Question on transition layer

[timzsta]

Is it possible for the transition layer to be greater then 500ft in height?

Example question:
QNH 1035. Transition altitude 3000ft. What is the vertical depth in feet of transition layer?

[Shunter]

Yes. Anywhere outside the UK where they've never heard of the quadrantal rule :)

[Spitoon]

You'll find that this can be a hugely contentious topic. It all depends on how you (or whoever decides what the T Level is) interprets 'first available FL'. It can be - as is - interpreted variously as the first FL ending in 0 or 5 that exists above the TA, the first FL ending in 0 or 5 that is at lest 500ft above the TA (a British thing in the main) or the first FL ending in 0 or 5 that is at least 1000ft above the TA.

In yet other places there is a TA and T Level set that, on a standard day, gives a T Layer of 2000ft or more. The idea of this is that you don't need to worry about changing the T Level unless the pressure is unusually low (did I get that the right way around???).

[timzsta]

My understanding is, well at least for the UK, that the Transition Level is the first flight level (ending 0 or 5) above the transition altitude. We don't fly at the transition level though - which is where I think some of the contention comes from. We do however fly at transition altitude - ie the SID's out of Heathrow where the TALT is 6000ft in the LTMA and the SID direct a climb to 6000ft.

The lowest usable flight level or "Minimum Flight Level" is the next one above the transition level.

In my example having reached 3000ft on QNH 1025 and then set 1013 - the altimiter (assuming 30ft/mb) will now read 2340 feet above the 1013 datum. Hence the transition level is FL25. The minimum available flight level would thus be FL30. Hence the depth of the layer is only 160 feet.

By making the minimum usable level FL30 if you fly at FL30 you will be seperated verticaly against somebody flying at the transition altitude.

[Spitoon]

tim, the problem is about what usable means. To me the lowest usable level is the first ending in a 5 or 0 above the TA - whether it is separated from traffic below the TA is a different issue. Some units consider that the lowest usable level is the first that guarantees separation from traffic at an altitude.

Certainly wherever I have done approach there has always been a TV showing, amongst other things, the Transition Level. As I recall, at one unit this was the lowest FL that existed whilst at all the others it was the lowest that provided separation from altitudes. The latter is convenient and saves the controller from having to think too hard about what FL he or she can allocate but loses a level that might be available. At my last unit the TV showed both the first available FL and the FL that guaranteed separation from alts - not a difficult calculation I grant you but when it was busy and thinking time was in short supply it was very useful to have it!

We don't fly at the transition level though...There are no rules about not using the lowest FL that exists - all the book says is that you don't cruise in the Transition Layer. From an ATC perspective there's no reason not to use the lowest FL as long as all the aeroplanes are separated.

In your example (I think you let a typo slip in second time around and I'm assuming a QNH of 1035), I agree that the T Layer is only 160 ft but I have no problem instructing an aircraft to fly at FL25.

But if we take your proposal to allocate FL30 as a minimum we only get 660ft between TA and FL30 so, in most airspaces/rules, the two levels are not separated.

PS - If I've got this last bit wrong I'll blame it on the rather good red wine that I have next to me.