Altimeter setting

[MARLAN_]

For those interested, I briefly talked to a Rhino pilot today about this topic.

They won't transition during cyclic ops.

They will transition to QNE when based out of a field with an IFR clearance (incl. flight >18K which is most flight)

[Yurgon]

There's a bit of terminology in use here that I'm curious about; and sorry to OP, I don't know the answer to the original question either, but it would be really cool to find that out with any authority. Sounds like a good question for the Fighter Pilot Podcast.

Now, as for the terminology:

"Local QNH" and "Standard QNH": not that Wikipedia is the reference, but I can't find any mention of a qualifier to "QNH" there, and the FAA website as well as the FAA's Aeronautical Information Manual (PDF download) also don't yield any search results for "local QNH" or "standard QNH". Seeing as QNH already has a pretty solid definition on its own, are you guys certain that the standard pressure setting is also known as "standard QNH"? To me that distinction sounds like an absolutely great way to get everyone confused.

"QNE = Standard Pressure Setting": Wikipedia defines this as

Quote

QNE is an aeronautical code Q code. The term refers to the indicated altitude at the landing runway threshold when 1013.25 mb or 29.92 inHg is set in the altimeter's Kollsman window. In other words, it is the pressure altitude at the landing runway threshold.

However, the FAA defines it as

Quote

QNE- The barometric pressure used for the standard altimeter setting (29.92 inches Hg.).

So, is QNE an altitude or a pressure setting? I've been trying for years to find any authoritative source for these different definitions; pilots told me QNE = standard pressure setting, but I keep finding sources that claim QNE is an altitude. The good news is there's little potential for confusion here, and I guess in context it should always be clear which of these someone refers to.

Either way, I think it's important to be aware of these rather odd and in part contradictory definitions.

[=475FG= Dawger]

32 minutes ago, Yurgon said:

There's a bit of terminology in use here that I'm curious about; and sorry to OP, I don't know the answer to the original question either, but it would be really cool to find that out with any authority. Sounds like a good question for the Fighter Pilot Podcast.

Now, as for the terminology:

"Local QNH" and "Standard QNH": not that Wikipedia is the reference, but I can't find any mention of a qualifier to "QNH" there, and the FAA website as well as the FAA's Aeronautical Information Manual (PDF download) also don't yield any search results for "local QNH" or "standard QNH". Seeing as QNH already has a pretty solid definition on its own, are you guys certain that the standard pressure setting is also known as "standard QNH"? To me that distinction sounds like an absolutely great way to get everyone confused.

"QNE = Standard Pressure Setting": Wikipedia defines this as

However, the FAA defines it as

So, is QNE an altitude or a pressure setting? I've been trying for years to find any authoritative source for these different definitions; pilots told me QNE = standard pressure setting, but I keep finding sources that claim QNE is an altitude. The good news is there's little potential for confusion here, and I guess in context it should always be clear which of these someone refers to.

Either way, I think it's important to be aware of these rather odd and in part contradictory definitions.

QNH, QNE and QFE in common aviation use are altimeter settings.

When you set each one they indicate your distance vertically from a particular reference datum.

QNE is standard atmospheric pressure , 29.92 in hg or 1013 mb and results in your distance above the standard atmosphere datum, referred to as your "flight level".

QNH is the local atmospheric pressure reduced to altitude above Mean Sea Level at the measuring station. This is what is meant when someone says "altitude"

QFE is local atmospheric pressure corrected for temperature. The altimeter will read zero at the measuring station elevation and this is referred to as "height"

[Frederf]

QNE (and all the others) are altimeter settings. The value is 29.92 inches or whatever.

They are defined as a pressure setting such that an altimeter will read X when at altitude Y.

I've never heard of standard QNH. Normally QNH is pount-specific, usually an airport. I assume standard is that. There are special QHNs. Britain has 20 zones with a regional QNH (technically regional pressure setting) and militaries have force QNH. You will absolutely know if your QNH is a special type so assume a normal type otherwise.

All QNHs are local. The locality can be bigger in special situations where coordination in altitude outside of a sungle airport is important. E.g. in Britain they measure QHN at several points in a region and then pick the safest one even if it's not super accurate at any particular airport. For tactical reasons jets can come from multiple airports and attack a place. The QNH given will be force-wide so everyone is the same even if it's not perfect for their origin base.

Edited July 25, 2022 by Frederf

[Yurgon]

10 hours ago, =475FG= Dawger said:

QNH, QNE and QFE in common aviation use are altimeter settings.

Source: A dude on the Internet.

I appreciate your response. The problem is that there are conflicting definitions, as outlined above. Simply writing down one of these definitions does not remove the ambiguity. But, again, thanks for your input. Like I said above, pilots tend to describe QNE as a pressure setting, not an altitude, so I'll take this as an additional point of data.

10 hours ago, Frederf said:

QNE (and all the others) are altimeter settings.

Same as above. Some sources say it is, some say it's an altitude. Your response is valued as well, but does not remove the contradicting definitions.

10 hours ago, Frederf said:

I've never heard of standard QNH.

It was referenced earlier in this thread:

On 6/6/2022 at 7:28 AM, Spock14 said:

So to make a long story short everything below Transition Level should be in local QNH and everything above Transition Altitude should be standard QNH.

I've heard it mentioned like this quite a few times over the years in the DCS context, and also found similar mentions in other aviation related forums, but could not find official mentions in FAA documentation or Wikipedia.

Hence the question: is this some kind of inofficial aviation lingo, or is it people on the Internet getting it wrong? In case it's the latter, I just wanted to raise awareness, and otherwise I'd just like to learn about it and see if anyone could point me to a source where it's defined like above.

10 hours ago, Frederf said:

All QNHs are local. The locality can be bigger in special situations where coordination in altitude outside of a sungle airport is important. [...]

Great explanation, thanks!

Edited July 25, 2022 by Yurgon

[MARLAN_]

58 minutes ago, Yurgon said:

Source: A dude on the Internet.

I appreciate your response. The problem is that there are conflicting definitions, as outlined above. Simply writing down one of these definitions does not remove the ambiguity. But, again, thanks for your input. Like I said above, pilots tend to describe QNE as a pressure setting, not an altitude, so I'll take this as an additional point of data.

Same as above. Some sources say it is, some say it's an altitude. Your response is valued as well, but does not remove the contradicting definitions.

It was referenced earlier in this thread:

I've heard it mentioned like this quite a few times over the years in the DCS context, and also found similar mentions in other aviation related forums, but could not find official mentions in FAA documentation or Wikipedia.

Hence the question: is this some kind of inofficial aviation lingo, or is it people on the Internet getting it wrong? In case it's the latter, I just wanted to raise awareness, and otherwise I'd just like to learn about it and see if anyone could point me to a source where it's defined like above.

Great explanation, thanks!

 

The definitions you posted don't conflict. It's important to note the verbiage "pressure altitude" which is not the same thing as altitude. QNH is corrected for non standard pressure, that is, it is local pressure. QNE is standard pressure.

QNH will indicate field elevation when you are on the field.

QNE will indicate pressure altitude when you are on the field.

QFE will indicate 0 when you are on the field.

For example, KLSV (Nellis AFB) field elevation is 1870, if you set QNH (e.g. 29.71) you'll see approximately that elevation.

If you set QNE (29.92) you'll probably see something closer to 2000' (depends of course) which is the pressure altitude.

It's also important to note that DCS indicates "QNH" incorrectly in the briefing & mission editor but it is actually a modified (in that its not from sea level) QFF.

Edited July 25, 2022 by MARLAN_

[=475FG= Dawger]

51 minutes ago, Yurgon said:

Source: A dude on the Internet.

I appreciate your response. The problem is that there are conflicting definitions, as outlined above. Simply writing down one of these definitions does not remove the ambiguity. But, again, thanks for your input. Like I said above, pilots tend to describe QNE as a pressure setting, not an altitude, so I'll take this as an additional point of data.

 

 

It helps if you understand what Q codes are. They originated with the need for shorthand methods to ask and answer questions using Morse code.

So each three letter group is, originally a specific question and response.

QNH is the question "What should I set on the subscale of my altimeter so that the instrument would indicate its elevation if my aircraft were on the ground at your station?" and the response "If you set the subscale of your altimeter to read ____ millibars, the instrument would indicate its elevation if your aircraft were on the ground at my station at ____ hours."

In real world use, you hear something like " Descend to five thousand, QNH 1011". You could ask "QNH?" and the response would be "QNH 1011" The time is implied as currently.

QFE is similar except the datum used is field elevation instead of Mean Sea Level.

QNE is a different sort of question and answer. If you were to say "QNE?" , you are asking " If I set my altimeter to standard pressure (1013 or 29.92) what would my altimeter read at your station?". This question has no practical use in aviation. No will ask for QNE. 

You will occasionally hear "set QNE" and "QNE set" (More often the crew will use the actual numbers) as part of a challenge and response between flight crew members.

In communication with ATC, if you are assigned a flight level, you are expected to set standard pressure on the altitude upon crossing the transition altitude in the US and immediately elsewhere.

[Yurgon]

28 minutes ago, MARLAN_ said:

If you set QNE (29.92) you'll probably see something closer to 2000' (depends of course) which is the pressure altitude.

So you are saying that QNE is a pressure setting, measured in inHG (or hPa), correct?

Meanwhile, a "pressure altitude" is not a "pressure setting".

In essence, QNE is either a pressure setting measured in inHG or a pressure altitude measured in feet.

Unless the terms "pressure setting" and "pressure altitude" actually refer to the same thing, these two definitions for QNE are contradicting one another.

14 minutes ago, =475FG= Dawger said:

QNE is a different sort of question and answer. If you were to say "QNE?" , you are asking " If I set my altimeter to standard pressure (1013 or 29.92) what would my altimeter read at your station?". This question has no practical use in aviation. No will ask for QNE. 

So, I guess that means that "officially" that's the way it is, but colloquially pilots would rather use the term QNE to mean the same thing as standard pressure setting, right?

14 minutes ago, =475FG= Dawger said:

It helps if you understand what Q codes are.

I appreciate that you're trying to help. But that was not the question. I understand what Q-codes are, I understand what they mean and how they're used. What I don't understand is why one of them has contradicting definitions, and why another one of them is sometimes used to mean one thing in one context and another thing in another context.

Edited July 25, 2022 by Yurgon

[=475FG= Dawger]

11 minutes ago, Yurgon said:

 

So, I guess that means that "officially" that's the way it is, but colloquially pilots would rather use the term QNE to mean the same thing as standard pressure setting, right?

I appreciate that you're trying to help. But that was not the question. I understand what Q-codes are, I understand what they mean and how they're used. What I don't understand is why one of them has contradicting definitions, and why another one of them is sometimes used to mean one thing in one context and another thing in another context.

 

The use of QNH and, consequently, QNE is something that has crept into aviation language outside of the United States. You won't ever hear either one coming from a US crew or controller. In the US, the local altimeter setting is referred to as "altimeter" or more correctly although less common "altimeter setting". As in, " Descend and maintain five thousand, altimeter 30.01". In the US, you set standard pressure when climbing through 18,000 feet. The trained challenge and response was always "Set <number>" and "<number> set"

In other parts of the world, QNH has crept/adopted into the lexicon and some pilots will use, incorrectly, QNE when they mean "standard atmospheric pressure".

It isn't correct but it is now defined as meaning "standard atmospheric pressure". Even the FAA defines QNE that way now.

So the short answer is QNE in aviation today always means "standard atmospheric pressure". 

QNE, in its original form, has no practical use in aviation and isn't used.

[Yurgon]

5 minutes ago, =475FG= Dawger said:

The use of QNH and, consequently, QNE is something that has crept into aviation language outside of the United States. You won't ever hear either one coming from a US crew or controller. In the US, the local altimeter setting is referred to as "altimeter" or more correctly although less common "altimeter setting". As in, " Descend and maintain five thousand, altimeter 30.01". In the US, you set standard pressure when climbing through 18,000 feet. The trained challenge and response was always "Set <number>" and "<number> set"

In other parts of the world, QNH has crept/adopted into the lexicon and some pilots will use, incorrectly, QNE when they mean "standard atmospheric pressure".

It isn't correct but it is now defined as meaning "standard atmospheric pressure". Even the FAA defines QNE that way now.

So the short answer is QNE in aviation today always means "standard atmospheric pressure". 

QNE, in its original form, has no practical use in aviation and isn't used.

Awesome, thanks!

[MARLAN_]

34 minutes ago, Yurgon said:

So you are saying that QNE is a pressure setting, measured in inHG (or hPa), correct?

Meanwhile, a "pressure altitude" is not a "pressure setting".

In essence, QNE is either a pressure setting measured in inHG or a pressure altitude measured in feet.

Unless the terms "pressure setting" and "pressure altitude" actually refer to the same thing, these two definitions for QNE are contradicting one another.

So, I guess that means that "officially" that's the way it is, but colloquially pilots would rather use the term QNE to mean the same thing as standard pressure setting, right?

I appreciate that you're trying to help. But that was not the question. I understand what Q-codes are, I understand what they mean and how they're used. What I don't understand is why one of them has contradicting definitions, and why another one of them is sometimes used to mean one thing in one context and another thing in another context.

 

QNE is a specific pressure setting, i.e. 29.92 or 1013.25 which will match the pressure altitude of a field. The pressure setting doesn't change, but the pressure altitude will.

QNH is a specific altitude for a field, so the altitude of the field doesn't change but the pressure setting does.

QNE is always 29.92 but the pressure altitude is different.

QNH will always be 1870' at Nellis but the pressure setting is different.

[Yurgon]

8 minutes ago, MARLAN_ said:

QNE is a specific pressure setting, i.e. 29.92 or 1013.25 which will match the pressure altitude of a field. The pressure setting doesn't change, but the pressure altitude will.

QNH is a specific altitude for a field, so the altitude of the field doesn't change but the pressure setting does.

QNE is always 29.92 but the pressure altitude is different.

QNH will always be 1870' at Nellis but the pressure setting is different.

I think at this point there's nothing left that you don't have backwards... 

[MARLAN_]

14 minutes ago, Yurgon said:

I think at this point there's nothing left that you don't have backwards... 

What do you mean? I'm just trying to help you understand it by explaining the other side of it.

[Yurgon]

7 minutes ago, MARLAN_ said:

What do you mean? I'm just trying to help you understand it by explaining the other side of it.

You literally just said QNH is an altitude. It's not. It's a pressure setting.

Nellis QNH is not 1850 feet, it's something like 29.96 inHg. When QNH is set, the altimeter will show 1850 feet at Nellis (or thereabouts).

You also just said QNE is a pressure setting, while the discussion has been about that particular question, and as Dawger just said, that's the colloquial or common use nowadays, but historically and officially, QNE is an altitude at standard pressure setting, and not the pressure setting itself.

I understand you're trying to help, but introducing fresh inaccuracies doesn't help a discussion that's all about the details.

[MARLAN_]

28 minutes ago, Yurgon said:

You literally just said QNH is an altitude. It's not. It's a pressure setting.

Nellis QNH is not 1850 feet, it's something like 29.96 inHg. When QNH is set, the altimeter will show 1850 feet at Nellis (or thereabouts).

You also just said QNE is a pressure setting, while the discussion has been about that particular question, and as Dawger just said, that's the colloquial or common use nowadays, but historically and officially, QNE is an altitude at standard pressure setting, and not the pressure setting itself.

I understand you're trying to help, but introducing fresh inaccuracies doesn't help a discussion that's all about the details.

QNH is an altitude, that is, the ISA MSL altitude of a field. But it's not commonly referred to as an altitude obviously, that is the inverse of the equation.

QNE is a pressure setting, that is, standard pressure, and this will correlate to the pressure altitude of a field. This is not the same as the altitude of a field.

https://en.wikipedia.org/wiki/Pressure_altitude

https://en.wikipedia.org/wiki/Atmospheric_pressure

Nellis is not "29.96" if you mean a set unchanging value, its a variable that changes with local pressure. Nellis QNH at the time of this last current report 29.95, which I'm sure you know since you just checked, but it was as low as the 29.70s yesterday. If you think this is a set number you're not understanding the material, and the fact you believe the Wikipedia and FAA definitions of QNE to be in conflict shows you aren't understanding the "details" as you put it.

I don't mean to be rude, but you are literally calling me out when you do not understand.

Edited July 25, 2022 by MARLAN_

[Yurgon]

15 minutes ago, MARLAN_ said:

QNH is an altitude, that is, the ISA MSL altitude of a field. But it's not commonly referred to as an altitude obviously, that is the inverse of the equation.

Okay, so if QNH is an altitude, and a pilot wanted to know Nellis' QNH, the response would be "Flight 123, Nellis Tower, QNH 1850 feet."

That's obviously not the response. A possible response is "Flight 123, Nellis Tower, QNH 29.96 (inHg)."

So how is QNH an altitude?

Just to be clear, I understand the altimeter will show an aircraft's altitude above mean sea level when QNH is set, and that this will correspond to the runway's elevation when the aircraft is at the runway threshold. That's the whole point. But per definition, QNH is a pressure setting, not an altitude.

18 minutes ago, MARLAN_ said:

QNE is a pressure setting, that is, standard pressure, and this will correlate to the pressure altitude of a field. This is not the same as the altitude of a field.

https://en.wikipedia.org/wiki/Pressure_altitude

Here is where it gets really funny. I actually quoted that very article you just linked in my first post above, and it literally says that QNE is an altitude. Did you even read that Wikipedia article?

The article then goes on to do some exemplary math, where the result is obviously a pressure altitude in feet.

21 minutes ago, MARLAN_ said:

Nellis is not "29.96" if you mean a set unchanging value [...]

I just made up a plausible QNH pressure setting value that could be current at Nellis, or really an other place in the world, at just about any time. It was obviously ill chosen to illustrate my point, the point being the QNH is not an altitude in feet, but rather a pressure setting in inHg.

You're trying to help by illustrating what QNH is used for, and that's appreciated. It just really doesn't help that you're now mixing up a definition with a use case.

[MARLAN_]

42 minutes ago, Yurgon said:

Okay, so if QNH is an altitude, and a pilot wanted to know Nellis' QNH, the response would be "Flight 123, Nellis Tower, QNH 1850 feet."

That's obviously not the response. A possible response is "Flight 123, Nellis Tower, QNH 29.96 (inHg)."

So how is QNH an altitude?

Like I said, I was explaining the inverse or other side of it to help you understand. It is not commonly referred to as an altitude, but it does mathematically relate to an altitude.

Setting for example, 29.70 inHg (assuming appropriate pressure/temperature conditions to match) will indicate 1850' when parked at Nellis . Setting your indicated altitude (at the field) to match 1850' will also indicate 29.70 inHg in your Kollsman window. They mathematically relate to each other and you can solve for one if you know the other (assuming you have pressure/temperature data available as well)

But yes, you are correct, you would never hear "QNH 1850 feet" on the radio, it is not used this way, I am only trying to show you the opposite side of the relation.

42 minutes ago, Yurgon said:

Here is where it gets really funny. I actually quoted that very article you just linked in my first post above, and it literally says that QNE is an altitude. Did you even read that Wikipedia article?

Yes, extensively, I am programming a dcs-weather-update program and I needed to use a lot of these equations to convert the incorrectly reported DCS: QNH to QFF, which includes using pressure altitude and QFE as part of the equations.

QNE is the pressure altitude, like I previously explained, they all relate to each other, and pressure altitude is not the same as (true) altitude. If you measured vertically from sea level to Nellis your measuring tape would read 1870' but your altimeter when set to 29.92 (QNE) would indicate pressure altitude which would be a different value depending how much the pressure & temperature differ from ISA on that given time, at that location.

42 minutes ago, Yurgon said:

You're trying to help by illustrating what QNH is used for, and that's appreciated. It just really doesn't help that you're now mixing up a definition with a use case.

The use case of QNH is to have consistent values based on local pressure such that obstacles or field elevations match charts because pressure/temperature changes, but measured elevation doesn't (but pressure altitude does). If an obstacle is 4000 feet, you want your altimeter to correctly read 4000 feet when at that obstacle based on the local pressure so you can consistently and safely avoid that obstacle and fly above it. If you had QNE set (29.92) instead of QNH, if pressure was low that day, your altimeter may indicate 4050' when you are in fact at 3850' and thus you would collide with that obstacle.

The use case of QNE is to uncouple local pressure to a set standard so that when above altitudes where earth obstacles are no longer a concern, but typically only other aircraft, you all have the same altimeter setting without needing to resync every time you enter a new airspace that has different local pressure.

Edited July 25, 2022 by MARLAN_

[Frederf]

2 hours ago, Yurgon said:

The problem is that there are conflicting definitions, as outlined above.

Then some are wrong. A properly stated definition should remove all ambiguity. Here's I'll quote the exact FAA and ICAO definitions for QNH.

• QNH− The barometric pressure as reported by a particular station. ( https://www.faa.gov/air_traffic/publications/atpubs/pcg_html/glossary-q.html )

QNH	Altimeter sub-scale setting to obtain elevation when on the ground (ICAO Document 8168, Vol 1, §1, Ch 2)

Re: "Standard QNH". That's not the correct term. I searched the thread. The setting that results in the altimeter displaying pressure altitude is called QNE. The person who said "standard QNH" should have said "standard altimeter" or "QNE" but not the incorrect mashed up "standard QNH". The person was using "QNH" as interchangeably with "altimeter setting" and thus "standard altimeter setting" become "standard QNH" which isn't correct. Some pilots, especially USA, refer to QNE as "standard setting" since it doesn't change with the weather.

Re: "Local QNH" QNH is in reference to a locality. QNH itself is a meaningless value unless you know where. Is it the QNH for Tokyo International Airport? Eiffel Tower, Paris? London Bridge, UK? In situations you'll required to reference "local QNH" which means use the QNH which corresponds to that locality.  If you're landing in Frankfurt, Germany use Frankfurt, Germany's QNH. Don't use the QNH from New Delhi, India. New Delhi's QNH isn't local.

Re: "Q__ is an altitude" It's not, at least the Q codes we're talking about, i.e QNE, QNH, QFF, QFE. There are dozens if not hundreds of Q codes that can be what temperature is it to what did you eat for breakfast. It's an old system when Morse was used to communicate so common concepts were greatly abbreviated.People are going to make that confusion because "altitude as displayed on the altimeter while set to the pressure reference Q___" is a mouthful. So they say I'm at "10,000 feet, QNH" or "10,000 feet QNE" or "10,000 feet QFE"  that those Q-codes aren't altitudes, but indications of what pressure setting is being referenced. Those brief phrases are shorter ways of saying "I'm at 10,000 feet as shown on my altimeter which is set to the QNH pressure setting". You might also see the "QFE datum" for example which is an elevation, feet meters. That is because the datum is a distance even though QFE itself is a pressure/pressure setting. The datums are part of the Q___ definitions but aren't the values themselves.

Pressure altitude is an altitude in that it's expressed as a distance although it's actually an indication of a pressure. There exists the International Standard Atmosphere (ISA) which is a scientific model of a typical "yardstick" atmosphere to which all other real atmospheres are compared. For example if it's 20°C today and standard is 15°C then it's "5°C hotter than standard". If standard pressure is 29.923" Hg and it's 30.023" Hg, then it's "0.100" Hg higher pressure than standard." We realize that the actual relationship of pressure to altitude is not the same as ISA on any given day, which is why calibration may be needed.

The barometric altimeter is simply a pressure gauge which has dial markings painted in distance. It only knows pressure and any displayed altitude is just a guess based on that pressure. If you put the gauge in a vacuum chamber in the lab and pumped out air you'd see the hands moving as if climbing. The relationship between pressure and altitude is 1:1. You give it a pressure and there's only one altitude that it can be and in reverse every altitude only has one corresponding pressure. For example in the ISA at 17,000' the pressure is 15.57'' Hg. So if you measure 15.57'' Hg you know you're at 17,000'. If every day the atmosphere was the same as the International Standard Atmosphere (ISA) then our altimeters would not need pressure setting knobs.

So we take this ISA barometric altimeter out into the real world and we take it up in an airplane to an actual (say as measured by radar from the ground) 17,000'. It sees the air pressure out the window and it should read 17,000', right? Maybe not. Say the pressure out the window is actually 14.94" Hg. The altimeter sees 14.94" and displays "you're at 18,000 feet." But we're not at 18, we're at 17. The altimeter is showing us what our altitude would be if we were in the standard atmosphere based on the measured actual pressure. That's pressure altitude. Pressure altitude is expressed in distance but it's not a distance to anywhere. If you found where the pressure altitude was 8,000' and 8,100' and then actually used a tape measure to find the true distance between these points, it might not be 100'.

[MARLAN_]

3 minutes ago, Frederf said:

Then some are wrong. A properly stated definition should remove all ambiguity. Here's I'll quote the exact FAA and ICAO definitions for QNH.

• QNH− The barometric pressure as reported by a particular station. ( https://www.faa.gov/air_traffic/publications/atpubs/pcg_html/glossary-q.html )

QNH	Altimeter sub-scale setting to obtain elevation when on the ground (ICAO Document 8168, Vol 1, §1, Ch 2)

Re: "Standard QNH". That's not the correct term. I searched the thread. The setting that results in the altimeter displaying pressure altitude is called QNE. The person who said "standard QNH" should have said "standard altimeter" or "QNE" but not the incorrect mashed up "standard QNH". The person was using "QNH" as interchangeably with "altimeter setting" and thus "standard altimeter setting" become "standard QNH" which isn't correct. Some pilots, especially USA, refer to QNE as "standard setting" since it doesn't change with the weather.

Re: "Local QNH" QNH is in reference to a locality. QNH itself is a meaningless value unless you know where. Is it the QNH for Tokyo International Airport? Eiffel Tower, Paris? London Bridge, UK? In situations you'll required to reference "local QNH" which means use the QNH which corresponds to that locality.  If you're landing in Frankfurt, Germany use Frankfurt, Germany's QNH. Don't use the QNH from New Delhi, India. New Delhi's QNH isn't local.

Re: "Q__ is an altitude" It's not, at least the Q codes we're talking about, i.e QNE, QNH, QFF, QFE. There are dozens if not hundreds of Q codes that can be what temperature is it to what did you eat for breakfast. It's an old system when Morse was used to communicate so common concepts were greatly abbreviated.People are going to make that confusion because "altitude as displayed on the altimeter while set to the pressure reference Q___" is a mouthful. So they say I'm at "10,000 feet, QNH" or "10,000 feet QNE" or "10,000 feet QFE"  that those Q-codes aren't altitudes, but indications of what pressure setting is being referenced. Those brief phrases are shorter ways of saying "I'm at 10,000 feet as shown on my altimeter which is set to the QNH pressure setting". You might also see the "QFE datum" for example which is an elevation, feet meters. That is because the datum is a distance even though QFE itself is a pressure/pressure setting. The datums are part of the Q___ definitions but aren't the values themselves.

Pressure altitude is an altitude in that it's expressed as a distance although it's actually an indication of a pressure. There exists the International Standard Atmosphere (ISA) which is a scientific model of a typical "yardstick" atmosphere to which all other real atmospheres are compared. For example if it's 20°C today and standard is 15°C then it's "5°C hotter than standard". If standard pressure is 29.923" Hg and it's 30.023" Hg, then it's "0.100" Hg higher pressure than standard." We realize that the actual relationship of pressure to altitude is not the same as ISA on any given day, which is why calibration may be needed.

The barometric altimeter is simply a pressure gauge which has dial markings painted in distance. It only knows pressure and any displayed altitude is just a guess based on that pressure. If you put the gauge in a vacuum chamber in the lab and pumped out air you'd see the hands moving as if climbing. The relationship between pressure and altitude is 1:1. You give it a pressure and there's only one altitude that it can be and in reverse every altitude only has one corresponding pressure. For example in the ISA at 17,000' the pressure is 15.57'' Hg. So if you measure 15.57'' Hg you know you're at 17,000'. If every day the atmosphere was the same as the International Standard Atmosphere (ISA) then our altimeters would not need pressure setting knobs.

So we take this ISA barometric altimeter out into the real world and we take it up in an airplane to an actual (say as measured by radar from the ground) 17,000'. It sees the air pressure out the window and it should read 17,000', right? Maybe not. Say the pressure out the window is actually 14.94" Hg. The altimeter sees 14.94" and displays "you're at 18,000 feet." But we're not at 18, we're at 17. The altimeter is showing us what our altitude would be if we were in the standard atmosphere based on the measured actual pressure. That's pressure altitude. Pressure altitude is expressed in distance but it's not a distance to anywhere. If you found where the pressure altitude was 8,000' and 8,100' and then actually used a tape measure to find the true distance between these points, it might not be 100'.

Well explained! Much better than I could explain it.

[Phantom711]

vor 17 Minuten schrieb Frederf:

Then some are wrong. A properly stated definition should remove all ambiguity. Here's I'll quote the exact FAA and ICAO definitions for QNH.

• QNH− The barometric pressure as reported by a particular station. ( https://www.faa.gov/air_traffic/publications/atpubs/pcg_html/glossary-q.html )

QNH	Altimeter sub-scale setting to obtain elevation when on the ground (ICAO Document 8168, Vol 1, §1, Ch 2)

Re: "Standard QNH". That's not the correct term. I searched the thread. The setting that results in the altimeter displaying pressure altitude is called QNE. The person who said "standard QNH" should have said "standard altimeter" or "QNE" but not the incorrect mashed up "standard QNH". The person was using "QNH" as interchangeably with "altimeter setting" and thus "standard altimeter setting" become "standard QNH" which isn't correct. Some pilots, especially USA, refer to QNE as "standard setting" since it doesn't change with the weather.

 

QNE is not a term that is internationally defined or officially used by ICAO (according my quick research on the internet).

I know, that it exists, but I`ve never heard any pilot say "set QNE" when climbing through transition altitude. At least it is common practice to call it "standard QNH".

[shagrat]

vor 1 Stunde schrieb Phantom711:

 

QNE is not a term that is internationally defined or officially used by ICAO (according my quick research on the internet).

I know, that it exists, but I`ve never heard any pilot say "set QNE" when climbing through transition altitude. At least it is common practice to call it "standard QNH".

If it's "common practice", that doesn't mean it is correct. Because per definition QNH is the pressure setting to show the Altitude above MSL, at a specific point/area. A "standard QNH" doesn't really make sense, that way.

QFE is the pressure setting to show height above a reference point on the ground (usually airport location or runway threshold).

Whereas QNE is the FL in reference to a pressure setting of 29.92 InHG/1013.25 hPa. So in reality FL0 may be above, below or at MSL, but FL180 is where the pressure altitude shows 18,000 ft on your gauge, no matter what your actual altitude MSL or height above ground is.

I found this website that explains it quite good:

https://skybrary.aero/articles/altimeter-pressure-settings

Edited July 25, 2022 by shagrat

[Phantom711]

@shagrat

Thanks, I know and understand all that. I was just referring to the fact, that at least to my experience nobody ever mentions "QNE" under that very name.

[Frederf]

There is no such thing as "standard QNH" as equivalent to QNE.

I don't believe Q-anything is spoken operationally but at least the FAA/ICAO controllers have to know Q-codes and they should pop up in the instructional material for pilots at the ATP level. They understand the concepts well enough.

[Yurgon]

3 hours ago, MARLAN_ said:

Like I said, I was explaining the inverse or other side of it to help you understand. It is not commonly referred to as an altitude, but it does mathematically relate to an altitude.

Glad we were able to reach this distinction.

3 hours ago, MARLAN_ said:

QNE is the pressure altitude, like I previously explained, they all relate to each other, and pressure altitude is not the same as (true) altitude.

I thank you for your explanations and your attempt to help me understand.

It would help if you chose your words carefully. QNE is either a pressure setting or an altitude. It can't be both. It's either measured in inHg or in feet.

There's no need to explain the relation between altitudes, air pressure differences and so on. I think everyone here understands that. The question is not "What is QNE and how do we use it?", the question is "How is QNE defined?". Or to be more precise, the question is "With 2 different definitions for QNE, which one is correct?"

3 hours ago, Frederf said:

Re: "Standard QNH". That's not the correct term.

Yeah, seems to me as well that someone is just using wrong terminology there.

3 hours ago, Frederf said:

Re: "Q__ is an altitude" It's not, at least the Q codes we're talking about, i.e QNE, QNH, QFF, QFE.

[...]

Pressure altitude is an altitude in that it's expressed as a distance although it's actually an indication of a pressure.

All that is fine. The underlying problem remains that one definition says "QNE is the pressure setting of 29.92 inHg" and another definition says "QNE is a runway threshold pressure altitude when the altimeter is set to 29.92 inHg". Which, once again, means that there are conflicting definitions where it is either a pressure setting (in inHg) or an altitude (in feet, at a specific place, at a specific time).

You just repeated one of these definitions while ignoring the fact that another one exists. I want to thank you for the great explanations. But we're no closer to solving the underlying problem here, which as far as I'm concerned was addressed in the meantime.

[Phantom711]

vor 38 Minuten schrieb Frederf:

I don't believe Q-anything is spoken operationally but at least the FAA/ICAO controllers have to know Q-codes and they should pop up in the instructional material for pilots at the ATP level. They understand the concepts well enough.

QNH is being reported as such in the weather report via ATIS or verbally from a controller.