Hi,

I always forget what i should pay attention to during climb (TA or TL) and same for an approach ?

Some SID and approach/STAR charts have them both . Also when flying online on Vatsim its common to have a TL with the METAR /pre-clearance request.

Am i correct that when cleared to a first FL by ATC from the chart normally is during climb i switch to STD 1013. and opposite during descend/approach .First TA i switch back to local QNH ?

Michael Moe 

 

Edited August 5, 20196 yr by Michael Moe

The TA is used during climb. It is the altitude at which you change from QNH and altitudes to using 1013mb and flight levels. The TL is used during descent. It is the flight level at which you change to QNH, or whatever local reference pressure is in use.

Whether you switch to STD during climb when cleared to a flight level or when you climb above the TA depends on the regulations for the airspace you are flying in. As I recall from reading these forums, in the US you stay on QNH until you climb above the TA, in Europe you change to STD when cleared to a flight level. Similarly for descending below TL and being cleared to an altitude.

Just to add that TA and TL are not always the default 18000 feet in FSX. (If you use the 'b' key to set the altimeter then it changes to STD above 18k) 

The easiest way to remember is remember where you are at in the phase of flight.

If I'm climbing and passing "altitudes", then I reset my altimeter to standard (29.92Hg or 1013mb) at the "Transition Altitude".  If I am descending and passing "flight levels", then I set my altimeter to QNH passing the "Transition Level". 

In the US and Canada, we use a single TA & TL, which is 18000' MSL or FL180.  In other places TA and TL are separated.  But here's a hint.  They are they same (i.e. TA=5000 & TL = FL050) when the station pressure (QNH) is standard or higher than standard.  If station pressure (QNH) is lower than standard, the TA and TL will differ by usually by 1000'.  

For example, today Hamburg Germany (EDDH) has the following METAR:

METAR: EDDH 052020Z 22003KT 180V250 CAVOK 19/16 Q1009 NOSIG

Standard is Q1013, so EDDH is below standard atmosphere.  Transition Altitude published on the SIDs for EDDH is 5000'.  Transition Level on the STARs and on the approaches is published as "By ATC".  Under lower than standard atmospheric conditions (e.g., QNH < 1013mb), the lowest usable FL is FL060.  Any lower and an aircraft with an altimeter set to 1013mb and level at FL050 would have less than 1000' separation from an aircraft with an altimeter set to QNH of 1009 and level at 4000'.  So in the case, the lowest usable FL is 060 and ATC and/or the ATIS would provide the transition level as "FL060".   In the case of lower than standard QNH, you almost always plan on the Transition Level being one flight level higher than the published Transition Altitude.  If the pressure becomes real low, then ATC may have to use a 2000' difference. 

In the US and Canada, the air traffic procedures prohibit using FL180 when the local QNH is below standard, and depending on the difference, higher flight levels above FL180 are not allowed for use.   

Where Transition Level is charted as "By ATC", ATC will either assign it or it will be included in the ATIS.  Transition Altitude is always on the chart, or at least I have never seen one that's "By ATC".  Not to say that there's one out there. 

Hope this helps,

Rich Boll