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Transition Altitude and Transition Level

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Transition Altitude and Transition Level

Postby Turbo » Wed Feb 18, 2015 8:49 pm

I know you can select your Transition Altitude however I want to know if it possible to select the Transition Level instead of OR selecting it automatically. Also Transition Level should be higher than the Transition Altitude.
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Re: Transition Altitude and Transition Level

Postby Turbo » Wed Feb 18, 2015 8:56 pm

Here some information on transition altitude and transition level

The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of altitude to the use of flight levels. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level. Above the TA, the aircraft altimeter pressure setting is normally adjusted to the standard pressure setting of 1013 hectopascals (millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level.

The transition level (TL) is the lowest flight level above the transition altitude. When descending below the transition level, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the QNH for the region or airfield. Aircraft are not normally assigned to fly at the transition level as this does not guarantee separation from other traffic flying (on QNH) at the transition altitude; the lowest usable flight level is the transition level plus 500 ft.
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Re: Transition Altitude and Transition Level

Postby islandmonkey » Wed Feb 18, 2015 9:06 pm

Yes, that is possible -- at the top where it shows the transition level, simply double click on the figure and it will display a dialog where you can change it (EHRD is 3000ft?).
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Re: Transition Altitude and Transition Level

Postby mickybadia » Wed Feb 18, 2015 9:10 pm

(turbo's info copy-paste)

Indeed.
Which to me kind of answers your own request. The TL depends on basically two things only: a (selected) TA and the current QNH. It is not to be "selected".

@islandmonkey: what you can select is the TA, not the TL.
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Re: Transition Altitude and Transition Level

Postby Turbo » Wed Feb 18, 2015 9:13 pm

Yes see what you mean but at the moment if I select TA as 3000 its gives me a TL of 2500, So ya thats wrong. At least let me insert the right information.
Last edited by Turbo on Wed Feb 18, 2015 9:22 pm, edited 1 time in total.
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Re: Transition Altitude and Transition Level

Postby Turbo » Wed Feb 18, 2015 9:15 pm

and every region is different so at least let the user add the proper TL
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Re: Transition Altitude and Transition Level

Postby mickybadia » Wed Feb 18, 2015 9:34 pm

Again, the TL is only function of (TA, QNH). And that does NOT depend on your region. However, TA is decided locally (hence the possibility to change it), and QNH changes all the time.
With a very high pressure it is normal to have low FL. In what way is that wrong?
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Re: Transition Altitude and Transition Level

Postby Turbo » Wed Feb 18, 2015 9:43 pm

The transition level (TL) is the lowest flight level above the transition altitude. Thats means the TL should be higher than the TA, therefore currently OR is wrong as it giving me a TL lower than the TA. Also currently the system does not take in account the QNH it is always -500 feet bellow TA. and finally the example TL table given on the wiki is just example, each country operates with different set of numbers. Easy way to fix this is by allowing users add there own TL information.
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Re: Transition Altitude and Transition Level

Postby mickybadia » Thu Feb 19, 2015 9:31 am

Not sure how this can be made any clearer.

Turbo wrote in Wed Feb 18, 2015 9:43 pm:the TL should be higher than the TA

For sure, by definition.

Turbo wrote:therefore currently OR is wrong as it giving me a TL lower than the TA.

What makes you thing it is lower?
If QNH is around 1030 hPa and your TA is 3,000 ft, it is normal to have TL=FL025. And that is *above* TA.

Turbo wrote: Also currently the system does not take in account the QNH it is always -500 feet bellow TA.

If you mean "100*TL - TA is always -500", that is also correct if QNH does not change. With fixed QNH, that difference is always the same, regardless of regions, regulations, etc. For it to turn positive (if I understand well what bothers you reading the figures), QNH must be lower than standard pressure.

For what this forum and Wolfram is interested in, the question rather is: was the QNH that high or is there a problem in TL calculation?
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Re: Transition Altitude and Transition Level

Postby wagnerw » Thu Feb 19, 2015 5:12 pm

Currently, it is not possible to define the transition level. It is calculated out of TA and the air pressure at the airport. I take the TA, convert it to standard pressure altitude and take the next FL above it.

Sorry, if I repeat what has been said: As much as I know:
TL is important if you come from above, TA if you start. TL is the next flightlevel above TA.

So if you come from above, at TL you tune in the airport pressure and this might mean, that your real altitude is higher than the flight level number, or less depending on the air pressure around your aircraft. Important is, that if you change from FL to altitude at TL, the TA will be below your current altitude. This is because TL is above TA.

If you come from below, altimeter set to airport pressure, you will change to standard pressure at TA. This can mean that you seem to be lower than before, but the TL will be above you, as TL is above TA and you changed the altimeter reference pressure only. You will have to raise to reach the TL.

Nevertheless the TL number might be seemingly below the the TA. Measured in AGL TL stays above TA.

I have decided to implement it this way, mainly because you know the real altitude of the terrain around your airport. When aircrafts want to land, are in danger to crash into terrain, they should be tuned to real altitude and should have left the standard pressure behind them.
TA is a fix value independently from air pressure, whereas TL changes with it. Another argument is that I thought, you have other things to do, than to calculate the TL for each new weather report.
I might be wrong, but I am not convinced to change this, yet.
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Re: Transition Altitude and Transition Level

Postby simbambim » Sun Feb 22, 2015 9:13 pm

mickybadia wrote in Thu Feb 19, 2015 9:31 am:If QNH is around 1030 hPa and your TA is 3,000 ft, it is normal to have TL=FL025. And that is *above* TA.


How is that?
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Re: Transition Altitude and Transition Level

Postby wagnerw » Mon Feb 23, 2015 6:48 am

Hi,

The calculation follows the rule on this page: https://en.wikipedia.org/wiki/Pressure_altitude.

In short this formula:
Pressure Altitude (PA) = Hgt + 1000 x (29.92 - altimeter setting)

You see the difference between standard pressure and pressure around your aircraft ('altimeter setting' should be the ATIS value == airport pressure) can be negative, if there is a high pressure situation. In this case, the pressure altitude is lower than your real altitude.
In low pressure, the pressure altitude is higher than the real altitude.

In long, I use hPa and round to the next 500 ft upwards.

See https://en.wikipedia.org/wiki/Flight_le ... n_altitude there is a nice table showing the effect of changing the reference pressure.

Your FL calculation is done like this:

TL in feet = 3000ft + 30ft/hPa * (1013-1030)hPa = 2490ft => TL025

This formula is not exact enough though, I used the pilots rule of thumb, in fact, the height difference per hPa is close to the ground about 27 feet per hPa. So the result should be 2541ft, roundet up to TL030 as depicted in wikipedias table. (see http://www.experimentalaircraft.info/fl ... ance-3.php too)
I will change this to 27 ft, but, there is also an temperature influence, that is not included...
Regardless of that, the algorithm seems to be correct.

If they talk about "TL must be above TA" they mean, if you have changed at TL from 29.92 to airport pressure, the TA must be below you. And if you change at TA from airport pressure to standard pressure, the TL must be above you.

It took me quite a while to understand it. I hope that I was able to explain it. Sorry, if not...

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Re: Transition Altitude and Transition Level

Postby mickybadia » Mon Feb 23, 2015 9:26 am

simbambim wrote in Sun Feb 22, 2015 9:13 pm:
mickybadia wrote in Thu Feb 19, 2015 9:31 am:If QNH is around 1030 hPa and your TA is 3,000 ft, it is normal to have TL=FL025. And that is *above* TA.

How is that?


If FL figures and AMSL readings in feet were the same, we would probably not be defining stupid things like a transition layer and changing vocabulary between ft and FL. The first thing to understand is that they are two DIFFERENT paradigms, hence figures are NOT comparable between the two, and they are never to be mixed. So there, FL025 and FL030 can easily be compared, and so can 2500ft and 3000ft, but "3,000 ft" and "FL025" are just incomparable in the general case.

In the necessary basics before understanding each paradigm (separately): flight altitudes in aviation are NEVER geometric; altimeters are barometers graduated in feet and based on the standard hPa decrease with geometric altitude increase (WW gave you the math).
The actual reading will depend on the "altimeter setting", which is the local sea-level pressure (QNH) you set on your altimeter.

Now in summary:
- the altitude paradigm is one where you change your altimeter setting (QNH) for your altitude indication to read your actual altitude (useful near ground)
- the flight level paradigm is the other way around: you forget about your actual altitude for your altimeter setting to match a standard fixed value, and that is 1013 hPa (useful for en-route control as this makes all values comparable without depending on local pressure)

Hope this helps understanding alt's vs. FLs.
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Re: Transition Altitude and Transition Level

Postby Omega » Mon Feb 23, 2015 6:21 pm

Hello,

It seems like some people here are spreading misinformation, so I thought I should stop that from happening.
First of, Wikipedia sure is a trustworthy source of information for some things but NOT for aviation! Therefore, I will be using official resources to support my statements.
ICAO DOC 4444 PANS-ATM (pg. 4-9): wrote:4.10.2 Determination of the transition level
4.10.2.1 The appropriate ATS unit shall establish the
transition level to be used in the vicinity of the aerodrome(s)
concerned and, when relevant, the terminal control area
(TMA) concerned, for the appropriate period of time on the
basis of QNH (altimeter sub-scale setting to obtain elevation
when on the ground) reports and forecast mean sea level
pressure, if required.
4.10.2.2 The transition level shall be the lowest flight
level available for use above the transition altitude established
for the aerodrome(s) concerned. Where a common transition
altitude has been established for two or more aerodromes
which are so closely located as to require coordinated
procedures, the appropriate ATS units shall establish a
common transition level to be used at any given time in the
vicinity of the aerodrome and, when relevant, in the TMA
concerned.

skybrary.aero wrote:In Europe and much of the rest of the world, the transition altitude varies from airport to airport. Again, it is a fixed value and is published on the airport documentation, approach plates, AIP etc. The transition level will "float" with the QNH and, again, the TL is the lowest assignable level (pressure altitude) that will guarantee minimum vertical separation from an aircraft at the highest assignable altitude using local QNH.


Based on these resources, the TL is always above TA.
AT and BELOW the TA, aircraft are flown at ALTITUDES; AT and ABOVE the TL, we use FLs. The TL is the LOWEST ASSIGNABLE FL above the TA. If TA3000 and TL25 then the transition layer is only about 70ft thick, which would be unsafe. Therefore in places such as the Netherlands, the TA is 3000 and when the QNH becomes >1050, the TL remains as FL30 to ensure a sufficient transition layer (this contradicts the table on wikipedia).

Furthermore, the determination of TL is NOT based on a "global formula/table". Each ATS unit has their own ways of determining it, it varies from region to region and depends a lot on local procedures.
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Re: Transition Altitude and Transition Level

Postby mickybadia » Tue Feb 24, 2015 12:32 pm

Hello everybody,

To be honest this is mostly in line with what has been stated here, as far as the alt/FL difference is concerned. One piece I even like and quote below as I reckon it has pedagogical value for those asking to learn about it:

the transition altitude [...] is a fixed value and is published on the airport documentation [...]. The transition level will "float" with the QNH


The detail that does contradict the previous posts is basically summarised in:

the TL is the lowest assignable level (pressure altitude) that will guarantee minimum vertical separation from an aircraft at the highest assignable altitude using local QNH


At this point we need to become picky enough (that's fine) to reveal a difference between two meanings contained in a recurrent ambiguity:
1. lowest FL above TA
2. lowest FL above TA that will be assigned by ATC

The expression "lowest assignable level" can be ambiguous as it can be interpreted as either of those two, but the second part of skybrary's sentence does suggest the second reading indeed. Similarly, the other excerpt reads "the lowest flight level available for use above the transition altitude", which this time sounds semantically closer to (1), though what comes after quite indirectly suggests (2) must be interpreted sometimes in case of coordination.

In any case WW and I were both clearly answering with reading 1 in mind, which I still believe suited the original post for two reasons:
- reading 2 gives no way for the radar to report an altitude between TA and TL if both are set manually and wide apart, and I understand the case here was on software implementation rather than local ATC coordination
- incidently, original poster was wondering more about the figures that confused him than ATC assignments to be expected

Besides, I have myself written elsewhere: "The transition level is by definition the lowest flight level that is still above the transition altitude. This does not mean the lowest assignable FL, which may take more vertical separation." I realise that I have used the unfortunate adjective "assignable" as well. I will be rephrasing that and perhaps replacing "by definition" to leave rightly room for reading 2.
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