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Guest PittsburghIII

Very low QNH

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Guest PittsburghIII

Hi, A couple of days ago, Scandinavia was hit by an extreme low pressure area. In Copenhagen the QNH was as low as 973 mb (could have been lower, I only did random readings). I noticed on approach that as soon as I decended through the transition altitude and hence reset my altimeter to local pressure, that all of a sudden I was already lower than lowest cleared altitude. My question is, how are situations like this handled in real life? I assume that the pilots in this case switch to local pressure a bit earlier to prevent decending too low. Is that so?ThanksBoazEKCH

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If you look at the jeppesen charts or any wat charts you have, y'll see that there are two different Trans altitudes. You have: TRANS ALT and TRANS LVL. TRANS ALT is were you switch from STD to local press setting and for all airports it's different. (in US it's 18000ft, in Germany it's 5000ft and etc.) but TRANS LVL is on ATC discretion and on such a low QNH they proberly gona give you an Altitude were you have to switch between STD to local press.Hope it helps,http://forums.avsim.net/user_files/107454.jpg

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Guest PittsburghIII

Hi Ramon, Thanks for the answer. That makes sense. It made me go back and check the charts for the airport I was approaching (EKCH -- I checked the AIS package given by SLV), and I can't see any information that states that the transition level is by ATC discretion. I would assume that this kind of information would be given either by the controller or by ATIS.Thanks,BoazEKCH

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Boaz,Ramon is right. ATC gives you a TL that will correspond to the TA at that particular QNH. I know this is true for European airspace and I guess this is true for all airspace even US/AU. It's especially important with very high pressures as you won't get the buffer zone you'd want.Cheers,


Mats Johansson
PMDG Flight Test Dept
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Transition Altitude: Altitude where you change from QNH to QNE (1013.25) .. this is a fixed altitude and published in AIP (f.e. belgium TA is 4500 ft, US is 18000 ft)Transition Level: Level where you change from QNE to QNH, this will change according to local QNH as to maintain at least 500 ft between Transition Altitude and Transition Level .. this area of airspace is called the Transition Layer and is not available for level flight ..In brief, TA is used for departure, TL is used for arrivals .. you switch from QNE to QNH as soon as ATC clears you to an altitude below TL ..Altitude = height measured with reference to QNHFlight Level = height measured with reference to QNEhope this clears it out .. feel free to ask if you want to know more

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