Transition Altitude
 

Is anyone able to explain upon what basis transition altitudes are calculated? TA seems to vary from 3000ft amsl in the UK to around 17,500ft amsl in the US. Having used the UK TA in everyday calculations for several years now I would really like to be able to explain why it is set at the value it is...


Many thanks in advance for any insights!

From memory, it has much to do with the highest terrain in the country you're flying in. The transition level is set so that any aircraft on the standard 1013.25, flying at the minimum FL, will always miss terrain. The transition altitude is usually 2,000 ft below the TL so that any aircraft operating on local QNH, even if the QNH is at an historic extreme, will be assured of minimum ATC separation from aircraft above it flying at a flight level.

I stand to be corrected.

However Ben Nevis stands at around 4400' amsl, so whose bright idea was it to make TA 3000' outside CAS? It appears one still needs to be aware of the terrain and flying above TL is certainly no guarantee of terrain clearance.

When I did my PPL in the States I was told that the US Transition Altitude was based on the height of Mt McKinlay in Alaska which is 6,194m or 20,400 feet in old money. Not quite sure how the maths works. I live in Malaysia and the TAs change here all the time. I think the US approach is a lot simpler and less dangerous.

Doesn't tell all but PANSOPS has this to say:

Interesting study here recommending a single European TA (with some background to the current situation) Eurocontrol

Don't know how current any of this is!

Isn't it 18k in USA?

The transition level in the US is 180.

The highest point in the contiguous US is not in the Rockies, but rather, in California's Sierra Nevadas. It is Mt. Whitney which stands 14,505 ft. The highest point in the US overall is Mt. McKinley (Denali) in Alaska which reaches 20,320 ft (so much for THAT theory! :eek:). There are 10 peaks in Alaska which are taller than Mt. Whitney, and six of them are above 16k.

The highest point in the US Rockies comes in at number 14 in the US peak list, at 14,400 ft.

Dave

Isn't the TA fixed at 18k and the TL variable depending on the pressure of the day?

I knew I said that too quickly with too little elaboration...:O

Hello

I don't know whether this is on the same lines as what you were on about, but the transition altitude differs for airfields in the UK e.g. in London TMA 6000ft, yet regional airfields 3000ft, because of SID's and STAR's - the 6000ft transition altitude allows separation of tracks and levels between the aircraft on the SID's and STAR'. Thats my understanding of the differences in them. Does that make sense, even if it could be along different lines??

TA and TRL
 

The TA (transition altitude) is determined by the CAA in every country, normally the same for the whole country, to make it easier for everybody. Here is where you fly on QNH.
Then we have the TRL (transition level). The TRL is the lowest flightlevel that can be used above the TA. Here is where you fly on standard setting, 1013,25. There are two schools. One is the US method where you have a fixed TRL and 2000 feet difference between the TA and the TRL. Here you never have to calculate the TRL but you lose an altitude (level).
In Europe we have a maximum of 499 feet between TA and TRL. This is calculated depending on air pressure and temperature. There is no separation between TA and TRL. This difference is called transition layer. For separation purposes you consider TA and TRL the same level. E.g. TA is 5000 feet and TRL FL 55 then the lowest flightlevel you can descend to is FL 70 to have separation.
As a general rule you can say that the higher up you fly the lower the pressure is.
If the QNH is 1013 or lower then the TRL is 55 or higher ( if TA is 5000 feet) and when the QNH is 1014 or higher the TRL is 50 or lower.
So, if you have e.g. QNH 1040 then TRL is FL 45 or lower but you are still flying above the TA which is 5000 feet on the QNH.
I hope this makes it clear for everybody!

Going from memory.....I think it's 17,000 ft TA in the US and Canada, making FL180 usable if the pressure is 1013/29.92 or higher, FL190 for 28.92 up to 29.91, etc etc

hence the transition altitude in Scotland being 6000ft amsl?

I stand to be corrected though...

Only in the Scottish TMA and Aberdeen CTA, elsewhere it's 3000'ALT.

Flying into Lgw the other day, we were cleared to 7000' on the Lgw Qnh which in 10 years of flying from Lgw has never happened before. WE did query the clearance but the Atco reaffirmed it.

Can any TC controllers enlighten me. The Qnh was 1013 as it happened.

Cheers
mcdhu

mcdhu:

If the controller wanted to provide 1000ft separation against something flying at 6000ft on the QNH, then he/she could have cleared you to an altitude of 7000ft. The fact that the QNH happened to be 1013 makes no difference. A similar situation can arise in times of low pressure (i.e. QNH less than 1013) when the lowest available flight level which gives at least 1000ft separation against 6000ft might be FL80. But if you are climbing underneath traffic at FL 80 you can be allocated FL70. It is separated from the one above but not of course from anything at 6000ft underneath.

I used to have this argument regularly with people who used to say: Low pressure, so FL70 does not exist! Rubbish:hmm:

Calculating the TA and TL
 

The transition altitude and the Transition level are set by the respective state mainly keeping in mind operational constraints and the average mean fluctuation of pressure. As a rule in places where pressure does not vary that much TA and TL are fixed. Thats how I was thought in my Ab initio a loooooooong time ago!!

eyeinthesky, I can't see where the problem is - TL is easy enough to calculate, surely?

MATS Part 1, Section 1, Chapter 6, Para 5.2, states:

5.2 Transition Level
Transition level is the lowest Flight Level available for use above the transition
altitude. It is determined from the table in Appendix A ...

... and Appendix A can be found here, at page 321


JD
:)

Eye in the sky is correct -

Yes JD - the TL is easy to calculate - any ATCO or Pilot that can't needs shot

- however Mcdhu states they were flying into LGW... they (LGW) may have had had traffic descending and passing 6000' - telling McDhu to descend to 7000' Alt knowing full well that at some point in the very near future Mchdu would have been descending further - on the QNH - is emminently sensible.

Unfortunately we can only speculate as to the situation at the time.

As for low pressure and FL70 not existing, EyeInTheSky is again perfectly correct - FL70 is not seperated from 6000' departure from EGLL, but is is seperated from FL80 etc and it is perfectly acceptable and sensible practice to use it on low pressure days- provided that when using it you are aware of what is is and is not seperated from!

Those people that cannot work out TL in their head would probably be best just to plod along and stick to the silly notion that it isn't available though!

JD: Thanks, but I think I may not have put it clearly enough! I was not talking about the calculation of the TL.
What I was referring to was colleagues in ATC who would argue that, because the minimum stack level in the LTMA (i.e. the lowest FL which gives AT LEAST 1000ft separation from ALT 6000') was FL80, you couldn't use FL70at all. But if I need to provide separation from traffic at FL80, then FL70 was fine. It just didn't also provide separation from ALT 6000'.

If you consider a QNH of 1012, then FL70 is only approx 970 ft above 6000ft (the maximum SID altitude in the LTMA) so is not available for use in the holding stacks, thus FL80 became the minimum. That meant that actually there was 1970ft separation between traffic at 6000ft on the SID and traffic in the hold. If I wanted to keep departing traffic climbing as much as possible I could climb it to FL70 (1000ft below the holding traffic) provided there was no confliction with other traffic underneath at ALT 6000ft. Some ATCs could not understand that logic...:ugh:

Edited to say: Thanks anotherthing, I was busy typing when you posted your reply!