Flight Level, Transition Layer and when to use it

[PompeyPaul]

Whilst doing all of the theory examinations I remember working on the transition layer. After 3000ft you changed to FL and switched the altimeter to 1013. There were also questions about whether FL30 existed on a particular day etc. All good fun.

However, thinking about it, I've not ever flown FL before. I put this down to the bad British weather, rarely having clouds higher than 3000ft. where I fly.

I was thinking though, when flying around the states at an altitude of 10,000ft and we still weren't on FL. I was then listening to the RT on United airlines today and coming into Heathrow we were at altitude 17,000ft., 5,000ft. The lowest FL I heard was FL250

So when do you switch to FL ? Or don't you ever ? Why did the books seem to suggest about 3000ft you switched to FL ?

[Keygrip]

Methinks this *might* have been better asked in the ATC forum - my guess is you would receive more professional answers.

Your call, leave it here and see what happens, or move it to ATC.

DO *NOT* duplicate it there. :E

[BackPacker]

Every country has its own transition altitude (TA) - and obviously the transition level (TL) is the lowest available FL above that.

In Europe, the most common TA seems to be 3000 feet. In the US/Canada, it's 18.000 feet. It's one of those things that you have to check in the AIP before you go flying somewhere.

And even the professionals get it wrong sometimes.

[mm_flynn]

So when do you switch to FL ? Or don't you ever ? Why did the books seem to suggest about 3000ft you switched to FL ?

Flight Level as a concept is more related to IFR than VFR flight.

Simplistically, once you are above the Transition altitude (which in many parts of the UK is 3000 ft - but in many parts of the South of England is higher) you must (IFR) or can (VFR) set 1013 as your reference pressure setting. This provides a simple way of insuring level separation of aircraft where the aircraft is sufficiently high that terrain separation is no longer an issue. (Although I believe there are areas of the UK where flying just above the transition altitude would not provide separation from terrain).

In the US it is much simpler, the transition level is 18000 feet and all airspace above 18000 feet (to about FL450 if I recall correctly) is Class A. This is why you are still on an altitude at 10000 feet in the US. IF your post is correct that a LHR inbound flight was cleared to 17000 feet, this seems wrong. I have never received a descent to an altitude above the Transition Level - always a descent to a FL. (If it was in the US, see previous point ;) )

[BackPacker]

you must (IFR) or can (VFR) set 1013 as your reference pressure setting

I would not consider setting 1013 optional, even if VFR. ATC will issue clearances and such, and will expect your altitude reports to be a FL instead of an altitude, once you are above the TA. Plus there's the quandrantical rule (UK) or semicircular rule (elsewhere) that requires you to fly VFR at specific FLs above 3000' AGL. If you fly a FL according to those rules, but have QNH still set on the altimeter you might be in for some serious surprises.

Not to say that I adjust my altimeter every time I go through the layer though. When I'm doing aerobatics training I typically request a block between "3000 feet and flight level 55" (because of the airspace above and below my usual training area). I leave the altimeter on QNH, and read the FLs off the transponder. It's a bit awkward to adjust the altimeter twice during a reverse half cuban...

[FREDAcheck]

Plus there's the quandrantical rule (UK) or semicircular rule (elsewhere) that requires you to fly VFR at specific FLs above 3000' AGL.
I don't think that's correct. The quadrantal rule (no "ic") applies to IFR. See Rules of the Air, Rule 34. Setting 1013 above transition level is optional for VFR flights outside CAS.

[mm_flynn]

I would not consider setting 1013 optional, even if VFR. ATC will issue clearances and such, and will expect your altitude reports to be a FL instead of an altitude, once you are above the TA. Plus there's the quandrantical rule (UK) or semicircular rule (elsewhere) that requires you to fly VFR at specific FLs above 3000' AGL.
The UK is slightly odd in that, for VFR, the quadrantal rule is recommended not mandatory in Class G (which is effectively the airspace we are talking about as most Class D is part of one of the designated CTZ/CTA/TMAs with higher TAs). When receiving a VFR ATC service, I always seem to need to switch to whatever they want (QNH, QFE, RPS) for them to not dump me - which makes sense.

[172driver]

Just to confuse you a bit more - outside the UK (I think pretty much everywhere), flying semicircular ALT/FL in the cruise is mandatory. No ifs or buts here, except a varying 'XXXX ft AGL' rule, IOW if you are below XXXX ft AGL, then no need to adhere to the semicircular rule. Also, you certainly set your altimeter to 1013 once above the TA. There are even places (Namibia being one), where ALL flying is done on 1013, except in a couple of TMAs. But I digress.....

[JW411]

Pompey Paul:

Let us try and keep this very simple.

The whole idea of having aircraft flying in controlled airspace separated by a common denominator (such as 1000 ft) is obviously predicated on all such aircraft having the same altimeter setting.

This setting is defined as 1013.25 mbs/hps or 29.92" in the USA and is known as QNE. Such a height when set is described as a Flight Level.

Now then, it has to be established, for very obvious reasons, at what height above the ground it is safe to set such a "false average ICAN standard" setting without making a nasty mess upon a nearby piece of cumulo granitus.

The height at which it is safe to change to 1013.25/29.92" is defined as the Transition Altitude.

How do we arrive at this figure?

It all has to do with the terrain around the airfield in question and the avoidance of collision with said terrain that would be possible before setting altimeters to the new "artificial setting". At the risk of being shot down in flames by the experts (I only went solo 51 years ago) the calculated Transition Altitude would be something like the sum of the highest ground within 25 nms plus 1500 ft plus 10% so that the usual UK airfield would come out to 3000 ft (or more in the frozen north).

So far, so good?

There are of course exceptions to this rule.

In the USA it was decided, many moons ago, that having different TAs for different airfields was probably too much for thick pilots to understand so they took Mt St Helens (before the earthquake), added 1500 ft plus 10% and so came out with a Transition Altitude of 18000 ft which will work whether you are flying out of Miami or Seattle.

Another exception is the London TMA.

So many locals and foreigners flying in and out of LHR, LGW, STN and LTN were failing to reset their altimeters at 3000 ft or thereabouts that it was decided to have a common TA of 6000 ft so that there was just a passing chance that everyone, regardless of race, colour or country of origin sitting in their whiz jets crammed full of whoosh going up at a rate of knots might just manage to get 1013.25/29.92" set before it became very important.

Now then, on the way down, we have to have a safe point at which we have to discard QNE and set QNH otherwise we could have a tangle with the hard bits on the way down having missed them on the way up.

The latest safe point at which to set QNH is known as the Transition Level.

The bit in between (and there has to be bit in between for obvious reasons) is called the Transition Layer.

Now then, our American friends coming off the pond in the early morning quite frequently respond to a descent clearance from London as "XXX Cleared descend 17000 ft". Our tried and trusted UK controllers will always remind them that they are cleared to "FL 170".

It is only when the clearance is to a height that is below the TL that the clearance would be, for example, "XXX cleared descend to 5000 ft QNH 998" would be given.

If you are really still interested, as soon as a clearance below TL is given by ATC, then it is acceptable to immediately set QNH upon the heightimeters.

[PompeyPaul]

That makes lots of sense.

Like I say, I never fly FL due to being VFR around Farnborough (not above 2,500ft mostly) and the weather.

I was just some what confused by the RT I heard. Although it was after a 12 hour flight so couldn't definately say there were clearences of 17,000ft there were certainly altitudes of 5,000ft being banded around. Your post explains why!

[modelman]

Just started my night qual and instructor said (IIRC) that night flight (UK at least) is considered IFR so above the transition level,the quad rule must be obeyed.With all the recent high pressure (1035Mb the other night),this can have a significant effect on the first available FL.
I can forsee that this could bring problems with respect to Class A and going into IMC (cloud:eek:).......
Will clarify that on next lesson.

MM

[Gertrude the Wombat]

Like I say, I never fly FL due to being VFR
Asked by ATC what level I was aiming at, VFR over East Anglia on a day with infinite visibility, I said "FL95" (instead of the "3000'" or so they were expecting), resulting in a giggle and a "remain clear of controlled airspace".

[Spitoon]

The international standards relating to Transition Altitude/Level leave a lot of scope for interpretation - hence many different arrangements. The rules for altimeter setting in the UK are described in the AIP - like many other things, other States have slightly differing rules. As I recall (without checking the good book) in simple terms, from an ATC perspective, on the way up, when cleared to a FL the pilot can set 1013 and make subsequent level reports as FLs, even if below Transition Altitude. On the way down, when cleared to an altitude, QNH may be set and level reports, even above Transition Level, may be made as altitudes. Some operators have SOPs that mean altimeter setting will be changed at different points in the flight. And terrain clearance is always a concern when down at levels where terrain clearance is not assured.

These generalisations really apply to IFR flights in controlled airspace. Other flights have to interpret the rules in a sensible manner appropriate to the flight profile.

My reading of the rules is that the quadrantal and semi-circular rules only apply to IFR flights.

And in the UK, night flight is not 'considered IFR' but must be conducted in compliance with the IFR. Rather fortunately, although the legislation is a bit convoluted, if the aircraft flies at or below 3,000 feet above mean sea level, at a speed of 140 knots IAS or less and remains clear of cloud, with the surface in sight and in a flight visibility of at least 5 km (yes, a bit like flying visually at night), and follows all the normal practices to do with ATC etc. it is pretty much in compliance with the IFR.

[172driver]

Spitoon et. al. just please be aware that the UK rules differ vastly from the rest of the world in many ways - the ones you describe are some of them.

[fireflybob]

It is only when the clearance is to a height that is below the TL that the clearance would be, for example, "XXX cleared descend to 5000 ft QNH 998" would be given.

If you are really still interested, as soon as a clearance below TL is given by ATC, then it is acceptable to immediately set QNH upon the heightimeters.

JW411, just one small correction to your erudite posting - for height read "altitude".

In aviation height and altitude are not the same thing - if you are flying on QFE the altimeter shows "height" if you are flying on QNH it shows "altitude"

[SNS3Guppy]

Transition is provided in some locations by ATIS or by ATC. It varies from day to day.

[BelArgUSA]

Will publish a 371 page book about the subject.
With my airline, had to lecture 3 hours on the subject for new hires.
And they all at least had a CPL/IR...
So for our friend PompeyPaul (I do not consider him one of the dummies).
And thinking of the UK/Europe environment.
Never flew VFR in UK, so excuse my lack of knowledge.
Do not know if UK does quadrantal or semi-circular altitudes/levels in VFR...
xxx
At low altitude, around airports, you will normally fly with a QNH.
Some guys flying touch and go do it QFE... Good idea for circuits.
xxx
If you climb higher, often above 3,000 ft QNH, you are passing the transition altitude.
Reset your altimeter to 1013.2 hPa (= QNE)... you are now in flight levels.
You should climb to (at least) the transition level. It is your lowest possible cruising level.
You cannot cruise from A to B in the transition layer airspace.
Transition layer is only for climb or descent.
And its "thickness" varies with high or low altimeter settings.
xxx
After a cruise, you descent through the transition level, say FL45.
Reset your altimeter to the local QNH for low altitude, and approach/landing.
xxx
Altimeter vocabulary -
Say "altitude" when you fly with an altimeter on QNH.
Say "height" when altimeter is on QFE.
Say "level" when cruising with altimeter in QNE.
xxx
Do not confuse yourself with the USA or or other procedures for metric airspace...
There are inches and millimeters of hPa to study about...
1013.2 hPa (former mB) = 29.92 inches = 760 millimeters.
That is off your subject.
xxx
One number for you to remember is 28.5 feet.
Every 1 hPa error in QNH setting will give you that 28.5 ft altitude error.
Many pilots round-it off to 30 feet.
xxx
:ok:
Happy contrails

[mm_flynn]

The height at which it is safe to change to 1013.25/29.92" is defined as the Transition Altitude.

How do we arrive at this figure?

It all has to do with the terrain around the airfield in question and the avoidance of collision with said terrain that would be possible before setting altimeters to the new "artificial setting". At the risk of being shot down in flames by the experts (I only went solo 51 years ago) the calculated Transition Altitude would be something like the sum of the highest ground within 25 nms plus 1500 ft plus 10% so that the usual UK airfield would come out to 3000 ft (or more in the frozen north).
The TA in significant parts of the UK provides less than 1500 feet of ground clearance and in 'mountains' it can provide negative ground clearance.

This is based on my understanding that the TA in the UK is 3000 feet unless it is specified as something else. The only specifications in the AIP are under a number of CTA/CTZ/TMAs - none of which are in the Welsh Mountains, the Lake District, the Highlands (the places where there is minimal or negative ground clearance at 3000ft.

[italianjon]

In the USA it was decided, many moons ago, that having different TAs for different airfields was probably too much for thick pilots to understand so they took Mt St Helens (before the earthquake), added 1500 ft plus 10% and so came out with a Transition Altitude of 18000 ft which will work whether you are flying out of Miami or Seattle.

Sadly, it appears that our beloved CAA may think the same is true for us... The TA will slowly be aligned to 6000ft across the whole UK at some point.

Personally I think this is a good idea, so my negative comment to the CAA should be taken very much tounge in cheek.

[PPRuNe Radar]

The UK CAA have been considering a common Transition Altitude since at least 1996, so it's nice to see them taking their time to get it right :ugh:

6000' would be a good altitude to pick as it is alread in place in some major pieces of airspace in the UK.

I was also taught that Standard Pressure Setting = 1013.2HPa or 29.92", whereas the term QNE was the reading on the altimeter on touchdown when the Standard Pressure Setting was used (sometimes called the Landing Altimeter Setting).

This would tie in with the Radiotelegraphy decode for QNE which is:

What indication will my altimeter give on landing at ... (place) at ... hours, my sub-scale being set to 1013.2 millibars (29.92 inches)?

[IO540]

Reading the above stuff does my head in :)

The answer must surely depend on whether one is trying to

1) answer an exam question (in which case I can't help because it will be heavily context and airspace/country dependent) or

2) is trying to fly safely.

On the 2nd one:

When outside controlled airspace, you can legally fly anywhere, at any height (subject to the 500ft/1000ft etc minimum rules etc) but fairly obviously you have to set the altimeter to either 1013mb (29.92") or the local airspace owner's QNH, according to whether the base of CAS is specified as feet or as a flight level.

If under an ATC service, and this includes flight in controlled airspace, you fly at a level/altitude and with an altimeter setting as negotiated with ATC.

Personally, when OCAS, I fly at "funny" levels e.g 3700ft because one is less likely to bump into somebody there. I would never fly at 2000ft or 3000ft for any length of time. But when approaching CAS and about to ask for a transit, then I sort myself out at one of these round numbers, to stop ATC wondering what the hell I am doing flying at 3700ft. So I might approach e.g. Solent doing my best to look good, at FL040.... only to have them grant me a transit at 4000ft on a QNH of XXXX :ugh: :) Like I say, you do whatever ATC would like you to do, and te rest of the time you do what you want to do.

Shall we start a thread on the minimum cloud separation, in different classes of airspace, in different countries, versus day/night? :) Oh and I nearly forgot... versus VFR/IFR flight.

[Spitoon]

172, I did try to stress that my answer was related to the UK, for the reason you mention. But I guess Pompy wants to get his head around the rules at home first.

[italianjon]

If you take PompeyPaul's post and answer the questions he asks, as he asks them... "So when do you switch to FL ? Or don't you ever ? Why did the books seem to suggest about 3000ft you switched to FL ?"

So when do you switch to FL?

Above the Transition Altitude (Whatever that is for the area you are in)

Or don't you ever ?

Depends where I am... I fly out of Popham, and London TMA and Solent TMA are 6000ft, so hardly ever fly FL as I don't fly higher than about 4000ft, and by using altitude I can ensure I remain OCAS. I do though if I am above 3000ft and crossing the channel, just be sure to call VFR on you initial to the French otherwise they always seem to assume IFR with a FL.

Why did the books seem to suggest about 3000ft you switched to FL ?

Because for most of the UK the TA is 3000ft. and normally outside CAS it is 3000ft unless a different TA is stated.... AND the less well known, (paraphrased) "Under Controlled airspace where the base is stated as an altitude" (E.g. under some of the Airways into and out of the London TMA whereby they are quoted at 5500ft. Would mean the TA is 6000ft - Otherwise you would be FL above 3000 and there would be no real way of ensuring you did not become a statisitic)