that's the case, why not have it change as the current pressure changes?
Bob G.

[tommchowat]

In a short answer, no!

Below the transition altitude (the altitude where you go from QNH to SPS [standard pressure setting]) aircraft work on QNH. Once they reach the transition altitude, which varies in different countries and can be found on the relevant charts, they switch to SPS, which is 1013.25MB, or 29.92 Inches of Mercury. These are then called flight levels. The main purpose of having flight levels are to maintain a set vertical distance between aircraft. That's why it's important to have SPS set when you need it set, if one aircraft is going head on towards another and they have different altimeter settings, they could collide. I can think of a couple of incidents where that has happened.

Basically, pressure can vary. A mass of colder air, for example, increases the pressure lapse rate and causes isobars to dip. A mass of warmer air causes the pressure lapse rate to decrease, making isobars expand. (Isobars - lines joining places of equal pressure). Therefore, the 600hpa level in a mass of cold air is lower than it is in a mass of warm air. The basic rule is, "From high to low, look out below", which means that when you're flying from an area of high temperature or pressure to an area of low temperature or pressure, your altimeter is overreading, and your true altitude is lower than what's being indicated to you. But, if all aircraft have SPS (1013.25, or 29.92) set they will all maintain their separation.

Every half hour the ATIS is updated and the QNH will be given. Once you're above transition altitude, you set SPS and forget about it. On the way back down, when you pass transition level (where you go from SPS back to QNH) you set the regional QNH (lowest forecast QNH for the area you're flying in) or the QNH for your aerodrome.

EDIT: Just noticed you're in the USA... replace every 'QNH' in my explanation with In HG ;)

[wolf2]

toms explanation is excellent. here in the u.s. its 18,000 ft msl. above that its just set 29.92 and forget about it.

[tommchowat]

I should also add for an example, FL190 would have a lower true altitude in a colder airmass or lower pressure area than it would have in a warmer airmass or higher pressure area. But if all aircraft are operating at SPS, it doesn't matter.

You could ask why SPS is not a different value, say, 30.12 or something, because theoretically, if all aircraft above transition altitude had this value set, they would maintain the same vertical distance. The short answer is that the altimeter (and several other instruments) is calibrated to ISA (International Standard Atmosphere). ISA assumes that mean sea level pressure is 1013/29.92. Of course it's not, and this varies all the time, but this is the datum point of reference.

ISA also assumes that the temperature lapse rate is 1.98 degrees celcius per 1000 feet of vertical distance. Using a formula in conjunction with the ISA temperature deviation (ie. a temperature at a certain height that does not conform to ISA) you can work out true altitude. But I won't go into that ;)

[bgets]

I understand the setting of 29.92" above FL 180 in the United States. My question is why does the ATC give me barometric pressure readings when he or she only has the pressure reading for their given place. I:E the airport etc. If I am at FL150 they tell me the pressure is such and such. How do they know? They are not taking readings at my altitude?
Bob G.

[tommchowat]

I think you've got the difference between altitude and flight level confused.

In the US, FL150 doesn't exist, as transition altitude is FL180, that is, the lowest useable flight level is FL180. Therefore what you have written as FL150 would be given as 15,000 feet in the US and you would therefore be using an In Hg setting on the altimeter as you are below transition altitude and not using SPS.

In the UK, transition altitude varies from 3000 - 6000 feet, so FL150 would exist, as it is above transition altitude.

If you are flying to an airport, when you approach it, you will be given the airport pressure setting by the controller, and be expected to change to it when you pass transition level. If you are below transition altitude and flying through an area, not necessarily to an airport within that area, the controller will give you the regional pressure setting.

[bgets]

Tom. I am sorry. I didn't mean to confuse you with FL and altitude. I understand transition etc. And I understand that ATC will give you the pressure when you are set up for appoach. Makes sense. What I have a question about is, why does ATC give me the pressure below transition when all I really need is what is the pressure at the facility I am going to land? Is this just a FSX thing, or do the controllers actually give you the pressure at 15000 feet etc? I notice of course while climbing or descending, if I keep using the B key, my pressure will change as I climb or descend or even change of coursee in level flight. If I have my AP set for 15000 feet and hit the B key and the pressure changes, I will notice that my aircraft will either have to climb or descend to maintain that altitude. Hope I am making sense.:)
Bob G.

[tommchowat]

I'm not sure how it works in FSX, haven't used default atc for years. In real life, say, approaching Chicago, you'll be given descent instructions. Let's say transition level is FL180. If you are at FL230 and using SPS, a descent clearance would sound like this; "Descend to fifteen thousand feet, altimeter 3006". You would carry out the descent and change from SPS to 3006 at transition level, which here is FL180. 3006 is the pressure at Chicago. Further descent clearance should only mention altitude, as pressure has already been given to you. If pressure changes, you'll be told.

Same goes for on the way up. You'll get your pressure from Chicago atis of 3006, and will climb on that up to transition altitude, after which you'll use SPS and be cleared to flight levels.

I don't know if fsx atc mentions the pressure every time they give you a new altitude, that might be throwing you. It's not like that in real life, you'll be given the pressure once, when you're passing through transition altitude or level, or if it changes. Or you can ask for it again if you've forgotten it.

Like I said before, if you're flying in the region below transition altitude but not landing anywhere near, you'll be given regional pressure, or get it from a weather chart for that day. The UK has 14 of these regions.

As for pressing the B key, don't do that. FS will want to constantly update the altimeter with pressure changes, especially if using real world weather. Just set inches of mercury below transition altitude and leave it unless you're told it has changed, and use SPS above transition altitude.

[mgh]

why does ATC give me the pressure below transition when all I really need is what is the pressure at the facility I am going to land? .

You will be given the pressure setting for the region. This ensures that all aircraft in the region use the same pressure setting so that aircraft with the same indicated altitude are flying at the same height. Not all aircraft in a region will be using the same airport.

In the UK, When below the Transition Altitude, pilots are to fly on the aerodrome QNH until established on final approach, at which point QFE or any other desired setting may be used. However the UK does not have a universal Transition Altitude but each major airport has its own, which is commonly 6000 ft.

[ladamson]

Tom. I am sorry. I didn't mean to confuse you with FL and altitude. I understand transition etc. And I understand that ATC will give you the pressure when you are set up for appoach. Makes sense. What I have a question about is, why does ATC give me the pressure below transition when all I really need is what is the pressure at the facility I am going to land? Is this just a FSX thing, or do the controllers actually give you the pressure at 15000 feet etc? I notice of course while climbing or descending, if I keep using the B key, my pressure will change as I climb or descend or even change of coursee in level flight. If I have my AP set for 15000 feet and hit the B key and the pressure changes, I will notice that my aircraft will either have to climb or descend to maintain that altitude. Hope I am making sense.:)
Bob G.

I don't really use FSX ATC much.....

But if you were on a cross country, reporting facilities along the way, may have different pressure readings, due to conditions. By updating your altimeter to the same reading, you and other aircraft will be showing the same altitude in the same vicinity. It's important for altitude seperation such as the odd & even + 500' for VFR flight, or what ever ATC assigns you for IFR.

If you landed at one of these "on the way" airports, with a new and different reading....your altimeter would match the airport elevation. It has nothing to do, with flying at 15,000'.

L.Adamson

edit: Since there were a few replies, while I typed mine, I'll add some more. In real life flight, I use XM Satellite weather with my GPS. It has a box on the main screen, that picks up the new pressure reading from nearby reporting facilities as I fly along. This way, I can continually update the altimeter setting, without having to talk to someone.

[bgets]

Ok, so my question is why don't the aircraft have the pressure reading change in real time. I:E. If you climb from 10,000 ft. to 16,000 ft, why can't the pressure reading change as it changes as you climb and adjust altitude accordingly? Once you reach 18,000 ft. have it change to 29.92" or the pilot may do this?
MGH, your response would make sense. I would think you would want everyone in the area to have the same pressure to maintain the proper altitude.
Bob G.

[mgh]

Ok, so my question is why don't the aircraft have the pressure reading change in real time. I:E. If you climb from 10,000 ft. to 16,000 ft, why can't the pressure reading change as it changes as you climb and adjust altitude accordingly?

The pressure setting (QNH) is the notional pressure at sea-level at any given time in a particular region and doesn't change with altitude. Altimeters are calibrated so that they read zero when at sea level with QNH set.

[bgets]

My physics classes taught me that atmospheric pressure will change with altitude and prevailing weather conditions. Cold front, warm front etc. at the same altitude may have different pressure readings.
Thanks all for your responses.
Bob G.

[Brett_Henderson]

.. There's still a little confusion:


Ok, so my question is why don't the aircraft have the pressure reading change in real time. I:E. If you climb from 10,000 ft. to 16,000 ft, why can't the pressure reading change as it changes as you climb and adjust altitude accordingly?

Lets seperate altimeter setting from pressure reading .. The change in read pressure as you climb is the altimeter itself. You wouldn't want some magical barometer following you during the climb so that you could keep adjusting the altimeter.. because it's the change of pressure that tells the altimeter that you're changing altitude. If you kept adjusting the altimeter setting, to match the atmospheric pressure AT your altidue, the altimeter reading would never change.

An altimeter setting is simply a method for making sure that everyone in the area is on the same page. A pilot at 5000msl passing under pilot at 10000msl will know that they are 5000 feet apart, because they're both using the same altimeter setting.

Theoretically, we could use the same altimeter setting all of the time .. pilots near enough each other to worry about colliding would obviously be within the same weather system, so their altimeters will be safe indicators of seperation. The problems would arise as you neared an airport. It's OK as far as collisions go, but can be problematic for things like minimum safe altitudes on the charts, because they are gauged in feet AGL (above ground level) but are printed in MSL (assuming and accurate MSL reading on the altimeter).. and of course TPA (traffic pattern altitude) needs to accurate in AGL.. and the need to know your altitude MSL when position reporting , or flying over mountains and water are important.


Once you reach 18,000 ft. have it change to 29.92" or the pilot may do this?


Not only may he, he must

For flight above transition altitude (in the US)... those aircraft are fast, and covering great distances.. they pass through too many weather systems too quickly for all of them to be on the same page.. hence a universal setting for them.

[NikeHerk67]

To add some thoughts on the subject.

Between 28.00 to 32.00 inches of mercury there is an altitude range of about 4000 feet.
The highest pressure ever recorded was 1087.0 mb or 32.6 inches of mercury. The lowest barometric pressure recorded was in a hurricane at 88.2 mb or 25.69 inches of mercury. The difference in altitude, if they were both measured at sea level, is 5743'.....quite a difference.

As far as giving the pressures at the various altitudes, it would be very difficult for a weather station to report all those differences when located at 1000' as example.
Can you visualize a bunch of weather balloons on tethers at various altitudes, up to 18000 feet, anchored at each weather station? :)

As long as the pressure is given from the same location (elevation reference point being the weather station) it doesn't matter what the actual pressure is within reporting distance around it. Everyone in the same vicinity will be off just the same and at all altitudes above the station location. The 'reference point" is what's important here. Let's say a storm moves in and the barometric pressure difference from one end point to the opposite end is 30.52 to 29.00 inches. All aircraft in the same vicinity would increase or decrease altitudes accordingly and all would maintain the same separation. As far as doing your own thing, such as getting your informaiotn from XM radio and not ATC or ATIS, I wouldn't recommed it.

The reason it is important to know 'exactly' what the pressure is at the airport weather station is for when the rubber meets the road (runway). An error of a 100' is huge in IFR approach when the decision height is 500' AGL.

At 18,000 feet a 500' difference between the actual pressure and 29.92 inches of mercury isn't that critical, unless you're flying near the Himalayas.

[mgh]

My physics classes taught me that atmospheric pressure will change with altitude and prevailing weather conditions. Cold front, warm front etc. at the same altitude may have different pressure readings.
Thanks all for your responses.
Bob G.

That is true but altimeters are calibrated to ICAO standard atmosphere. This is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It consists of tables of values at various altitudes. At sea-level the trmperature is assumed to be 15 dec C and the pressure 1013 hPa (29.92 in Hg). At 10,000 ft the temperature is assumed to be -4.8 deg C and the pressure 697 hPa (10.10 in Hg). At any given time the actual atmos[phere is most unlikely to match the standard one because of the reasons you mention.

A basic altimeter subject to a pressure 0f 697 hPa will read 10,000 ft. This however doesn't mean the aircraft is actually at 10,000 ft, But, this means that all aircraft all at the same (unknown) height above sea level.

[lnuss]

My physics classes taught me that atmospheric pressure will change with altitude and prevailing weather conditions. Cold front, warm front etc. at the same altitude may have different pressure readings.
Thanks all for your responses.
Bob G.

Your physics classes taught you correctly and, for example, the pressure at around 18,000 feet is about half that at sea level. But you are missing the point for an altimeter setting. An altimeter is a barometer calibrated in altitude readings, rather than pressure readings. The Kollsman window (and it's associated knob) allow limited adjustment of the reading, to adjust to the ground station reading. If you attempted to change the altimeter setting to reflect ambient conditions, you'd likely always have the altimeter reading zero feet (or nearly so), regardless of altitude, which isn't very helpful.

So the altimeter setting (U.S. style, therefore FS style) is actually the reading of the ground station pressure, corrected to sea level*, and should let the pilot adjust his altimeter such that if he descended to ground level at that station, that his altimeter would read the station's altitude above sea level.

As Brett and NikeHerc have explained above, the whole purpose of all this rigmarole is to keep aircraft in the air separated from each other and aircraft separated from the ground. So accuracy on the ground is what is needed.

--------------------------------
* If a station were at, say, 5000 feet, the ACTUAL pressure reading might be around 24 inches of mercury, or a little less. So they recalibrate the barometers they use to read an equivalent sea level pressure. In Denver, for example, altimeter settings given to a pilot would still be around 29-30 inches of mercury, even though the actual pressure at that station is in the 23-24 inch neighborhood. Even the weatherman on TV in Denver will give current barometric readings as corrected to sea level.

[bgets]

That all makes sense. So when I am flying at say 8,000 ft and the weather is very unstable, like a T-storm, FSX will continue to pop up with the message that my altimeter is set wrong, please adjust the setting by pressing the B key, that's not correct in real life then?
Bob G.

[ladamson]

As far as doing your own thing, such as getting your informaiotn from XM radio and not ATC or ATIS, I wouldn't recommed it.



XM weather..................IS AWOS, ASOS, or ATIS. It's the new way of doing things. I can dial in a distant airport, and get the full report in print. It's a subscription service.

edit: And you'll also have a much better picture of the weather patterns, including animations. Far better than a briefing over the phone or comm. It's all real time (within a few minutes) while in flight. And it's extremely popular these days.

L.Adamson

[Brett_Henderson]

As far as giving the pressures at the various altitudes, it would be very difficult for a weather station to report all those differences when located at 1000' as example.
Can you visualize a bunch of weather balloons on tethers at various altitudes, up to 18000 feet, anchored at each weather station?



Aside from being nearly impossible.. it would be dangerous and confusing. I'm assuming you meant each baloon would have an altitude-calibrated barometer.. as in, a baloon at 5000msl would use the same barometer that an airport at 5000msl would use. Can you imagine the ATIS/AWOS/ASOS broadcast that states several altimeter settings for different altitudes ? And think about the poor pilot.. He's level at a certain altitude based on an altimeter set BEFORE getting to that altitude. It's bizzare catch-22... he'd trying to use an altimeter setting for his current altitude, that's an altimeter reading from another altitude and a different altimeter setting.. :confused:


***the rest of this post is for those still trying to understand altimeter settings ***

The altimeter setting that a pilot gets from an airport, is what the barometric pressure would be at sea-level, regardless of the airport's actual elevation.

See the attached image... it shows three airports that are relatively close to each other.. one at sea-level; one in a 2000 foot deep canyon; one on a 2000 foot high mountain. It assumes a 'standard' day (29.92 barometric pressure at sea-level), but the theory applies to any set of weather conditions.

If you were at the sea-level airport.. your every-day barometer would read 29.92. If you took that same barometer imediately down to the airport in the canyon, it would read 31.29.. and then if you took it up to the mountain airport, it would read 27.91... (if you stuck a little card behind the barometer needle calibrated in feet.. the barometer would be functioning as an altimeter.. because that's all that an altimeter is.. a very sensitive, very accurate barometer)

Now, in actuality (as mentioned); a barometer at any airport is calibrated to display what the BP (barometric pressure) would be at sea-level. So.. as you fly by these airports, even though the BP at each airport is different.. the altimeter setting they'd give you would be the same (29.92 in this example).

To carry this discussion a bit further from a pilot's perspective...

Let's say you land at an airport.. as you taxi to parking, your altimeter will obviously read the airport's field-elevation. Let's say you return the next morning and there is no ATIS/AWOS/ASOS .. A new weather system has likely moved in, so your altimeter is now reading something other than field elevation. Since you already KNOW the field elevation, you can simply spin the altimeter setting knob until the altimeter reads the field-elevation.. The number now displayed in the little setting window, is the same number that the ATIS/ASOS/AWOS would have given you as an altimeter setting...

Carrying it even further.. for the sake of theory.. let's say that you're flying an airplane with a ridiculously accurate manifold-pressure gauge. When a piston engine is not running, its manifold-pressure is equal to the barometric pressure. If the airport is near sea-level.. your hyper-accurate manifold-pressure gauge could be used to get a reasonable altimeter setting.. :cool:

[ladamson]

That all makes sense. So when I am flying at say 8,000 ft and the weather is very unstable, like a T-storm, FSX will continue to pop up with the message that my altimeter is set wrong, please adjust the setting by pressing the B key, that's not correct in real life then?
Bob G.

I'm going to assume, that you started with a defalt or saved flight, which had a specific altimeter setting, and then it's corrected by using "real weather", or you've flown into a new area, in which real weather has made an update.

As my airplane sits in the hangar, the altimeter may change quite a bit, just due to weather conditions. Before flight, I'll set it to the airports elevation of 4603', which is also about what I'll get if I set it to the latest report. Yesterday, it was showing over 5000' msl. due to current weather, and not being adjusted.

L.Adamson

[Tim_A]

In real life, you'll normally only get told about pressure changes when you talk to a new controller (or somebody else does, if you're monitoring the frequency). You'll have some idea of what the pressure is likely to be doing from looking at synoptic charts before the flight (these charts show surface level pressure isobars, frontal systems etc, so you'll know beforehand if you're going from (say) 1024mb to 995mb (a fairly extreme example, but it's representative of the difference between a high pressure day and a low pressure day).

Now, as someone mentioned earlier in the thread, there's a rule of thumb "High to low: look out below!" Which basically means if you're flying towards low pressure, you will be gradually descending even though your altimeter stays at 8000ft. Thus, with the example numbers, if I'm at 8000ft on 1024, by the time I get to my destination, if I leave my altimeter set to 1024 and maintain 8000ft on the altimeter, it will read 8000ft, but my actual altitude will be 7140ft. I'll have descended by 860ft!

You can see this on the ground too -- even in FS, with real weather set -- as the local pressure changes, the altimeter will go up and down. Save the flight, with the altimeter set to field elevation; come back tomorrow and it will say something different. You'll find a lot of RW pilots (GA ones anyhow) are quite anal about watching the weather...

Every time you make initial contact with a controller, they will give you the altimeter setting. That's universal throughout the world. They may call it 'QNH' or 'altimeter', and give it in HPa or inches (in any combination), but they still give it. It becomes second nature, whenever someone tells you the current pressure, to check it and set it. BTW, you can turn off the nag in FS (Realism->Display Flying Tips).

In the US and Europe, where controllers and other traffic are plentiful, knowing how high you are is easy. But out in the wilds, the only tools you might have are knowing the elevation of the field you took off from, and the TV weather forecast. Then, knowing what happens when you fly across a pressure gradient (high to low etc) could be the difference between missing the mountain and hitting it.

[ladamson]

In the US and Europe, where controllers and other traffic are plentiful, knowing how high you are is easy. But out in the wilds, the only tools you might have are knowing the elevation of the field you took off from, and the TV weather forecast. Then, knowing what happens when you fly across a pressure gradient (high to low etc) could be the difference between missing the mountain and hitting it.

As to mountains, you'd be surprised as to how good a GPS database has got these days, at least in the U.S., when corrected with WAAS. I can bring up the Garmin 696 panel page, and the GPS altimeter will flow exactly with the airplanes altimeter for an amount of time. It will eventually differ, due to a different barametric setting. Of course, the terrain is also shown on the GPS.

See you all later. Going mountain flying with a friend. I'm not legal at the moment due to an accident.

L.Adamson

[BushPilot]

Lets say you're at some airstrip in Denver Colorado. You're are sitting in your plane ready to depart and you have an actual barometer with you. The barometer will read something like 24.98 inHG because this airstrip is exactly 5000ft above sea level!

When you call up ATC to get the altimeter setting, they will not tell you 24.98 inHg but they will say something like "altimeter 29.98." (this value is corrected from actual pressure at this airport, so that when you put it in the Kollsman window, the altimeter reads height above sea level (true altitude)

So when you put 29.98 the altimeter will read 5000ft above sea level because all navigation charts use true altitude as reference. (correct me if I am wrong)

If you use the setting from the barometer which is next to you (24.98) and put it in the Kollsman window, and not call ATC, your altimeter will read "zero" and that will be dangerous!

[skylab]

I flew for an airline briefly where we carried a third altimeter aboard that was set to Station Pressure for each approach. The Engineer would call the Station and get the setting from the person on the ground and we would then set the 3rd altimeter. This merely acted as a 'back-up' and would read 'zero' on landing if the pressure didn't change. I don't recall ever using it during any approach but probably looked at it and compared it anyway. But...that was almost fifty [50] years ago !!!:)

[NikeHerk67]

Back in 1972, when I first started flying lessons, we set the altimeter to the airport elevation. After reading this thread I remeber that now. We were located in country and never conversed with ATC. The flight plan was filed VFR for a remote area for lesson practice.

What would happen in the following scenario, and who is responsible for keeping separation other than ATC? Remember, some GA aircraft don’t even have radios.

Aircraft A is flying west and the pilot recieves a barometric pressure reading from his arrival airport A, located 50nm west of his location, of 29.67 inHg.

Aircraft B is flying east and the pilot recieves a barometric pressure reading from his arrival airport B, located 50nm east of his location, of 30.15 inHg.

Both aircraft are at the same location and in this example, the two airports are located 100nm apart from each other.

Each pilot sets the altimeter for his respective airport and that difference in barametric pressure makes a difference of 500’ elevation.

I realize that would be an extreme change in barometric pressure; however, even with a lesser difference in readings, there’s also a pilot error possibility of +or – 100’ from actual altitude. What is the possibility of something like this occurring?

[Brett_Henderson]

I realize that would be an extreme change in barometric pressure; however, even with a lesser difference in readings, there’s also a pilot error possibility of +or – 100’ from actual altitude. What is the possibility of something like this occurring?


It's very possible... and why cruising altitudes are set up to have aircraft on opposing headings seperated by 1000 feet..

[deor]

"why does ATC give me the pressure below transition when all I really need is what is the pressure at the facility I am going to land?"

Possibly because when you are flying cross country at what your altimeter shows as 6000 feet, you may come across mountains which are 5500 feet high and it is good for you to know that your altimeter is showing your true altitude!