Altimeter setting procedure for FL/Altitude.

[pplmuppet]

Hi All,
I have what is seemingly a basic question but one which i never coverd during my flight training PPL, either in practive or Theory. It concerns the QNH/1013.2 mb setting procedure when going through the transition altitude.
However i have several questions relating to it. Please bear with me. And would be greatful for explanations.
1. where do you find out what the trans level/altitude is for the local area?
2. how exactly do you know what the layer is, ie thickness?
3. when do you change from QNH to STD 1013.2 and vice versa?
4. calc of MSA on a FL (dependant upon local QNH), more critical when QNH is lower than STD i assume.
5. So what happens when the next available FL is below MSA based on semi circular rule, (i am based in France and German and there is no quadrantal rule here).* Do you have to fly on the next highest FL? Or alternatively, if the MSA is high enough, then do you have to move to FL's, thence the complication of semi-circular rules and flying higher than necessary and potentially not seeing the ground if flying VFR?
thanks in advance.

[Capt Claret]

pplmuppet

I can only give an Australian perspective, not having operated in Europe or the US. In general terms I'd expect that the basics would be the same.

1. where do you find out what the trans level/altitude is for the local area?

The transition altitude is published on Jeppesen Approach charts, and would normally be found in the state's Aeronautical Information Publication (AIP). I would expect that the TA/TL would not change within a country.

2. how exactly do you know what the layer is, ie thickness?

In the Australian AIP, in the ATC section, under altimetry, is an explanation that:

Transition Altitude (TA) is 10,000' on QNH (Trans Alt does change from country to country)
Transition Level (TL) is FL110 (1013.2hPa)
Cruise flight is not permitted in the Transition Layer - that layer between the TA and the TL.
To retain a minimum 1000' buffer between TA & TL, when the underlying QNH is less than 1013, FL110 is not available for cruise flight. If the QNH is less than 997hPa FL115 is not available for cruise flight and if the QNH < 980hPa FL120 is not available for cruise flight.

3. when do you change from QNH to STD 1013.2 and vice versa?

Changing from QNH to STD Pressure should not normally occur before you reach the TA.

Changing from STD Press to QNH can occur once one is cleared to an altitude, as opposed to a FL. It is wise to consider leaving this change until approaching TL, incase your descent is interupted and you have to level off at a FL. This saves mad resetting of the altimeter setting, or forgetting to do so, and possibly compromising separation with another aircraft.

4. calc of MSA on a FL (dependant upon local QNH), more critical when QNH is lower than STD i assume.

With the highest MSA in the country, something like A080, I've never given this one any consideration.

5. So what happens when the next available FL is below MSA based on semi circular rule, (i am based in France and German and there is no quadrantal rule here).* Do you have to fly on the next highest FL? Or alternatively, if the MSA is high enough, then do you have to move to FL's, thence the complication of semi-circular rules and flying higher than necessary and potentially not seeing the ground if flying VFR?

I would guess you would cruise at MSA if allowed, or go to the FL closest to, BUT above the MSA.

[Farmer 1]

1. where do you find out what the trans level/altitude is for the local area?
If you don’t have the charts, ask ATC. Transition Level is dependent on the pressure, so that will vary. It is the first available Flight Level above the Transition Altitude.

2. how exactly do you know what the layer is, ie thickness?
I don’t know why you feel you need to know this, but it will always have a maximum depth, depending on whether you are using quadrantal or semicircular levels, i.e. 500 or 1,000 ft. I suspect that statement might be a light-the-blue-touchpaper job. Each pilot seems to have his own strongly-held views on that particular subject, but that is how I understand the U.K. situation.

3. when do you change from QNH to STD 1013.2 and vice versa?
In the U.K. I learned that I could change to Standard within 1,500 ft before reaching the Transition Altitude, and to QNH at any time during the descent, once cleared to an altitude. Capt Claret’s advice seems most wise if you start from a high cruising level.

4. calc of MSA on a FL (dependant upon local QNH), more critical when QNH is lower than STD i assume.
If you have only one altimeter, you need to calculate and remember the correction to apply from one setting to the other. With two gauges, you have both altitude and flight level readouts, of course. (How are you at sucking eggs.)

5. So what happens when the next available FL is below MSA based on semi circular rule, (i am based in France and German and there is no quadrantal rule here).* Do you have to fly on the next highest FL? Or alternatively, if the MSA is high enough, then do you have to move to FL's, thence the complication of semi-circular rules and flying higher than necessary and potentially not seeing the ground if flying VFR?
Can you have a FL lower than the MSA? If you are VFR, you are not governed by Flight Levels, so the world is your lobster. If you are IFR, then you must be above the MSA, and if that means you are above/in cloud, then so be it.

This subject always seemed to me to be far more complicated than it should be, and very easy to make a simple error. Remember, Flight Levels have nothing to do with the aircraft’s heading, but are dependent on magnetic track.

[DC-8]

The transition altitude is published on Jeppesen Approach charts, and would normally be found in the state's Aeronautical Information Publication (AIP). I would expect that the TA/TL would not change within a country

Just for curiosity. In Spain the TA is not the same for the whole country. For all aerodromes is 6000ft, but there's an exception, Granada (LEGR) where it is 7000ft. Do you know if this also happens in other countries or is it really rare?

[BOAC]

It is normal for TA to go up if there is a high MSA in the area - like Granada.

[pplmuppet]

Many thanks for the take on this. Especially the Altimeter setting procedure, in particular coming down to the TL.
However for extra background information, and why i thought it was necessary to 'worrie' about the MSA. It was my understanding that even VFR, MSA is still very relevant. I am doing my flying in the south of France, where the TA is 5000'. Which i believe is standard in France unless indicated on a chart, (regions of Cherbourg are 3000'). However, when i look at at Jepp chart LF-7 for example, it is filled with those minimum grid area altitudes? (I am assuming AMSL), so when i am flying in an area of 54, and the TA is 5000', tis si where the problem is. Ok i understand that in VFR and excellent VMC that there is 'nothing to worry about, just keep your eyes open', but fo me that is not sufficient. So in this case, the next FL is 7500 West or 6500 East mag heading. Only semi-circular in use in France.
To the question 'why is it that i wish to know the layer'? Ahhh well curiosity i suppose, also this way i can calculate what the next available FL would be for my MSA.
Then i have another question, how regularly do people fly VFR under the MSA? Obviously weather conditions depending?
Also i do know that the UK has varying TA's depending upon the Airfield, i believe, but don;t quote me, it varies from 3000' to 6000'.
thanks again for your help...now i am less confused...i think :ugh: :confused:

[JW411]

I've said this before but I will say it again. The Transition Altitude for a given airfield is a function of the terrain surrounding the said airfield. Obviously it has to be safe to change to 1013.25 at such a height so that it will not leave the aircraft concerned in any danger by flying on the Standard Setting.

Now if I were to use a "Very Ballpark Figure" (and I am being very simplistic) this would equate to the highest ground in the vicinity plus 10% plus 1500 feet.

Therefore in the UK most of the airfields would end up with a TA of 3000 ft unless they are situated in the hills.

The Transition Level will depend on the QNH but would ordinarly be some 500 ft to 1000 ft above TA. This ensures that the altimeter is reset to QNH from the "unreal" Standard Setting of 1013.25 before terrain avoidance becomes a problem.

Some countries like the USA have decided to have a nationwide standard TA. In the USA it is 18000 ft. This is probably Mount St Helens plus 1500 ft plus 10% and it means that from Florida to Washington State you are going to be safe.

In the UK the TA within the London TMA has been standardised to 6000 ft. This is a practical consideration and has little to do with terrain clearance.

The depth of the Transition Layer is of no practical value to you and is purely a function of the existing QNH.

When should I change from one setting to the other? In controlled airspace in the climb as soon as you are cleared to a Flight Level you can set 1013.25 on your altimeter. (Your Mode S transponder is already showing your height as a flight level).

In the descent, as soon as ATC clear you to an Altitude as opposed to a FL then you may set QNH.

I shall now don my flak jacket and retire from the scene!

[INLAK]

I
When should I change from one setting to the other? In controlled airspace in the climb as soon as you are cleared to a Flight Level you can set 1013.25 on your altimeter. (Your Mode S transponder is already showing your height as a flight level).
In the descent, as soon as ATC clear you to an Altitude as opposed to a FL then you may set QNH.
I shall now don my flak jacket and retire from the scene!

No flak from me!
I firmly believe that this is the safest and best way to do it. However, my present company says that the change should be made on passing the Transition Altitude/Level, and they're not really open to the idea of changing it! In my opinion, leaving the change until passing the TA/TL is asking for it to be forgotten.

[JW411]

INLAK:

It would be interesting to know how your company fares in the altitude bust tables?

Changing when cleared to a FL on the way up and changing to QNH as soon as cleared to an altitude on the way down ensures that you don't forget to change altimeter settings whilst occupied doing something else. I can see no reason for not doing it this way and ATC are perfectly happy with the idea.

Thank God that our SOPs are simple!

[5milesbaby]

As a London controller (all inside CAS) I was taught during training what JW411 says, that is a pilot will change to QNH/standard setting once cleared to the corresponding level beyond the transition level. However, certain exceptions apply as always, one I remember is that if in the same clearance ATC ask for a level check relating to the current pressure setting, ie a/c at FL200 given: descend to altitude 3000ft QNH ****, report passing FL100. In this instance the change shouldn't occur until passed FL100.
But it was 10 years ago that I went through training.......... :ouch:

[BOAC]

Then i have another question, how regularly do people fly VFR under the MSA? Obviously weather conditions depending- I experienced this question when I arrived at a flying club in Scotland many yeras ago as CFI. Cross-country 'students' were being told to plan their flights at MSA, with the result that a lot of them were just not getting them flown! I changed that.

The ONLY time you need to do anything about MSA is when you cannot see the terrain towards which you are flying. If, for example, you enter an area of poor visibility, you should consider climbing to the appropriate MSA, as well as changing course for better weather, etc. If you are 'lost' but visual with the ground, you MAY choose to climb to MSA 'in case', although if you can see the terrian there really is no need, but you should always be AWARE of what it is.

PPLM - you may plan your flight at any height in VMC, remembering rules of the air minimum heights, possible engine problems and airspace restrictions. Either set your altimeter to the area QNH or keep it updated with 'local' QNHs from ATC units.

[Gerund]

We should be clear here that the reporting of vertical position by reference to Flight Level when below Transition Altitude, but cleared to climb above Transition Altitude, is UK SPECIFIC and a filed difference from ICAO Standards.

You will be incorrect to adopt this approach in other countries where with no filed difference, the normal rule, requiring the change to 1013.2 when passing Transition Altitude, is MANDATORY.

There is no substitute for reading the ICAO Rules of the Air and MAKING SURE you read country differences before going there. For example, flying to Spain in a jet.... you change to QNH at commencement of the letdown.

(Since the UK has more filed differences from ICAO than any other country bar the USA, it is well worth aquainting yourself with all the oddities here. If you know the unique differences we have adopted, it will wave a red flag for you in the appropriate areas when you go elsewhere. For example, if you know that the quandrantal rule, VFR criteria, and so on, are filed differences, you will know you need to do something else when not in the UK.)

... just seen the post by BOAC. Just watch out in France for 'remembering rules of the air minimum heights'. France has filed differences from ICAO!