Jump to content

QFE vs. QNH


PE_Tigar

Recommended Posts

No your are right. The western countries use QNH which is the corrected pressure value for your altimeter to indicate the elevation above MSL(Mean Sea Level) at the corresponding airport.

 

QFE is just the value for what you get the indication of height above the reference point(airport).

Dont know if russians use QFE all the time, but normally QFE is only used if flying in the vicinity of your airport(for example glider Pilots that dont intent go fly cross-country).

 

For QNE its the definition of pressure at sea-level in the ISA (International Standard Atmosphere(29.92inch/hg or 1013.25hPa, and 15°C).

This value is ONLY used in cruise flight, above the transition altitude!(USA generally 18.000ft, in europe this value differs, since Im from germany I can give u examples: standard value 5.000ft but may be defined higher due to mountainous regions in the CHARTS. When flying above the transition altitude you call your heights in flight level( 10.000ft is FL100). When descending back below the transition level your turn your altimeter back to the QNH(You get this from either ATC or the ATIS(Automatic Terminal Information Service), the transition level is although stated in the ATIS for given by ATC/Charts, in germany its for standardvalues either FL60(QNH > 1013) or FL70 (QNH < 1013)

 

I hope I didnt just tell you what you allready know or jsut didnt want to know :)

 

Greetz


Edited by SoulJah
Link to comment
Share on other sites

Smokin Hole, I've got a CPL with IR, you don't have to explain to me which is which (BTW QNH is defined as local air pressure normalized to the MSL, but whatever).

 

SoulJah - thx - I guess it would be great if ED could fix ATC to give us QNH rather than QFE, would be more useful. BTW - transition altitude vs. transition level - transition alt is variable, given by ATC, in thousands of feet - when climbing you change to QNE at that value. Transition level - given in the chart, you switch to QNH from QNE at that level. So when I approach say, Batumi, and the ATC dude gives me QFE it's pretty much useless - QNH would be much better, I have radio altimeter for getting the height above the terrain.


Edited by PE_Tigar
  • Like 1
Link to comment
Share on other sites

Originally posted by one of our IP's - 74th_Sloth, this post will explain all you need to know. Regards

 

'T'

 

**********************************************************

 

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!

 

------------------------------------------------------------------------

 

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 wink.gif

 

 

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

 

------------------------------------------------------------------------

 

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.

 

------------------------------------------------------------------------

 

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.

 

------------------------------------------------------------------------

 

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.)

 

------------------------------------------------------------------------

 

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!)

 

------------------------------------------------------------------------

 

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.

 

------------------------------------------------------------------------

 

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 wink.gif

 

Spoiler:

 

 

 

 

------------------------------------------------------------------------

 

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 wink.gif Fly fast, take risks! Happy flying!

------------------------------------------------------------------------

  • Like 1

 

Come pay us a visit on YouTube - search for HELI SHED

Main Banner.PNG

Link to comment
Share on other sites

BTW - transition altitude vs. transition level - transition alt is variable, given by ATC, in thousands of feet - when climbing you change to QNE at that value. Transition level - given in the chart, you switch to QNH from QNE at that level.

 

 

May I ask in what country you have your CPL ?

 

I have my ATPL(A)(Frozen) in germany and in germany the transition altitude is given in the chart.(see annex)

 

The transition lvl is calculated with the actual pressure. Which makes sense I mean a QNH below 1013 mean when you switch from QNE to QNE your altitude decreases so you are respecitvly lower to the ground so you change at FL70 when you have a pressure above 1013 your altitude increases and u are safe to fly at FL60. But its possible its just germany we are doing rules that noone needs and udnerstands a fixed lvl of FL70 would allthough be safe enough.

SID.thumb.jpg.b5636364403664c60787245a9667f60f.jpg

Link to comment
Share on other sites

SoulJah - I stand corrected - it is as you've said it of course, I've mixed up the two. I got my CPL in the Czech Republic and it's exactly the same, TL is given in ATIS and seems that it's a practice in Europe. As ATIS in the US does not contain the info on TL, I'd guess that it's fixed at some value (like FL180?) the same way as the transition alt.


Edited by PE_Tigar
Link to comment
Share on other sites

Tyger - so if I understand correctly the calls we get from ATC on contact in DCS are actually what you get in real combat situation? If so I'll just shut up and fly :).

 

No, not at all. The A-10 requires you to set the altimeter to field elevation prior to takeoff in order to take a snapshot during the takeoff roll that calibrates the LASTE computer's altitude inputs. Setting the altimeter to QFE will completely bork the system.

 

The same is true in the target area. The anticipated altimeter setting at the target is included in the mission brief by the weather shop. Altimeter setting over/near the target can also be passed by other aircraft, or a ground party (JTAC/CCT/CWT).

"They've got us surrounded again - those poor bastards!" - Lt. Col. Creighton Abrams

Link to comment
Share on other sites

It´s the other way around.

In Portugal usually transition level varies with barometic pressure and it is given by ATC " Speedbird 315 descend to and maintain 3000´ inbound PG transition level 50 qnh 1015 expect NDB app rwy 35" (nevermind the values.)

 

Transition altitude is usually set by the charts :thumbup:

 

 

Smokin Hole, I've got a CPL with IR, you don't have to explain to me which is which (BTW QNH is defined as local air pressure normalized to the MSL, but whatever).

 

SoulJah - thx - I guess it would be great if ED could fix ATC to give us QNH rather than QFE, would be more useful. BTW - transition altitude vs. transition level - transition alt is variable, given by ATC, in thousands of feet - when climbing you change to QNE at that value. Transition level - given in the chart, you switch to QNH from QNE at that level. So when I approach say, Batumi, and the ATC dude gives me QFE it's pretty much useless - QNH would be much better, I have radio altimeter for getting the height above the terrain.

Link to comment
Share on other sites

No, not at all. The A-10 requires you to set the altimeter to field elevation prior to takeoff in order to take a snapshot during the takeoff roll that calibrates the LASTE computer's altitude inputs. Setting the altimeter to QFE will completely bork the system.

 

The same is true in the target area. The anticipated altimeter setting at the target is included in the mission brief by the weather shop. Altimeter setting over/near the target can also be passed by other aircraft, or a ground party (JTAC/CCT/CWT).

 

Is that why adjusting my altimeter in-flight causes my aircraft to pitch wildly for a moment?

[sIGPIC][/sIGPIC]

Link to comment
Share on other sites

Is that why adjusting my altimeter in-flight causes my aircraft to pitch wildly for a moment?

 

isn't it because your AP is set to hold a certain altittude, and changing your altimeter will tell the Ap ''hey, I'm not at the same pressure, so my altitude must've changed... I'll pitch the plane to set back the correct altitude'' ?

Do you think that getting 9 women pregnant will get you a baby in 1 month?

[sIGPIC][/sIGPIC]

Mobo: Asus P8P67 deluxe Monitor: Lg 22'' 1920*1080

CPU: i7 2600k@ 4.8Ghz +Zalman CNPS9900 max

Keyboard: Logitech G15

GPU:GTX 980 Strix Mouse: Sidewinder X8

PSU: Corsair TX750w Gaming Devices: Saytek X52, TrackIr5

RAM: Mushkin 2x4gb ddr3 9-9-9-24 @1600mhz

Case: 690 SSD: Intel X25m 80gb

 

Link to comment
Share on other sites

No, not at all. The A-10 requires you to set the altimeter to field elevation prior to takeoff in order to take a snapshot during the takeoff roll that calibrates the LASTE computer's altitude inputs. Setting the altimeter to QFE will completely bork the system.

 

The same is true in the target area. The anticipated altimeter setting at the target is included in the mission brief by the weather shop. Altimeter setting over/near the target can also be passed by other aircraft, or a ground party (JTAC/CCT/CWT).

 

Well that's what I get for not reading the manual (I guess this is in the manual, right?) - I mean here I am, wondering for the gazilionth time why the A/P is not working until some time into the flight, and here's why. Thanks! Another question I have in that case is what altimeter setting does the AWACS use for its calls?


Edited by PE_Tigar
Link to comment
Share on other sites

Still trying to wrap my head around this. I think we need to set the altimeter to airfield elevation in DCS. I think I got that. Now we're given target elevation information by JTAC or the briefing. Now In the case of dynamic weather we might have a different pressure setting over the target area. I haven't played around with JTAC much, but do we really need to readjust the altimeter at this point? I don't recall if JTAC gives us pressure over the target area so not sure if it's even modeled.

Link to comment
Share on other sites

Is that why adjusting my altimeter in-flight causes my aircraft to pitch wildly for a moment?

 

If the LAAP is engaged in any form of ALT HLD mode, then yes. It's maintaining the barometric altitude indicated by the altimeter at the time of engagement. Changing the reference pressure with the BARO knob will cause the LAAP to sense a delta between actual and desired altitude, and it will attempt to null the delta using elevator input.

"They've got us surrounded again - those poor bastards!" - Lt. Col. Creighton Abrams

Link to comment
Share on other sites

Still trying to wrap my head around this. I think we need to set the altimeter to airfield elevation in DCS. I think I got that. Now we're given target elevation information by JTAC or the briefing. Now In the case of dynamic weather we might have a different pressure setting over the target area. I haven't played around with JTAC much, but do we really need to readjust the altimeter at this point? I don't recall if JTAC gives us pressure over the target area so not sure if it's even modeled.

 

DCS ATC wants you to set the altimeter to read "0" when you're on the ground (QFE). In the real world, military jets don't like to do that. In the A-10, there are operational reasons why you shouldn't do it.

 

In the target area, you do the best you can with what you have. If the JTAC can give you altimeter setting (in DCS, he can't), use it. You could get it from another jet operating in the area, or you can use the forecast value generated by the meteorologists before you took off.

 

You want the altimeter setting to be as accurate as possible in order to give the LASTE computer the best data available for ballistics computations. When you leave the target and your main concern is not hitting other airplanes, go back to 29.92.

"They've got us surrounded again - those poor bastards!" - Lt. Col. Creighton Abrams

Link to comment
Share on other sites

Yikes, Fred! I know I typed "US" in there the first time I wrote that sentence. I must have deleted it by accident during editing.

 

I swear, sometimes I spend more time correcting my iPhone's "autocorrections" than i do composing my thoughts...

"They've got us surrounded again - those poor bastards!" - Lt. Col. Creighton Abrams

Link to comment
Share on other sites

I guess it would be great if ED could fix ATC to give us QNH rather than QFE, would be more useful.

 

It would depend on which side you are flying on. IIRC, the Russians use QFE most of the time, but yeah, it would be great.

Nice plane on that gun...

OS764 P930@4 MBUD3R M6GB G5870 SSDX25 CAntec1200 HTMHW

Link to comment
Share on other sites

It would depend on which side you are flying on. IIRC, the Russians use QFE most of the time, but yeah, it would be great.

 

Had a little chat with a friend who used to fly with both Warsaw Pact and NATO and here's what I learned:

 

- Russians use QFE, but so do Brits and the French, according to him (!)

 

- Germans and the US - QNH

 

- AWACS calls are on QNE

Link to comment
Share on other sites

DCS ATC wants you to set the altimeter to read "0" when you're on the ground (QFE). In the real world, military jets don't like to do that. In the A-10, there are operational reasons why you shouldn't do it.

 

In the target area, you do the best you can with what you have. If the JTAC can give you altimeter setting (in DCS, he can't), use it. You could get it from another jet operating in the area, or you can use the forecast value generated by the meteorologists before you took off.

 

You want the altimeter setting to be as accurate as possible in order to give the LASTE computer the best data available for ballistics computations. When you leave the target and your main concern is not hitting other airplanes, go back to 29.92.

 

Thanks - this is really helpful. BTW, when you use "automatic startup" in A-10C I'd assume that it sets the altimeter automatically? Because I've had that nagging autopilot issue for a long time, and others in my squad had not - but I'm the only dork in the squad insisting on not using the auto-start :).

Link to comment
Share on other sites

...BTW, when you use "automatic startup" in A-10C I'd assume that it sets the altimeter automatically?....

 

No it doesn't. The auto startup does the bare minimum required to start the aircraft up to a fully operational state.

 

Spoiler

Intel 13900K (5Ghz), 64Gb 6400Mhz, MSi RTX 3090, Schiit Modi/Magi DAC/AMP, ASUS PG43UQ, Hotas Warthog, RealSimulator FSSB3, 2x TM MFDs + DCS MFDs, MFG Crosswinds, Elgato Steamdeck XL

 

Link to comment
Share on other sites

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...