18000' and FL180
 

Greetings,

This is mainly for my north american buddies, but the rest can still pitch in their .02 (insert currency) worth...

I always thought that TA and TL should be seperated by the appropriate minimum vertical seperation, but I keep seeing documents talking about TL FL180 and TA 18000'. How can those be the "same"? Is it because in the climb pilots change to 2992 passing 18000' (not 17000') and in descent change to altimeter setting through FL180?

Just wondering if it is technically correct to say TL FL180 and TA 18000' at the same time. Looking at some european procedures their TL/TAs are not the same with 1000' seperation between the 2. :suspect:

Well, they'd only be the same if the SL pressure was 29.92. I was always under the impression that the TA was a given, but that the TL will vary depending on the SL pressure.

Hi Smurf;

I have no idea what a TL and a TA are, but as far as ATC is concerned over here. It is FL180...

regards

Scott

Transition Layer=TL, Transition Altitude=TA.

Reynolds you are right. Scott, the Transition Altitude must be as the name suggests an altitude, so is it 17,000 ft?? Whatever the altitude is, it is the altitude noone is allowed to cruise at, due to the changing between 1013 and the area QNH (sorry I don't know inches only hectopascals, but same **** different bucket). Here it is 14,000 ft, so guys either cruise at 13,000 ft on local QNH, or at FL150 on 1013 (2992?). If the area QNH is extremely low then the transition layer is bigger and that affects what levels are available to be cruised at. I am assuming it is the same in the USA, just everything would be termed in inches instead of Hectopascals/Millibars.

A question I do have to everyone is why the world over we don't have a standardised transition altitude. Wouldn't that be a huge improvement to safety. We here have neighboring FIR's that operate with a different transition altitude, and when QNH is 994 as it is now, it makes like a 500ft difference depending on when and where the aircraft switches QNH, to abide by the new transition layer, which makes that 1,000ft standard look very small indeed.

There seem to be some state-to-state differences in the definition of transition level. Some states build in separation between the transition altitude and the transition level. Others, the UK for example, don't.

Scott, true but not quite...When it drops below 2992 FL180 is no more.

And right you are, we never use TA around here, but I'm just wondering how do pilots transition from altitude to FL and vice versa. Passing 17K? Passing FL180?

In Oz, A100 is the transition altitude.
FL110 is normal transition level.
Cruising in the transition layer (between TA & TL) is not allowed.
To retain the minimum of 1000 ft buffer, FL115 becomes TL when QNH is below 1013, FL120 when below 997 etc.

ANSA,

In the above instance, TL = Transition Level

Invictus

Thanks Invictus, I got that now. I would imagine that all around the world the concept is the same ie. The transition altitude is constant, and as the QNH drops the transition level goes up, to in effect maintain a minimum 1,000 ft separation between the transition altitude and level.ie. The transition layer will always remain a minimum 1,000 ft.

My problem is where, one FIR has a transition level of FL100 and abutting that is an FIR with a transition level of FL140, and the problem of aircraft switching from one to another at varying points along their route, and the very real possibility of nose to nose aircraft operating in the same area but on different QNH settings, as it is now, 19 hectopascals different. For example, one aircraft operating at 11,000 ft on 994 hpa and opposite direction traffic at FL100 on 1013 hpa. I think I have that the right way around. I think you understand my concern. My question again is, why isn't it a safety driven priority, to get all FIR's the world over working with the same transition level?

Ahhhhhh, I see what y'all are talking about. In the US we use FL180 as the transition or really 17,999. You are supposed to change the alt. to 2992 upon going through or reaching FL180. As to it not being there when the alt. goes below 2992. That isn't really true. FL180 can still be used, it just can't be used for separation with another aircraft at 170. You could still assign it for whatever reason as long as you weren't separating from someone else below at 170 or lower depending on the alt...

regards

Scott

I was taught (and this is in low-performance types) to set 1013 at the start of a climb which is to finish at a FL, and set QNH at the start of a descent which is to finish at an altitude (or QFE/height). Gives problems if you have a change of plan half way but it's one less thing to remember during the climb/descent and helps avoid busts. Even more important I guess if your climb rate is in the 1000's of fpm...

Tim

Tim, I get the impression that is what most pilots do, but I have seen this cause problems where the aircraft is cleared initially from a FL to an altitude, and then subsequently stopped off at a FL, 1,000ft above opposite direction traffic and the pilot has forgot to switch QNH back to 1013. As I said when there is a 20 hectopascal differential between area QNH and 1013, this can make a big difference to the altitude of the aircraft. Also causes problems when the aircraft is on climb through an altitude but has been cleared to a FL and you are trying to verify Mode C.

Alas not .... 11,000 on 994 = 11,570 on 1013. And 10,000 on 1013 = 9,430 on 994.

Ergo, no separation problem :)

The 'altitude' will always be recieved as a Flight Level. Some ATC systems will then convert this to an 'altitude' based on the QNH which has been set in the radar processing system.

At our ATC Centre, we are provided with a simple conversion factor on the sector briefing screens which we can then apply to either a displayed Flight Level or displayed Altitude if the pilot reports on the wrong datum. Our datum is always applied to a displayed Flight Level to give the altitude. Thus +3 Conversion Factor means we add 300' to the Flight Level value and we have the true 'altitude'. If we wanted the Flight Level value of an aircraft displaying an altitude then we would reverse the value, namely subtract 300' (or -3 Conversion Factor). It's not rocket science and everyone copes with making the calculation (if required) in a millisecond.

The UK altimeter setting rules are relatively simple. Cleared to a Flight Level, set Standard. Cleared to an altitude, set QNH. If ATC then have to stop you off at an intermediate level using the opposite datum, then it's up to them to ensure that the pilot has the correct datum reset.

I am fascinated to read the above comments from our Australian and American colleagues.
In the UK the Transition Level is the first available Flight Level above the Transition Altitude; this makes the Transition Layer anything from 1 foot to 499 feet, but never more than that.
The TA and TL are clearly not separated, and are in place merely to denote the points at which reference is made to either Flight Level or altitude.
Within controlled airspace at least, altitudes are not used above the TL,and Flight Levels are not used below the TA.

Manu gives a book answer but just in case it ever happens to a pilot reading this, a controller can ask the pilot to set any altimeter setting and fly at an appropraite indicated level. The aim of the game is to separate aircraft (either from each other or from terrain).

It may not be viewed as good practice but it does happen. This may mean that an aircraft is asked to fly at an altitude above TA. Theoretically, an aircraft below TA could be assigned a FL but whilst altitudes above TA are not uncommon, I don't think I've ever seen the latter.

There are many other factors to be considered but it can be done - inside or outside CAS!

Scott, are you allowed to use F180 when it drops below 2992? Up here we can't, ops manual says next one would be F190 and so on....

Hi Smurf;

Technically, the book states for separation purposes... So it is an assignable altitude, but isn't used...

regards

Scott

Spitoon, have to admit I never have been taught or seen what you say as happening. My experience in Oz and the Middle East has always been that below the transition level, all aircraft must always be on Area QNH, and above must always be on 1013. I knew that it was common practice for aircraft to switch to Area QNH when cleared to an altitude, but I thought that was a shortcut by the pilots that wasn't exactly Koshe. Since our radar system gives us Mode C based on the entered Area QNH into the system when aircraft are below the transition level, it will cause inaacuracies when separating between two aircraft, one that is correctly on Area QNH and the other that is incorrectly on 1013.

PPRUNE radar, ok, I got it the wrong way around, but we have had a reduction in separation occur here, purely because one aircraft switched to 1013 prior to leaving our FIR, due to the fact that 10,000ft in our FIR, becomes FL100 once crossed the boundary. Again I have to admit to never thinking that if you level off an aircraft to an intermediate FL after previously having cleared that aircraft to an altitude, that it is the ATC who has to ensure that the aircraft hasn't already switched to Area QNH. Since I was always led to believe that the aircraft should switch to or from 1013 on passing the transition level and not before, I never saw the need to ask the pilot which setting he was on.

As I said before, there seems to be a lot of different procedures operating throughout the world, and it would seem to make sense to me that all countries operated with the same rules and the same transition level. Then again, I would also think that everyone could operate with either hectopascals or inches but that's all too hard I guess.

Terrain!! Terrain!!
 

Is not the selection of the Transition Altitude governed to a certain extent by the height of the highest terrain in the relevant FIR.
ie The TA should be higher than the highest terrain . I think that this is the case in most places with a few obvious exceptions ...Tibet, Switzerland ??

ANSA, what I was descrbing wasn't taught at the college that I went to - only by others during OJT. I guess it's only likely to happen around an aerodrome rather than en-route (certainly in the UK). The most common reason in my experience is when the TA is low, say 3000 ft, and inbound vectored traffic is being separated from traffic that is holding at relatively low level. The easiest way to be sure of getting vertical separation is to put the inbound on QNH a little earlier than usual and use altitudes above the TA.

It's one of those things that I should probably explain more to the pilots but I generally don't have the time. Occasionally a pilot will query it but most don't - perhaps this is because in the UK we have different TAs in different areas - 6000 ft just down the road at London, for example - so the pilots are not unused to hearing the sort of altitude assignments that I might use.

Like I said, I wouldn't argue that it's good practice but I don't think there are any rules against it and it sometimes saves lots of mental gymnastics if some of your aircraft are flying close to TA for some reason.