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In the US, the transition altitude is standardized at 18,000ft, however in Europe it is different for every airport.

Why is it not standardized like it is in the US? I could see situations where someone has their altimeter calibrated differently, and thus giving different altimeter readings, which would seem dangerous, especially for smaller GA aircraft that might not have TCAS?

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    $\begingroup$ It seems that there is US and Canada in one side, and "Europe and much of the rest of the world" one the other side. Quite strange given that feet is used because of the US influence $\endgroup$
    – Manu H
    Nov 30, 2016 at 14:51
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    $\begingroup$ For starters, US is a country, Europe is not. $\endgroup$
    – DeltaLima
    Nov 30, 2016 at 15:13
  • $\begingroup$ Can you give an example where non-standard transition altitude would be a problem? $\endgroup$ Nov 30, 2016 at 16:46
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    $\begingroup$ TA is not "different for every airport". Some countries have a common TA used throughout the country, others have defined areas within which a certain TA is used. $\endgroup$ Nov 30, 2016 at 18:41
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    $\begingroup$ @lightbord That generally won't happen, because TA's don't change any where near large airports. Even so, air traffic controllers know how to handle this, and will have procedures to ensure vertical separation in areas with different TA's. $\endgroup$ Nov 30, 2016 at 20:07

2 Answers 2

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You're right that having a common transition altitude is better- efforts are underway in Europe to set a common transition altitude. Multiple studies have been carried out by Eurocontrol and it has been noted that it is better to have a single transition altitude. For example, from an aircrew prespective:

The multiplicity of transition altitudes and the national rules and procedures make the European environment very complex. This can lead to a lack of altitude awareness and altimeter mis-settings and is operationally unsatisfactory.

and

Establishment of a common transition altitude has a clear safety benefit.

Similarly, from the ATC perspective:

The establishment of a common TA for ECAC States and the EUR Region is a fundamental element in achieving the goal of a unified sky and the safety policy of reducing risks to the greatest degree practicable.

The reason such an unsatisfactory state of affairs have persisted is that people have become used to that- the second document makes that point:

ATC providers, as is human nature, grow comfortable with what they are most familiar.

The main reason is that there is no requirement for any common transition altitude. The relevant ICAO document simply states:

2.1.2.1 A transition altitude shall normally be specified for each aerodrome by the State in which the aerodrome is located

and

2.1.2.3 As far as possible, a common transition altitude should be established: ...

As a common transition altitude is not exactly a requirement (in-spite of its advantages) different European states have historically established their own TAs, to be decided by the aerodromes or the regulatory authorities.

As they have become comfortable using it (and no major accidents have happened because of this), there is no serious move towards the establishment of a common transition altitude (through arguably, this is the case with much of the world- common transition altitude is not that common). In the UK, CAA has launched a consultation process to raise the transition altitude to 18,000 ft, which notes that,

... it has been agreed that the TA in the UK will be raised to 18,000ft.

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  • $\begingroup$ 'A common transition altitude is better' better than what, and for whom? I am not sure it is. $\endgroup$
    – DeltaLima
    Nov 30, 2016 at 16:00
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    $\begingroup$ @DeltaLima better for avoiding confusion among pilots and ATC. Say you're in an area where the TA is 7000ft, and moving into an area where it is 7500ft. You're at FL70, but due to a severe pressure your actual altitude is 7600ft. Now you report in "blahblah with you FL70", ATC is confused as they see you at FL75. $\endgroup$
    – jwenting
    Dec 1, 2016 at 10:01
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    $\begingroup$ @jwenting If you are FL070, ATC will not see you at FL075, but perhaps at 7500ft. But that is not my point. Take for example the Netherlands: maximum elevation is approximately 1000ft AMSL, the highest obstacle is approx. 1500ft AMSL. The transition height is 3000ft for IFR, 3500ft for VFR. The aircraft in your example would both be flying at standard pressure setting. If the transition altitude would be raised to 18 000ft, both aircraft may have had different settings if the pressure in the area's would differ. A higher TA would cause more workload and more potential for errors. $\endgroup$
    – DeltaLima
    Dec 1, 2016 at 11:16
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It is the pilot’s responsibility to know what the TA is where he is flying. Typically it is a fixed value for an entire FIR and/or country, which makes this relatively easy.

There is a buffer of at least 1000ft between TA and TL specifically to prevent the safety issue you describe (including allowable altimeter errors). Also, ATC’s terminology will tell you whether to set QNH or QNE, even if you didn’t bother to look up the TA as you should have. So, varying TAs simply isn’t a significant problem in practice.

While it would seem marginally easier for everyone to have a common TA, that disregards the reason why different TAs exist in the first place: terrain. A low, flat country can reap the benefits of a low TA, for both pilots and ATC, while a mountainous one can’t.

Also, not all of the “US” has a TA of 18kft. The US-managed Oceanic Control Areas, which aren’t that far offshore and include part of the Caribbean, have a TA as low as 6kft; there’s no reason for it to be higher over the oceans where there is no terrain to worry about—and often no available altimeter setting to safely fly below the TA, so the low TA allows a low TL and thus more controlled airspace for IFR flights.

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