QNH and Flight Level

[17thhour]

Quote:
"There would be a possibility of conflict if traffic cruising above the transition altitude were on Regional QNH and aircraft only slightly above it were on the 1013 reference datum. For this reason there is a layer above the transition altitude at which cruising flight should not occur, to ensure satisfactory seperation of at least 500ft."

^^ an extract from the air pilots manual.
And what exactly is the "air pilots manual" please?

Ok I presume this refers to Trevor Thom?

Whilst I have every respect for Thom's work not everything you read in a book is correct! It is surely what is specified in legislation and/or the state AIP which is the definitive guide

Yes im referring to Trevor Thom, and what, are you not agreeing with what is being said? It makes sense to me and theres noting in the AIP or legislation about it, just like the quadrantle rule is optional for VFR, we still do it even though its not "legislation."-well some of us do anyway.

[JonG]

FL30 is below the transition altitude of 3000ft amsl. The first available flight level is FL35 with a transition layer of 440ft. So FL50 would be the answer.

[Spitoon]

Originally Posted by 17thhour

Yes im referring to Trevor Thom, and what, are you not agreeing with what is being said? It makes sense to me and theres noting in the AIP or legislation about it, just like the quadrantle rule is optional for VFR, we still do it even though its not "legislation."-well some of us do anyway.

Your comments, and the extract that you refer to may be considered to be good practise.

The real problem is that for all that may be good or bad about the rest of the book, the extract that you reproduce is mixing two different issues. The pilot of an aircraft flying under VFR without receiving any form of air traffic control service is responsible for collision avoidance - this is achieved through a combination of techniques and good practises including awareness of the implications of differing altimeter settings. Separation is an ATC function and is something that the controller provides. In the vertical plane, the separation that is normally applied is 1000ft or more - the controller will use a variety of techniques to ensure that this minimum vertical separation is achieved, regardless of the altimeter settings that are used by the aircraft concerned.

In a known traffic environment the controller may instruct an aircraft to fly at an altitude above the Transition Altitude, or at a FL when below the Transition level, or may instruct an aircraft to fly level in the Transition Level. Such clearances may not be good practise (or may not be possible for other reasons) but the point is that it's up to the controller how he or she manages the traffic in order to achieve vertical separation when it's required.

Mr Thom sadly mixes what some may consider to be a good idea (for collision avoidance staying 500ft above other aircraft seems to be a good idea if you can do it), with the ATC function which provides separation (and where 500ft usually will not be 'satisfactory'!).

[modelman]

Millibars

G-EMMA

Does CAP 413 have any reference to adding mb after 3 digit QNH such as niner niner tree???

Check out CAP413 Chapter 3 Section 1.1.4. which does indeed state the word 'millibars' should be appended to pressures below 1000Mb.

Good to see you back

MM

[LH2]

unfortunately I used a couple of examples that perhap don't illustrate my point regarding the exams very well

For a better example of a question where they test your knowledge of English rather than your understanding of the subject at hand, please see the "Chart question" thread over at the Professional Training branch, in particular EK4457's answer, and his correct reasoning over which one is the "CAA" answer.

Sadly, there is the small detail that, even disregarding the fact that a number of vital pieces of information are missing from the question and therefore assumptions must be made, all of the given answers are bound to be blatantly incorrect, as they are.

This is made possible because in their zeal to make things simple so that even pilots can understand, they give you the wrong formula for the scale factor (their method only works at the equator).

They do teach you, correctly, that on a transverse mercator the scale along the prime meridian stays the same, and that it expands away from it. What they unfortunately forget to tell you is that the rate at which the scale factor expands is also a function of latitude. Which would be fine, it's not something you really need to know. Except that then they go on and ask you questions about it. And then they want the wrong answer.

So in short, you can't afford to know the subject if you expect to pass your exams easily, as I'm sure the aerodynamicists, meteorologists, mechanics, and engineers amongst you know so well. Gets a bit frustrating sometimes, but then again, would I be able to come up with a better system? Would I be able to push it through the bureaucrats, politicians, lobbists, etc., etc., so that it ever sees the light of day? I don't think I could, and most certainly I'm not going to try. So I for one just took it on the chin and got on with it.

By the way, the correct answer for 80N030E is 1.00045493 (0.04549% error )

[Shunter]

DFC,

If you sit in your aircraft with QNH 1020 set, then dial it back down to 1013, the indicated altitude decreases, therefore if QNH >1013 the altitude is above its corresponding flight level and vice versa. In such light, please educate me as to why a low QNH is of any concern as regards flight levels.

[Another_CFI]

Shunter. In the example you give of QNH = 1020 assume that you are flying at an altitude of 5000ft. Resetting the altimeter to 1013 will decrease the reading on the altimeter by 210ft (assume 1mb=30ft) therefore the altimeter will be reading 4790ft. If the aircraft is now climbed to FL50 its altitude will be 5210ft.

Therefore if the QNH is 1013 or greater and the altimeter is set to 1013 to fly at flight levels the actual altitude of the aircraft is greater than indicated. The converse is true if the QNH is below 1013.

Assume that the QNH is 963mb and the aircraft is flying at an altitude of 3000ft. Changing the subscale setting to 1013 will make the altimeter read 4500. If the aircraft is climbed by 500ft to fly at FL50 is actual altitude will be 3500ft.

In the example above with QNH = 1020 FL50 equated to an altitude of 5120ft whilst with a QNH = 963mb FL50 equated to an altitude of 3500ft.

[Shunter]

That'll teach me to go posting on headscrew subjects first thing in the morning. DFC is of course right...

If you fly along at 3000ft on Q1015, then adjust to Q1013 your indicated altitude decreases, hence you must climb to reach the corresponding flight level. In such light in the context of the question, FL30 is correct.