Am I correct in thinking that QNH represents current barometric pressure at mean sea level for the local area(hence indicated altitude is read as AMSL)? This, in turn, means that at sea level, the altimeter would indicate zero altitude(approximately, given that I can never be at MEAN sea level!)? Or, to take another example, (which I found in an article explaining Transition Altitude/Level), if QNH is 1043hPa, then my altimeter will detect 1013hPa ACTUAL barometric pressure at 900 feet AMSL approximately. If I now change QNH momentarily to 1013hPa, the altimeter will read 0 feet AMSL?Follow up question: At what QNH value will the altimeter display the correct airport elevation when the aircraft is on the ground at the airport in question? I notice that in FS, it is always 1013hPa, or 29.92 in of mercury.Summarizing: the barometric altimeter compares the actual outside air pressure with the pressure reference/index in the subscale. The difference, translated to altitude(30 feet increase in altitude approx for every 1hPa drop in pressure), is displayed on the altimeter. For this reason, if the pressure reading and subscale index are the same, the altimeter will read 0'. EDIT: Forgot to sign my name!, so....Thanks,Frank

"Am I correct in thinking that QNH represents current barometric pressure at mean sea level for the local area(hence indicated altitude is read as AMSL)? This, in turn, means that at sea level, the altimeter would indicate zero altitude(approximately, given that I can never be at MEAN sea level!)?"That

Thanks for your reply, Tord!Yes, my post is confusing, because my thinking is a bit confused at this point. I did change the example from the article(here: www.hoppie.nl/tatl/tatl.pdf). One of its main points is to explain the need for a Transition Layer between Transition Altitdue and Transition Level, to prevent loss of vertical separation between aircraft. An example(I'll try to be clearer than last time :-)): QNH = 983hPa, TA = 5000'. 1hPa = about 30' altitude.Aircraft A has passed the TA and is at FL 060. Aircraft B is just passing the TA, 5000' and resets his altimeter to 1013hPa, which now reads 5900'. Although you cannot directly compare altitudes and flight levels, it is obvious that there has been a dangerous loss of separation between the two aircraft! For this reason, it is necessary to have a cushion of at least 1000'(more, depending on QNH) to ensure that there is sufficient separation between aircraft at or near the TA, and aircraft at the lowest Flight Level.I'll stop there, because it is probably getting confusing again! :-)Thanks again for clarifying my understanding about how the altimeter works.BR,Frank

Well done, Frank - you seem to be grasping the basics of altimetry pretty well. There are even some aviation professionals who never do manage it!To picture what's going on, you need to understand that all a pressure altimeter does is indicate how far above (or below) whichever point you set it to zero you are. This "zero point" is known as the datum. If the datum is sea level (QNH) then the altimeter will show you your vertical distance above sea level - your altitude .If the datum is the aerodrome level (QFE) then the altimeter shows your vertical distance above the aerodrome - your height. You'll now see that although the terms "altitude" and "height" are often used interchageably in general conversation, they have very specific meanings in altimetry - altitude above sea level (amsl) but height above aerodrome level(aal).Finally, if your datum is the 1013.2 hPa/Mb pressure line, your altimeter will show your vertical distance from that pressure line - flight level.You're correct in saying that with a QNH of 1043 hPa, if you set 1013.2 as you passed approximately 900ft above sea level, your altimeter would read zero. This is because with 1013.2 set, your altimeter is showing flight levels and FL00 is at the 1013.2 pressure line just as 0ft on QNH is at sea level and 0ft on QFE is at aerodrome level. It works the other way too. If the QNH was 983hPa, and you changed to 1013.2 at 900ft amsl, your altimeter would show 1800ft (FL18) because the 1013.2 hPa pressure line would be approximately 900ft below sea level so you'd be 1800ft above it. All clear so far? :>)You're also correct about the problems of separation between the TA and the TL that you mention in your second post. In the example you give, with a QNH of 983, the Transition Level would actually be FL80 to avoid this very problem. If the QNH dropped even lower, you'd end up with a TL of FL90 - I'm sure, with the knowledge you have already, you can work mout what the pressure would have to be for that to happen.To use a TL of FL70 with a Transition Altitude of 6000ft, you'd need a QNH of 1014 hPa or greater to provide the 1000ft separation you're using. If the QNH is high enough, you could even have a Transition Altitude of 6000ft and a Transition Level of FL60 and still have a 1000ft or more between them! Can you work out what the QNH value would have to be to allow this?Thanks for posting on my favourite subject! :>))BestPete

Ah, good old Hoppie! I know the author, Jeroen, from the PS1 forum.Jeroen does a good job at explaining the differences so I won

Just to add to the excellent explanations on offer so far:-"Or, to take another example, (which I found in an article explaining Transition Altitude/Level), if QNH is 1043hPa, then my altimeter will detect 1013hPa ACTUAL barometric pressure at 900 feet AMSL approximately. If I now change QNH momentarily to 1013hPa, the altimeter will read 0 feet AMSL?"Not necessarily. Altimiters are calibrated to an ISA atmosphere. They assume a suface pressure of 1013.2millibars, roughly 27'/millibar (pilots assume 30'/millibar for mental calculations) and a suface temprature of 15 deg C with a reduction in air temprature of roughly 1.98 deg C / 1000'. As you know, the QNH is calculated to provide the airfield elevation on the ground at that airfield. If the atmosphere between the airfield and sea level is not ISA (could be warmer/colder than ISA or higher/lower pressure than 1013) then the alty will show less/more than zero MSL.Meterologist use QFF that assume zero at seal level then calculates up (the reverse of QNH).I assume you know about QFE, used in the UK. An altimiter set to an airfield's QFE shows zero at that airfield' level. You can approximately calculate the QFE by dividing the height of the aifield by 30. Then reduce your QNH by the result.Then there is QNE. QNE is the height at the threshold for an altimiter set to 1013 millibars. This is used in extremely high airfields where some altimiters do not have a low enough setting.When dealing with QNH and FL flying in mountainous regions it can be important to try and assses you TRUE ALTITUDE. If the air is colder than ISA your altimiter will always overread and visa-versa. The rule of thumb correction is calculated as: Indicated Altitude - (Indicated Altitude * 0.04 * ISA Temp Dev/10)One final thing is that the Transition Level does not change because of pressure variations. What changes is the minimum flight level. Not as pedantic as it sounds but in the UK outside CAS, the TA is 3000'. IFR, you must fly to a flight level (1013 on the alty). VFR it is optional. If the QNH is 1023 with a TA of 3000' then there is 10 millibar difference between the QNH and FL. 10*30 = 300' so the instead of the minimum FL being FL35 (as it would in ISA conditions), it would be FL40 as FL35+300' = FL38 which isn't a valid level so then next lowest available is FL40. Of course that is further compounded by the minimum level by your track which will have its own minimum FL according to the Semi-circular and Quadrantal rules....but I digress :)Probably more information than you were asking for, but I quite enjoy waffling on so there you go :DCheers,Kef.P.S. Also, it is inconcievable that any professional pilot does not understand altimitry! (sorry, I couldn't let that one go!) :)

>>P.S. Also, it is inconcievable that any professional pilot does not understand altimitry! (sorry, I couldn't let that one go!) :) Oh, and by the way, I never mentioned the word "pilots". ;-)PP

Damn...I should have come to this forum when I was training for my PPL :)

"One final thing is that the Transition Level does not change because of pressure variations. What changes is the minimum flight level."English isn

Hi Tord,You are absolutely correct! The TL will change according to the minimum FL and my previous explanation is lacking. Further explanation is required: What won't change is the Transition Altitude. The Transition Altitude is the altitude below which, in the climb, the pilot will report his height according to the QNH and above which he will report his height according to FL. However, generally the pilot will just set 1013 as he starts the climb and leave the second altimiter on QNH (if he has two). If there is a large pressure differential then his level may well be much lower than his QNH but the pilot will already have determined the minimum flight level at which he could level out.On the way down, in the descent, the minimum Transition Level comes into play. That is the height with the alty set to 1013 that the pilot must change over to QNH and below which he much report his height in QNH. Generally, the pilot will set the QNH as they start their descent from cruise altitude...although this isn't a rules.For example, East Midlands has a TA of 4000' and a TL as directed by ATC. The Open FIR in the UK has a TA of 3000' and a TL determined by the pilot.In summary, you need to know the TA on the up and the TL on the way down!Clear as mud probably!Cheers,Kef.

Thanks, now I

No, FAR in US require adjusting the pressure altimeter from local barometric pressure to 1013.2 millibars (29.92" Hg) as you climb past 18,000 indicated altitude, and to switch from 29.92 to local barometric pressure as you descend through 18,000' indicated altitude.

Pete (and Gentlemen),(yes, of course, Pete, you're a gentleman, too :-)),Thank you all for the explanations, corrections, and further clarifications. Although I often teeter on the brink of information overload, I enthusiastically welcome all attempts to help me(and hopefully, others) better understand everything to do with aviation.Pete, QNH=1047hPa will do it, I think? At TA=6000', resetting the altimeter to STD will result in a reading of 4980' approx,((1047-1013) x 30). So, you will still have 1000' vertical separation from an aircraft at FL060.Thanks, everyone, for a most interesting thread.BR,Frank

I'm sure that's correct for the US, Rudi, but, clearly, this discussion is not about US procedures. As far as procedures in the UK go, both Kef and Tord are correct. The UK Manual of Air Traffic Services Part 1 contains the following:"NOTE 1 When an aircraft has been cleared to climb from an altitude to a flight level vertical position will be reported in terms of flight level unless intermediate altitude reports have been specifically requested by ATC. Similarly when a pilot is descending from a flight level to an altitude the pilot will change to the aerodrome QNH unless further flight level vacating reports have been requested by ATC, in which case the QNH will be set following the final flight level vacating report."The same information is also to be found in the UK AIP. To give some practical examples with a TA of 6000ft and a TL of FL70:At 4000ft, cleared to FL80 - set 1013.2 mB/hPa on leaving 4000ftAt 4000ft, cleared to FL80 to report passing 5000ft - set 1013.2 on passing 5000ft.At FL90, cleared to 4000ft, set QNH onleaving FL90At FL90, cleared to 4000ft to report passing FL70 - set QNH on passing FL70Pete