Transition altitude – the higher the better?
On 06-06-2011, in Buzzwords explained, by steve
Altimetry primer
Although I know full well that no pilot or air traffic controller needs a reminder of what Transition Altitude or Transition Level means, I will recap quickly for the benefit of those readers not directly concerned with aircraft altimetry. When we fly, we determine the vertical distance from the surface of the Earth using an instrument called the barometric altimeter which measures the outside air pressure which drops with altitude and this can be displayed as a vertical distance from a datum point expressed in feet or meters.
Of course this is a measurement which can give widely different values at the same vertical position depending on the actual atmospheric pressure and the datum point at which the measurement starts. In order to account for the differences in atmospheric pressure, altimeters require that we dial in the pressure value we want it to use as the actual reference. Here we have two options: with an aircraft on the ground, we can dial in the actual pressure (identified by the old Q-code QFE) at the location of the aerodrome concerned resulting in the altimeter showing 0 feet, regardless of the elevation of the aerodrome. So at an aerodrome 1000 feet above sea level, the altimeter will still show 0 feet altitude. More commonly, the pressure value entered into the altimeter is that converted to mean sea level (QNH) resulting in the altimeter showing 1000 feet altitude when the aircraft is on the ground at this aerodrome.
In the past especially Russian and UK operators insisted on having the QFE value given to them on final because they used this to land, preferring an altimeter showing 0 when on the ground to one showing aerodrome elevation… With the advent of radio altimeters, which always show the vertical distance of the aircraft from the terrain below, everyone moved to the near exclusive use of QNH which does have certain advantages.
Before proceeding further, let’s agree the exact meaning of four terms: altitude, flight level, height, level and elevation.
Altitude is the vertical distance of a level, a point or an object considered as a point from mean sea level.
Flight level signifies a surface of constant atmospheric pressure which is related to a specific pressure datum, 1013.2 hectopaslcals and which is separated from other such surfaces by specific pressure intervals.
Height is the vertical distance of a level, a point or an object considered as a point from a specified datum.
Level is a generic term relating to the vertical position of an aircraft in flight and meaning variously, height, altitude or flight level.
Elevation finally is the vertical distance of a point or a level, on or affixed to the surface of the earth, measured from mean sea level.
I guess the above definitions, coming from ICAO, are enough to make one’s head spin but they are actually quite logical and they do provide a good framework for measuring your vertical position in the air.
But what does all this mean?
To get your altitude you set you altimeter to QNH (pressure converted to mean sea level) and it will show your vertical distance from mean sea level.
To get your height you set your altimeter to QFE (local pressure) and it will show your vertical distance from the reference datum, in this case from the aerodrome who’s QFE you are using.
If you set your altimeter to 1013.2 hectopascals, it will show Flight levels which only have a relative meaning in terms of the vertical distance from the ground but all aircraft in each other’s vicinity on the same Flight level will be on the same level compared to each other… important for en-route separation provision of course.
Level is thus just a generic term used (like in the sentence above) when there is no need to specifically indicate whether one means altitude or height or Flight level.
Finally, elevation is a term that is not connected to altimetry as such but which is still significant since it indicates things we want to avoid flying into or need to keep in mind when landing, such as aerodrome elevation (the highest point of the landing area).
Transition altitude, transition level and the transition layer
Generalizing a bit, we could say that flying at relatively low levels, we use altitudes and when higher up, we fly on flight levels. The important thing, apart from avoiding obstacles and terrain, is to ensure that aircraft constituting relevant traffic for each other use the same altimeter setting so that if one says I am at 3000 feet and the other says I am at 4000 feet, there should actually be something very close to 1000 feet between their levels. In other words, they must both have their altimeters set to the same QNH value. In case of Flight levels, they must both have their altimeters set to the standard value of 1013.2 hPa so that if one were of FL100 and the other at FL110, there would still be 1000 feet between them vertically.
But how do we get from the realms of feet and QNH to Flight levels and standard setting and vice versa?
Well, when climbing, reaching the so called transition altitude signals that it is time to change the altimeter to the standard setting. The transition altitude is after all the altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes and above it… well, you know..
Conversely, when descending, at some point we reach the transition level which is the lowest flight level available for use above the transition altitude. Descending further, it is time to set the altimeter to the local QNH value which will normally be communicated by ATC and is also available via other means (e.g. the ATIS).
It will be clear from the above that there is a kind of no-man’s land between the transition altitude and the transition level, the so called transition layer where no aircraft in level flight will be found bar exceptional circumstances.
Where do we find the transition altitude? In most of Europe it will be somewhere around 4000 feet with of course notable exceptions in the region of the Alps and other high mountains. The transition level is consequently at around FL 60 but this depends on the pressure and it may have to be set as high as FL 80 if the pressure is very low (to ensure the same relative distance from the transition altitude which depends on the local QNH of course).
In the United States the transition altitude is typically at 18,000 feet.
Is there a problem with the transition altitude?
Well, perhaps you will be surprised to hear that there have been numerous loss of separation incidents, some of them quite serious, in Europe in which altimeter setting errors were found to be major contributing factors.
While the concept and use of the transition altitude may sound simple and straightforward, there are two aspects of the transition altitude as implemented in Europe that harbor the potential for mistakes.
First of all, there is no uniform transition altitude in Europe. Clearly, States with high mountains will always have a different transition altitude from those with no mountains to speak of but even these latter have not come to any kind of agreement as to where the transition altitude should be. Such disparity and eventual uncertainty in the cockpit can easily lead to errors.
Then there is the process of changing the altimeter setting itself. With a transition altitude around 5000 feet, the change has to be made during a phase of the flight that involves high workload, another factor that brings with it the potential for errors.
Taking them together, those two aspects of the altimeter setting procedures do constitute a measurable risk to safety.
What is the solution?
The answer is relatively simple: raise the transition altitude to a higher level, agreed on a European scale, with exceptions only where high terrain makes the commonly agreed level inappropriate.
However, this is easier said than done.
The matter has been on the agenda of various international organizations at least since May 2000 when the Technical Director of the UK Guild of Air Pilots and Air Navigators first raised the subject at EUROCONTROL. A subsequent survey found that there was indeed a wide variation of transition altitudes in Europe and the variations extended even to considerable differences from the applicable ICAO provisions related to the establishment of transition altitudes.
It also became apparent that the specific local interpretation of the transition altitude was used at some places, especially in the military context, as a kind of airspace organization and “default” separation tool and hence any proposal to change it was being opposed not so much on the ground of the transition altitude as such but the impact the change would have on the airspace organization built around it.
As the years dragged on it became increasingly apparent that while raising the transition altitude to around 12,000 feet might be acceptable for some States, having a region-wide practically identical transition altitude was basically out of the question.
However, the nature of the problem is such that only the combination of both raising and harmonizing the transition altitude can bring the desired safety improvement.
New impetus
In the meantime various studies had shown that any raise up to and including 10,000 feet would yield basically no improvement while going to 18,000 feet (as used in the US) would in fact be the most effective solution. This is now also supported by IFALPA as the altitude best meeting the pilots’ needs.
Of course it also became clear that if non-uniform transition altitudes differing from each other by one or two thousand feet were a problem, non-uniform transition altitudes differing from each other by more than 10,000 feet would be an even bigger problem to safety.
While there were States who would have implemented 18,000 feet without delay, they had to hold back because of neighboring States who were reluctant to accept any change.
Getting fed up with the obfuscation, the UK and Ireland, taking advantage of their somewhat “isolated” airspace, decided to unilaterally implement 18,000 feet as the transition altitude starting in 2013.
More recently even the European Commission has joined the fray stating that they considered the time politically and technically ripe for putting an end to the transition altitude saga by implementing a safe solution meeting the industry’s requirements.
The question of transition altitude is on the agenda of most FABs also tough this is a double edged weapon since transition altitude harmonized on the FAB level represents only a half solution, as mentioned earlier.
A solution in sight finally?
All studies performed over the years delivered conclusive evidence that 18,000 feet is a good choice for transition level wherever terrain allows. While the 18,000 feet transition level in the US has not been without incidents, the level of safety demonstrated over there is also convincing. One should also keep in mind that the UK and Ireland must have performed all necessary analyses of the options before coming to their decision to go for 18,000 feet. It is also clear, well, even common sense, that not harmonizing the transition level would only perpetuate a situation that has been shown to be a safety risk.
With the matter once again on the agenda of the appropriate working groups and with the EC also lending its weight to push for a solution it is not unthinkable that a safe and efficient solution will finally be agreed and the matter can be put to rest once and for all.
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Chris says: Yes, Steve,
It should read HEIGHT, not ALTITUDE, you are right.
By the way, I personally like these graphs on the website, as they are very well made!
A very good and explicit way to explain the issue at hand!
I am in favor of 18000 feet, as this works quite well in Northern America.
Chris
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alexf says: Steve,
While I agree that a higher transition level would probably be a good idea, one should note that it isn’t entirely ‘cost free’. A higher TL means that more en route aircraft (though not many jets, they cruise well above 180) will have to continually reset their altimeters as they transit the various zones, a bore, even though they seem to accept it in the USA. The really big advantage, apart from the hoped for reduction in level busts, I suspect will be easier airspace design and more predictable flight paths of FMS STARs; currently having to accommodate the level variation near the terminal is a big constraint on practical flight paths.
Over 30 years ago British Airways was formed by a merger of BOAC and BEA. You’ve guessed it, the first used QNH for landing, the latter QFE. The then head of BA’s navigation, Ron Bridge, and I were tasked with coming up with a single world wide solution, Of course the clue was in the title, a world wide solution meant it had to be QNH as QFE was said not to be available everywhere. So that’s what we recommended. But in doing so, it dawned on us what a stupid argument it was, one that was entirely due to the lack of progress in altimeter design since the thing was invented. It should have been possible to get the thing to switch from FL, to Altitude, to Height by a touch of a button, the only inputs needed were field/threshold elevation and the pressure. This was impossible on purely barometric instruments, but trivial in an electro-mechanical device or an electronic display. Clearly everyone had been busier arguing than thinking. Of course the suggestion got nowhere because the answer was needed IMMEDIATElY.
Fast forward to the present, and we still haven’t altered the basic altimeter functions, but now is not the time to fiddle with it, we need to be much bolder. It is ridiculous that aviation in the 21st Century is so dependent on technology from the 14th, 18th and 20th; I refer to Magnetic heading, baro altimetry and VHF voice comms. The first should be replaced by true heading, which most modern aircraft calculate before adding variation from a table of dubious validity. Geo Height (from GNSS) should become the vertical reference, no need to worry about the datum and TL then! And unambiguous communication requires more than VHF voice to be safe.
That should be enough to be getting on with.
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Elmar says: Dear Steve,
Dear Mr.Alexf,
I’ve read the article with great interest as well as comment of Mr.Alexf. Excellent material to think over!
A while ago I read that IFALPA was supporting 10000 ft for TA in Europe and with very reasonable explanation why. It’s strange that IFALPA changed their mind.
GNSS is a wonderful tool but it will take some time to be used by all aircraft and it will be a complete system change with a new vertical reference. I’m afraid it will not happen tomorrow. Meanwhile why not move a little forward with common TA in Europe? Look at Ukraine: they have recently implemented TA 10000 ft for the whole of their country!
And finally, Mr.Alexf, why are you speaking of a “higher transition level” but not “higher transition altitude”? Is it TA that is common and fixed at 18000 in the USA? Some people believe that it is FL180 that is common TL for the whole of the USA. Can you please clarify? -
alexf says:
Dear Elmar,
I confess to some sloppy shorthand, using Transition Level instead of Altitude and 180 instead of 18000ft. I promise I do know the difference, TA is fixed, TL is variable.. but the arguments in favour of higher values for both are clear. I can’t comment on where IFALPA’s position is right now, I don’t know. I would just say this is yet another example of pilots’ interests being quite different from controllers’. For Pilots, harmonisation is highy desirable, for controllers local efficiency or convenience is what matters, the tesnion is obvious.
I fully accept that a transition to some form of GNSS as primary vertical measurement is not likely to happen soon, I just wanted to point out that some ‘traditional’ arguments in aviation, that go on for decades, producing really deep divisions, have solutions that lie outside the argument in question. I recall early in my career long discussions about how runway visibility should be reported and whether Slant Visual Range was needed and if so exactly how to do it. There were good arguments all around, but the practical solution that removed most of the sting from the debate and made it not worth pursuing was to certificate a reliable autoland system. I really do think we should be looking at the implications of geo-referenced altimetry, not least because for once, there is an obvious and widely available back up that doesn’t need to be funded by the tax payer! Is it even on the SESAR/NextGen agenda?
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To the best of my knowledge neither SESAR or NextGen are looking into geo-referenced altimetry.
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[...] upper limit, what happens there? Then there is also the below taken from the following website: Roger-Wilco | Transition altitude "Is there a problem with the transition altitude? Well, perhaps you will be surprised to hear [...]
There is an error on the figure showing the level relative to the aerodrome and runway QFE. The topmost sentence should of course read: “Height measured with reference to…”.