QFE question

[marcus2us]

Ok, prior to take off ATC give you a QFE for example "2987" ok, I set my set my altititude to 2987 and rolling. after I take off and turning towards my my waypoint away from the base do I need to readjust my altitude for the mission? :helpsmilie:

[RodBorza]

Nope. Errr...well, I think you should, the problem is that you don't have this info and this is no airliner. Anyway, in the end, the only good thing QFE value is good for is for landings.

When you adjust the QFE value, in reality you are "trimming" the altimeter to the current airport zero setting. When you take off and come back to land at the same airport, there is no problem, but it starts to make a difference when you choose to divert to other airports.

[Frostiken]

<Q-code can of worms and arguing here>

 

 

:D

 

Honestly your best bet is to just leave it at 29.92 so it's standardized with everyone else. The particulars of what Q-code to use vary greatly depending on where you're flying, who you're flying with, and naturally they would all be on the same page.

 

I have oddly noticed that many of our F-15Es have their altimeters set to 2992 more often than not... but there are exceptions found. Just last week I noticed one was at 2880 or thereabouts...

[EtherealN]

If ATC gives you 2987 as an altitude, you shouldn't set your altitude to 2987. You should set your altimeters pressure setting to 2987, which will then cause you to show 0 feet altitude. (Q Field Elevation.)

 

At altitude, if you operate with aircraft from other fields, you should set altimeter to standard atmosphere - ie 2992. This way you'll all show the same and can deconflict easily.

 

If you have the information available, you can always set up your altimeter to a QFE for the target area, but this isn't as useful nowadays since the A-10C can take pretty precise elevation data from the DTS.

[EtherealN]

I personally prefer to operate on QNH as well, mainly because I'm used to that from RL. I use hPa IRL though and always feel sort of confused by the inHg - it's mostly a question of getting used to it though. IRL I fly entirely metric, so when I transitioned from russian birds to american in the simulators I was completely lost and couldn't really "feel" the numbers. That fixes itself after a while though.

[Ryrrar]

errrr what setting is QNH? :P 2992?

[rud]

not that I'm trying to make light of this discussion, but next time someone asks me "how complex is DCS A-10C?" I'm going to tell them "well it's been over 6 months and online we're still talking about the altimeter" :book:

[winz]

Nope, that's the QNE, standard pressure at sea level. The altimeter then reads altitude above sea level at standard pressure.

 

For QNH you need to get the actual pressure at sea level that you'll set on the altimeter from an external source (tower i.e.). But there is no such source available...all are reporting QFE

[WH_Boomer]

So you use one of the many airfield reference guides to get field elevation, ie. Tbilisi is 1528'

Then adjust altitude to match. Taadaa,, QNH.. Which is most commonly used by US pilots.

[BlueRidgeDx]

Is it possible to disrupt the bombing solution calculation by cranking the altimeter to some screw altitude? I know there's the EGI blending, baro/delta altitude source switch, etc. which normally means that the bombing calculation uses the actual height (instead of whatever the altimeter is set to). But isn't there some combination of switches and settings where the bomb calculation is entirely dependent on the altimeter?

 

Yes, in BARO mode, the barometric computations are based off of the altitude displayed on the altimeter at takeoff. If you crank the altimeter to QFE that will seriously degrade the weapons delivery capablity, because IFFCC ballistics will be based on an incorrect reference altitude.

 

The altimeter must be set to read Touchdown Zone elevation prior to takeoff in order to ensure an accurate starting point for altitude computations. Ideally, with QNH set, the altimeter should read within a few feet of field elevation, which for simulation purposes is "close enough".

 

Assuming you set the altimeter properly before takeoff, BARO is typically the best available altitude source because it uses GPS to constantly correct the altitude computation, assuming that the GPS the minimum required accuracy.

 

If the GPS has low accuracy, or is unavailable, then changing the altimeter setting while in BARO mode will result in erroneous computations and inaccurate weapons delivery.

[Slothface]

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. This 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 for all services use.

 

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

 

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

 

QNH = The pressure at mean sea level. 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 the 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 ;)

 

 

 

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.

 

 

 

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

 

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!

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

Edited June 29, 2011 by Slothface

[Frostiken]

Awesome, thanks Slothmug.

[Slothface]

Awesome, thanks Slothmug.

 

:thumbup: No probs Frosty.

[tangodownNZ]

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. This 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 for all services use.

 

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

 

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

 

QNH = The pressure at mean sea level. 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 the 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 ;)

 

 

 

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.

 

 

 

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

 

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!

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

 

This is very good. This is all anybody needs to read really. Thank you

[Nooch]

My guess is that QFE is only used when performing IFR landings. Is this correct?

[laumaya]

Thanks Slothface :thumbup:

[Flamin_Squirrel]

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.

 

Good explanation but this bit isn't quite right. ATC will report QNH in real time, not forecast. If you're talking to ATC at a specific airfield then they'll report QNH at that airfield at that current time.

 

You might be thinking of regional QNH. This is the lowest pressure reported by all the weather stations in that region (this is true for the UK at least). Again this will be updated in real time.

 

My guess is that QFE is only used when performing IFR landings. Is this correct?

 

It depends on the country and operator. The following may not hold 100% true in all cases, but civil UK ops use QFE for VFR operations in the circuit, QNH for everything else (including IFR flight/approaches). Approach plates reference altitude (to be used with QNH).

 

Military operators work differently; they use QFE in aerodrome airspace whether you're in the circuit or not. I'm not sure how this affects their IFR approaches though.

[Yurgon]

I think I actually never read this amazing post, Slothface. Would have cleared up a lot of things a lot sooner for me. ;) :thumbup:

 

I have one tiny remark, though. QNE isn't actually a pressure setting. QNE is a datum's altitude at Standard Pressure Setting.

 

In other words, if I set my altimeter to SPS (1013.25 hPa/29.92 inHg), and it shows 1500 feet on the runway, than that airport's QNE at that particular moment is 1500 feet. Basically, SPS and QNE are linked, but they're very much different, and if a pilot ever received a QNE from a tower (not that that's in any way common, though), it really makes all the difference in the world what to do with that information:

"Hawg 1-1, cleared for IFR approach Whatever 5, runway 24 left, QNE 1200 feet" would mean that on touchdown, the altimeter would read 1200 feet if it is set to SPS.

 

However, that begs an interesting question: since SPS means altitudes will be given as flight levels, would it need to be "FL 12" in the above example? I honestly don't know, but I'm pretty sure QNE is hardly ever used anyway. :smartass:

[Nooch]

It's pretty simple. If you are under 10k feet you use QNH (or QFE in some rare cases). If you go above 10k feet you switch to QNE. Please correct me if I'm wrong but as far as I know, this is how it works.

[Flamin_Squirrel]

It's pretty simple. If you are under 10k feet you use QNH (or QFE in some rare cases). If you go above 10k feet you switch to QNE. Please correct me if I'm wrong but as far as I know, this is how it works.

 

I'm afraid it's definitely not that simple. Transition between QNH and QNE varies from country to country for a start.

[Nooch]

I'm afraid it's definitely not that simple. Transition between QNH and QNE varies from country to country for a start.

 

That's what I suspected... Why make it simple and standardized after all...? :D

[Yurgon]

Again, it's not the transition between QNH and QNE. It's the transition between QNH and Standard Pressure Setting. QNE is not a pressure setting, it's an altitude.

 

The transition altitude (from QNH to SPS) does indeed depend on country and/or local regs. I believe the US use 18,000 feet as transition altitude.

 

The transition level (from SPS back to QNH) depends on the transition altitude and the actual QNH. Wikipedia has a nice table to calculate the transition level based on transition altitude and QNH.

[David OC]

I came accross this older investigation report the other day, interesting how easy you could make a mistake when distracted, even for two pilots in the cockpit. Incorrect configuration involving an Airbus A320

 

I'm sure this has happened a few times before, would be way more scary in very bad weather when you realise:cry:.

 

.

[DaveRindner]

Just set altimeter to zero prior to takeoff.

 

There is a very simple to handle QFE, assuming you are recovering at your departure field. After startup, and taxi to runway. As you seat at taxiway at runway entry, the tower clears you and gives you QFE setting. Set that on your altimeter, you know it is right if AGL needle is at zero. Really it is that simple. The intention , of course, is to make sure you safely recover, in IMC , without functional radar altimeter, without HUD, but with functional ILS, and to make visual recoveries safer.

In RW, QFE is set (in flight) prior to recovery ,at field that was not departure field, or every 100 miles or so. When set to a field, that field is at ZERO altitude, as far as pilot is concerned. With QFE properly set, and keptup with throughout flight, and functional nav, you should be able to fly through low altitude valley flight plan with low (but not zero) visibility , or at night without goggles. You just have to practice, and thank Lord that it is only a simulator.

[kylekatarn720]