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QNH, QFE, QNE and QFF explained! :)


Slothface

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I posted the below in another thread but thought it actually might be of benefit as a thread of its own to help explain things for those who are interested in the who, what where and why! Please bear in mind I'm no instructor and my way of explaining things might not be as clear or easy to understand as I'd hoped. But please read and enjoy. Apologies for inaccuracies. I'm quoting what I can remember from my own past studies. Also, apologies if this has been covered before. I haven't found a thread similar and maybe it will spark interest or resolve mis-understandings to a new wave of simmers. Enjoy!

 

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There is a lot of confusion for peeps as to what QNH, QFE, QNE and QFF settings all mean. Not sure if it's been explained before but I'll just do a quick info filler post for those new to altimeter pressure functions. Its a lot of info to take in but hopefully will ease the pain and I will summarise at the end.

 

History -These 'Q codes' originate from days way back when. Voice radio was hard to make out clearly at times, especially on HF frequencies. The reversion back to using morse code was then required to establish a clear method of communication. These Q codes were a set of 3 letters beginning with a Q. They were set up to transmit a sentence quickly with just morsing 3 letters. The Q originally was to signify a question. QRB = What is your distance. QRC = what is your true bearing etc.

 

As the codes developed they also incorporated statements such as QFE = Pressure at particular observation station (an airfield/port/oil rig/etc).

 

Just out of interest, Q codes reserved for aviation use are QAA–QNZ. There are reserved sets of codes for maritime and sets used by all services.

 

Q codes we use on a day to day basis in aviation relate to headings too, QDM, QDR, QTE, QFU and QUJ. They all have a standard meaning attached to them and allowed the old morse operators to transmit info far more quickly. I wont go into these in this thread... maybe another future thread if there is enough interest ;)

 

 

So, history lesson out the way... what do the pressure setting Q codes actually mean?

 

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QNH = The pressure measured at station then reduced down to mean sea level pressure. When set on your altimeter it will read your ALTITUDE. Sat on the tarmac at your airfield the altimeter will display the airfields elevation above mean sea level.

 

This is the most commonly used pressure setting in the commercial world. Its probably the most useful setting to have, as nearly all aviation references to elevation are in relation to mean sea level. The mountain peaks on a map, airfield elevation, target elevation, minimum safe altitudes enroute etc. Incidently, QNH is given as a regional pressure setting and should be updated with new ones if you leave its area of reference into a new QNH pressure region. The QNH is the LOWEST FORECAST pressure at mean sea level for a given day to ensure that safe terrain seperation is maintained regardless of the days variation in pressure.

 

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QFE = Is mean sea level pressure corrected for temperature, adjusted for a specific site or datum like an airfield, being the most obvious example. When this is set on your altimeter, it will read your HEIGHT not altitude. It will read zero at airfield elevation and after take off will read your HEIGHT above that specific airfield. If you fly to another airfield of different elevation and/or different QFE pressure, you will have to ensure you reset that particular airfields QFE if you want your altimeter to read zero on touchdown.

 

QFE is very good for new pilots who are remaining in the circuit around an airfield and keeps things simple for that task.

 

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QFE Example: Airfield A with elevation 250ft above mean sea level. Airfield B elevation 300ft AMSL. A to B = 10miles. Assuming a uniform atmospheric pressure in the region.

 

Take off from A, altimeter reads 0ft on runway and after take-off reads HEIGHT above airfield A. Go and land at B and your altimeter will read 50ft on the runway. This is because B's HEIGHT is 50ft higher then A.

 

In this example, if we set the regional QNH, then the altimeter will read ALTITUDE and therefore the airfields altitude AMSL. Airfield A, altimeter will read 250ft. Airfield B will read 300ft. This is why QNH is the primary pressure setting used in aviation at lower levels. It is far simpler working in a setting that gives ALTITUDE, so you can reference your vertical position from everything on a map or chart. (All airfield plates (charts) have their altitudes AMSL on the plate.)

 

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This is all good and well knowing that QNH is the best pressure setting to use in a region for vertical situational awareness. But it is not always possible to get the regional pressure setting QNH from accurate means and a reliable network of meteo stations. Remote airfields and isolated combat zones are just 2 examples where it'd be difficult to get an accurate QNH when you dont have access to good forecasts and numerous pressure sensing stations.

 

If pressure info isn't available then you can get QFE easily by selecting an altimeter setting that reads zero on the airfield. The number in the altimeter pressure window is your QFE.

 

To get QNH, you just need to know your elevation AMSL and set that in your altimeter. Airfield elevation = 250ft. Set altimeter to read 250ft. Pressure in the altimeter pressure window shows your QNH. (You have to remember that this wont be the lowest forecast QNH pressure for the day and just be cautious at low level. But thats why a radio altimeter is handy!)

 

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There are 2 other Q codes used for aviation pressure settings

 

QNE = the Internation Standard Atmosphere (ISA). It is the average mean sea level pressure around the globe. It is planet earths mean atmospheric pressure at sea level basically. This pressure setting is refered to as STANDARD in aviation. STANDARD is set from QNH when climbing up through the "Transition Level". Your altimeter will then read your FLIGHT LEVEL. A reading of 25,000ft is FL250. 5,000ft = FL050. 13,500ft = FL135.

 

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The last one isn't really used in day to day aviation knowingly by pilots. To avoid info overload I have hidden it if you feel the above is alread a lot to digest ;)

 

 

 

QFF = the actual pressure at a measuring station, reduced to mean sea level. It in essence is the opposite to QFE where QFE is the MSL pressure adjusted for the height of the staion. QFF is what is used on meteorological synoptics charts with isobars.

 

 

 

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Summary

QNH = Altitude (AMSL)

QNE = Flight Level

QFE = Height (AGL)

QFF = Not used for altimeter settings

 

In reality, QNH and STANDARD are the 2 most commonly used pressure settings. The other 2 - QFE is good for very local aerial work such as circuits at a specific airfield but not much use for wide area flight. QFF is used in meteorological weather charts.

 

 

 

 

Hope this helps at least one person out there ;) Fly fast, take risks! Happy flying!

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Edited by Slothface
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Awesome read, thanks a ton.

 

One question, and excuse me if your explanation explains it, but maybe I need it in more simpler terms? lol

 

QNE = the Internation Standard Atmosphere (ISA). It is the average mean sea level pressure around the globe. It is planet earths mean atmospheric pressure at sea level basically. This pressure setting is refered to as STANDARD in aviation. STANDARD is set from QNH when climbing up through the "Transition Level". Your altimeter will then read your FLIGHT LEVEL. A reading of 25,000ft is FL250. 5,000ft = FL050. 13,500ft = FL135.

 

How do you set the QNE? What altimeter setting is it? Is it the 2992 setting once you hit 25k?

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Thanks a lot!

 

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Awesome read, thanks a ton.

 

One question, and excuse me if your explanation explains it, but maybe I need it in more simpler terms? lol

 

 

 

How do you set the QNE? What altimeter setting is it? Is it the 2992 setting once you hit 25k?

 

No such thing as a stupid question ;)

 

ISA for STANDARD/QNE is exactly this: 1 ISA atmosphere = 1.01325 bar.

 

In aviation we use millibars abbreviated as mb (which is exactly the same value as Hectopascals abbreviated as HPa). The US use inches of mercury (inHg)

 

1.01325 bars = 1013.25 Millibars (mb) (Hectopascals (HPa))

 

So STANDARD/QNE pressure setting is 1013mb on an alitimeter BUT...

 

As this sim aircraft is american, it will use inHg for pressure settings so...

 

ISA for STANDARD/QNE is exactly this: 1 ISA atmosphere = 29.92126 inches of Mercury. So STANDARD/QNE pressure setting is 29.92inHg on the A-10C alitimeter.

 

So basically Ryrrar, 2992 is correct! :)


Edited by Slothface

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Thanks for the positive feedback guys! :thumbup:

Sloth

 

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@ =4c=Nikola: Thanks for that! I didn't know that about Russian aviation.

 

Another interesting point I guess would be to point out the Rusky's and Chinamen of this world (basically all the Commies ;)), use metrics - METERS instead of FEET for their altimeter scales. I guess most of you who converted from the Black Shark would have noticed :)

Sloth

 

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Btw one lil nitpicky part about QNH...

 

QNH is not the pressure measured at sea. It's measured at (any given meteorological site) and by formula calculated back to sea level.

 

As I said... minor nitpicky thing but a well :)

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Indeed, If meteo station is at 1000ft, you cannot measure pressure at sea level because, obviously, it's underground :P

 

They should just dig a 1000 foot well and lower a pressure sensor down to the bottom. :D

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Nice, informative article.

 

 

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29.92hg @ 15c is a "standard day" in general aviation. You get that drilled into you and it will never leave you.

 

The distinction between QFE and QNH (other than field elevation correction) is essentially the difference between pressure altitude and density altitude, where density altitude is pressure altitude corrected for nonstandard temperature. Why worry about density altitude? Simple, it drastically affects your performance. Not only engine performance, but aerodynamic performance as well. So knowing both is important.

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PS.

 

Sort your avatar out!

 

;-)

 

Roger Boss! ;)

Sloth

 

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Btw one lil nitpicky part about QNH...

 

QNH is not the pressure measured at sea. It's measured at (any given meteorological site) and by formula calculated back to sea level.

 

As I said... minor nitpicky thing but a well :)

 

Thanks mate. I didn't actually specify it as that but have edited the QNH section to reflect your correct fat ;)

 

Let me know if you spot anything else or there is another QNH section I need to amend. Thanks again!

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  • 3 years later...

Good resurrection! :D

 

Seriously, though, why isn't this stickied? I figured out most of what Slothface has written, but this is the nicest summary on the topic I've read so far. This thread would have saved me from spending at least an hour on Wikipedia and here in the forum to figure it all out. :thumbup:

 

In case it's of help to anyone, there's a simple way to remember at least two of these Q-codes:

 

QFE: Field Elevation (Altimeter reads 0 at airfield)

QNH: Nautical Height (Altitimeter reads 0 at sea level)

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  • 2 years later...
In the case of the french Mirage 2000C where you can set HG and mbar pressure is the same story or you have to do something different?

 

Check the manual. What you're looking for can be found in "Chapter 9: Flight Instruments" and then "Altitude Indicator".

 

Use the barometric adjustment knob to set the desired QFE, QNH or QNE in millibars (mbar).

 

The DCS ATC only provides QFE, and I think it does so in inHG, but if you understand the original post, you'll know how to do the proper setting while still on the ground. :thumbup:

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What are your procedures with the altimeter settings?

 

For myself I usually just fly with QFE from my departure throughout the whole mission and all altitudes (and heights :music_whistling: ).

 

So do you set your altimeter to standard if climbing or descending through transition altitudes or levels? Or maybe even calculating the QNH yourself?

 

 

BTW does the altimeter setting does have any impact on weapons delivery? E.g. the cluster units?? Never experienced in DCS but maybe I.R.L. there is something to know about.


Edited by AnimalMother711

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Just to chime in...

 

QFE is only useful within about 10-15 miles out from the airfield the QFE relates to as it only gives you the height reference to the touchdown point on the runway. On departure you should switch to the Standard Altimeter Setting (SAS) of 1013hPa/29.92in.Hg/760mm.Hg when passing the transition height (= Transition Altitude - Airfield elevation) Transition altitude/height varies amongst countries, in the UK it's 3000ft in the US it's 18,000ft.

 

If you are not climbing that high to make the switch then you should switch to the RPS which is the lowest forecast QNH in a given Altimeter Setting Region. This will be normally worked out by the weather guys and will be part of the pre flight weather brief. In DCS this will be a single number based on what was selected in the mission editor. 29.92in.Hg is default.

 

QNH is used exactly the same as QFE the difference being that your altimeter will read a silly number on touchdown (yes i know it's airfield elevation but who wants to land at 397ft when you could just easily land at 0ft)

 

In DCS every airfield QNH will be the same due to the weather modelling.

 

QNE is NOT 1013hPa/29.92in.Hg/760mm.Hg, that is called the Standard Altimeter Setting (SAS), this is a very common misconception. QNE is actually a number in feet that the altimeter will display on touchdown with 1013hPa/29.92in.Hg/760mm.Hg set. It is rarely ever used (I've never seen it used before, although we came close once to the point we got all the documents ready). However it will only be used if the airfield QFE/QNH is above or below the pressure range available to the pilots altimeter (in the UK that is typically below 950hPa and above 1050hPa).

 

As the pilot cannot get the altimeter to read the QFE/QNH he will then just set SAS and the ATC will give him the number his altimeter will read when he touches down based on the difference between QFE/QNH and the SAS. This cannot be worked out in the air without knowledge of what the QFE/QNH is (if your trying to work out what it is with QNH you also need airfield elevation)

 

As an additional point military pilots tend to ALWAYS use QFE and never QNH, so in DCS there is no reason to even wonder about what the QNH is. There is also nothing stopping you from having the SAS set below the TA or QFE set above the TA providing it is under instruction from an authorised control station i.e. ATC/Naval C2/Airborne C2.

 

Another point worth noting is that in combat there will be a Force QNH which will be a pre mission agreed upon Altimeter setting that all pilots will fly on regardless of altitude, so no changing for the TA. Departures and arrivals will remain on QFE however, this just means that one persons Angels 30 is the same as another persons Angels 30.

 

EDIT: Spelling


Edited by Ventus_Clu
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As an additional point military pilots tend to ALWAYS use QFE and never QNH, so in DCS there is no reason to even wonder about what the QNH is. There is also nothing stopping you from having the SAS set below the TA or QFE set above the TA providing it is under instruction from an authorised control station i.e. ATC/Naval C2/Airborne C2.

 

True for the UK with our flat terrain around bases, low terrain elevation in general, and low TA/TL, but it's not universal. QNH use is very much normal in the US. And as has been stated already in other discussions, setting QNH before takeoff is a requirement in the A-10C as IFFCC uses it for calibration purposes (not modelled in DCS however).

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Just to chime in...

 

QFE is only useful within about 10-15 miles out from the airfield the QFE relates to as it only gives you the height reference to the touchdown point on the runway. On departure you should switch to the Standard Altimeter Setting (SAS) of 1013hPa/29.92in.Hg/760mm.Hg when passing the transition height (= Transition Altitude - Airfield elevation) Transition altitude/height varies amongst countries, in the UK it's 3000ft in the US it's 18,000ft.

 

If you are not climbing that high to make the switch then you should switch to the RPS which is the lowest forecast QNH in a given Altimeter Setting Region. This will be normally worked out by the weather guys and will be part of the pre flight weather brief. In DCS this will be a single number based on what was selected in the mission editor. 29.92in.Hg is default.

 

QNH is used exactly the same as QFE the difference being that your altimeter will read a silly number on touchdown (yes i know it's airfield elevation but who wants to land at 397ft when you could just easily land at 0ft)

 

In DCS every airfield QNH will be the same due to the weather modelling.

 

QNE is NOT 1013hPa/29.92in.Hg/760mm.Hg, that is called the Standard Altimeter Setting (SAS), this is a very common misconception. QNE is actually a number in feet that the altimeter will display on touchdown with 1013hPa/29.92in.Hg/760mm.Hg set. It is rarely ever used (I've never seen it used before, although we came close once to the point we got all the documents ready). However it will only be used if the airfield QFE/QNH is above or below the pressure range available to the pilots altimeter (in the UK that is typically below 950hPa and above 1050hPa).

 

As the pilot cannot get the altimeter to read the QFE/QNH he will then just set SAS and the ATC will give him the number his altimeter will read when he touches down based on the difference between QFE/QNH and the SAS. This cannot be worked out in the air without knowledge of what the QFE/QNH is (if your trying to work out what it is with QNH you also need airfield elevation)

 

As an additional point military pilots tend to ALWAYS use QFE and never QNH, so in DCS there is no reason to even wonder about what the QNH is. There is also nothing stopping you from having the SAS set below the TA or QFE set above the TA providing it is under instruction from an authorised control station i.e. ATC/Naval C2/Airborne C2.

 

Another point worth noting is that in combat there will be a Force QNH which will be a pre mission agreed upon Altimeter setting that all pilots will fly on regardless of altitude, so no changing for the TA. Departures and arrivals will remain on QFE however, this just means that one persons Angels 30 is the same as another persons Angels 30.

 

EDIT: Spelling

 

Just one point. Using QNH is very much a regional thing. Saying that all military pilots only use QFE is a blatant lie, since it very much depends on the country and operating procedures. Here in Finland QFE is never used except on airshows (to know the exact height above the runway for vertical maneuvers). Here the only 2 settings used are Standard and QNH even in military ops.

 

Otherwise a great explanatory post. Very well formulated.

 

Regards,

MikeMikeJuliet

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