Do you know the difference between QNH and QFE?

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I know you know, but just in case:It is setting for Altimeters.....QNH is if you set your Altimeter with reference to Mean Sea Level.QFE is if Altimeter is set to Airport Elevation on which aircraft is going to Land.QNE is Altimeter setting to Standard value that is 29.92 in of Hg.Or:'Q' Codes/ Communication codes.To set the days pressure of aerodrome.QFE - Query flight elevation.Setting the prevailing days pressure in the altimeter to make it read zero while landing and take off.QNE - Query normal elevation.Setting the standard pressure at sea level of 1013 mb or 29.92 inch of Hg to make the altimeter read elevation of the respective aerodrome or airfield.QNH - Query normal height.Setting the pressure scale of the altimeter to read airfield height above the sea level.

Ok, Ill step in here with some info.First a bit of side info. The TA acorss europe varies, even within the uk there are 4 different values. There is at present work taking place to harmonise the TA across the whole of the UK, and eventually across the whole of the uk, with values like 10000, 14000 and 18000ft being discussed.Some parts of the world publish both a fixed TA and also a fixed TL other parts have a fixed TA and the TL varies dependant upon the local QNH at the airfield.In the US you would change from standard pressure to local qnh on passing the TL of 18000ft.In the UK and most(not all parts of europe) You set Local qnh when cleared to an altitude even if above the TL still, should for some reason you get stopped off before reaching the TL, the controller would reiterate to adjust again to a FL, or would compensate and stop you off at an altitude that remains seperated from ac at a FL(not ideal situation)The transponder is set to standard, and controllers radars are configured to the local QNH and it automatically calculates the TL, and thus displays the level/altitude readout based on this. it wouldnt matter what your altimeter was set to, the radar would still show your level based on the transponder, so may read strange, but would still be represented as a FL or altitude, in low pressure and High Pressure situations, youll actually see an aircraft descending level readout do a strange thing as it passes through the TL, so could actually show a climb, or in one sweep look like it just dropped 800ft.To put this into context why this is done and to scramble your brains a bit - take an airport in the UK with a TA of 6000ft(this is a fixed value regardless of pressure) the local QNH that day is 1033hpa that would give you a TL of FL55. So if you take into account some peoples views on here that you wouldnt change to local QNH until passing the TL then if you were cleared to descend to 6000ft to maintain - you would never set the local QNH as you never reach FL55 from a cockpit perspective. Now in essence if you descended to 6000 but on standard with a QNH of 1033 there would be no seperation issues as at FL60 as you would be youd be well above an aircraft at 5000ft and anything at FL70 you would be a 1000ft beneath, so there is in essence no safety risk - however you are now a lot higher. Look at the situation the other way round, you are departing on a sid which ends at 6000ft, same local QNH etc, in essence you climb to an altitude as thats what the SID is published at the TA is 6000ft, but the TL is FL55 so again peoples views here would suggest that you would set standard pressure on passing FL55 which is the TL, however that would now mean you are climbing to FL060 and not the published 6000ft!For low pressure situations the way ATC get around seperation issues, is they publish whats know as a Min stack level - so with low pressure FL070 woudnt be 1000ft away from 6000ft so the lowest level we would descend someone to would be FL080 or even FL90 if really low - to ensure separation from aircraft at the TA of 6000ft so say pressure was 980 although the TL is FL70 you would never be given descent to that level as you wouldnt be seperated from ac at 6000ft, so the stop of wouldnt happed, Id descend from FL080 to altitude 6000ft, if i cocked something up id need to stop you at 7000ft, but then anything at fl080 wouldnt be seperated.So to clarify - if your descent is given as an atltiude you set local QNH regardless of where you are in relation to the TL/TA - remeber you can always set the standby altimeter to an alternative value for reference.again regarless of the TL, climbing out if at or below the TA you will set local QNH, if given a climb to a FL you will then set standard, unless as part of the SID you are required to level of at an altitude, then you would set it on passing the TA - this is why the instruction "Climb NOW Fl090" was introduced the NOW means ignore any step restrictions on the SID and climb directly to the FL unrestricted and set Standard pressure at that point.QFE is hardly ever used at most airports, most circuit heights are published with reference to an altidude on the QNH and take into account the elevation, so circuit height wouldnt be 1000ftQNH if you had a airfield elevation of 600ft! some smaller airfields still use QFE, but anywhere bigger it causes more issues as you end up with aircraft on two different pressures and confuses radar controllers!(and panicks them cus they think something is lower than it should be)Confused!!!??? now you can see why the work is taking place to try and harmonise the application of TA/TL across europe! try being a radar controller and watching a height read out say 062, 060, 058, 055, 60 57 when you cleared something to descend!