hi there i think i need some help with this subject as i am learning about flight levels (FL100) at the moment

in fact it really is getting confusing so can anyone explain what flight levels are and why we use them or in fact any information on flight levels would help me no end for example FL100 (flight level 100)

thanks in advance

Very simple terms - take a flight level, multiply by 100, and that gives you the approximate altitude in feet. So FL100 is 10,000'.

But it's not quite that simple. In a world without flight levels, you use the altimeter to tell how high you are. But the altimeter needs to know the air pressure at sea level (the QNH) in order to tell you your altitude, or your height above the sea.

What this means is that as you fly around the country, you need to keep setting your altimeter to different pressure settings, because pressure varies from place to place (as well as from day to day, or from hour to hour). Not a huge problem for a C150 doing 90kts in the local area, but a big problem for a B747 doing 500kts across a couple of continents.

So instead, flight levels are used. They are not used at very low levels, because at very low levels it's important to know your altitude with some degree of accuracy, to ensure you are a safe height above the ground. But once you get to an altitude where you can be confident about being clear of the ground, you don't need to worry about the regional variation of these pressure settings. So, instead, everyone sets their altimeter to a standard setting (1013.3mb), and leaves it there for the rest of the flight until they are ready to descend below this safe altitude again.

When an altitude is flown with this standard pressure setting set on the altimeter, it is refered to as a Flight Level.

The altitude at which you can start using flight levels is known as the Transition Altitude. It varies from country to country. In the UK, it is 3000' in most of the country, but there are parts of the country where it is different.

That's a pretty good summary, but it's not complete - I could go on and give more detail if you've got any more questions.

FFF
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Flight level

Expression of a level with reference to the standard altimeter setting (1013.2 mb / 29.92 in) above the transition altitude. (normally 3000ft in the UK, 6000ft in the London / Scottish TMAs, 18000ft in the USA).

A flight level is simply the level above Transition altitude, with the last two zeros chopped off.

FL55 - 5500ft (pronounced Flight Level Five Five)

FL100 - 10,000ft (Pronounced Flight Level one hundred)

FL185 - 18,500ft (Flight Level one eight five)

FL200 - 20,000ft (Flight Level two hundred)

Make sense so far?

hello

well first of all thanks for the quick reply and must say both of the post have shed some massive light on what i need to know so again may i thank you. :ok:

Oh and HO HO HO merry christmas

It is difficult to believe, but there is actually an entire book (http://www.amazon.co.uk/exec/obidos/ASIN/0025273108/qid=1103759978/ref=sr_8_xs_ap_i1_xgl/202-0286146-5050276) devoted to this fascinating subject. :p

One other point I'd add to the excellent descriptions above is that you don't HAVE to use flight levels just because you're above the transition altitude, of course (in the UK anyway). It's quite acceptable to fly around at 4000 or 5000 feet on the local QNH.

The only time I'd use a flight level is perhaps on a longer flight where I'll be crossing an ASR boundary and don't want to have to adjust my altitude when I do so.

Also, where I fly there are quite a few airways that step down to as low as 3000ft - the bases of these are measured in flight leveles down to (I think) FL50, so it's useful to perhaps have 1013.2 set on the second altimiter to make sure you're staying well clear of these.

Also, be aware that the height (transition altitude) at which an ascending aircraft might change to using flight levels is not the same as the height (transition level) at which it would change when descending.

This can be quite confusing for anyone who is reading it from a book for the first time. I remember (not so long ago) reading it and looking at the included diagram, explaining basically what was mentioned above. I must admit, I did have to read it a few times for it to sink in :sad:

Regards,

Maz

Maz,

I remember having problems understanding it too. That's because the recommended PPL textbooks aren't exactly written in language that makes things as clear as it could. FFF, could you maybe write some new ones?

FFF, could you maybe write some new ones?
Yes, agreed! Take up a dual career mate :E

You'd need some reinforced bookshelves...

:p

is flying on a local QNH near the base of an airway when the barometer is (now I always get this bit wrong) low.

This means that you and all the aicraft, separating themselves on the standard pressure setting, using FL's are vertically closer together.

The use of a second altimeter set to 1013.2 to check proximity is good advice.

Or sumfink loike that.

Sir George Cayley

I am probably about to get this wrong too but I think you are wrong Sir George. The lower the pressure the lower your actual altitude (pressure low - look out below) so the dangerous time is when flying below the base of an airway when the QNH is GREATER than 1013. Mind you - you said you would be wrong so you're actually right :-)

Just to sort out a few of the comments

Knobby
One other point I'd add to the excellent descriptions above is that you don't HAVE to use flight levels just because you're above the transition altitude, of course (in the UK anyway).
You do if you're IFR
Also, where I fly there are quite a few airways that step down to as low as 3000ft - the bases of these are measured in flight leveles down to (I think) FL50,
Make your mind up - is it 3000ft or FL50? Either way it doesn't matter. If the base of an airway (or any Controlled Airspace) is defined as a Flight Level and you're close to it then get 1013 on an altimeter so you don't penetrate.

If it's defined as an altitude then put the relevant aerodrome QNH (Under the London TMA - London QNH, available from the many Heathrow ATIS broadcasts) and NOT the regional pressure setting as you will penetrate CAS. (Regional Pressure setting don't forget is the lowest forecast pressure in an area for h+1 to h+2 - h being the time of promulgation)
Also, be aware that the height (transition altitude) at which an ascending aircraft might change to using flight levels is not the same as the height (transition level) at which it would change when descending. Not necessarily. Common practice when descending to an altitude is normally to set QNH on the altimeter when commencing descent, unless requested to report passing a specific Flight Level on the way down. Otherwise, especially in times of estreme low pressure, you might fly through the assigned altitude.

Student-PA28-161 I hope this helps. I have made it basic, but there are probably quite a few out there struggling to come to terms with altimetry, so...........................

This is my idiot's guide to altimetry and I make no apology about the fact that there is a distinct UK bias as that's the bit of sky I fly in.

When you are planning to land and takeoff from an airfield, most (but not all) light aircraft use the QFE. Think of the E as elevation and thus you know how high you are above the field.

Then some inconsiderate fool decided to veer away from the airfield and it no longer mattered how high they were above the airfield. The charts had hills, masts and all sorts of other nasty objects marked for the pilot to avoid. On the charts, these are all given in heights above the sea level which is basically altitude. Thus we have QNH. QNH is the pressure setting that tells you your altitude, your height above the sea level. If you know how far above sea level you are and how high above sea level the hill is then you can easily deduce if you will hit it or not. Likewise it is nice when aircraft have the same pressure setting so they can avoid each other. This is the favoured setting most light aircraft pilots use. Think of QNH as how to avoid Hills and Hitting things and you won't go far wrong.

Visualise a large circle with a small bullseye in the middle with the airfield being the Bull. Close to the bull (airfield) you want the QFE for your elevation relative to the airfield. As head out away from the bullseye away into the circle you want the QNH so that you can avoid your hills etc. Next imagine that each and every airfield in the UK has one of these circles with a bullseye overlaid over the top. Some will be close together and some far apart. Now imagine you are flying from one airfield to another.

Take off in your bullseye with QFE and then use QNH en-route to avoid hills etc. Assuming the two fields were close together, as you close on your destination you cross from the circle surrounding you departure point into the circle surrounding your destination. So you set your destination airfield's QNH so that you can avoid the hills etc. in that area. Finally you set your Destination QFE in order to join the circuit and land.

However, if your departure and destination points are far apart you have two choices. You could always set the QNH of each airfield you pass en-route and this would give you constant updates of the most accurate pressure setting available at that time in that area. But that increases workload and we don't like that. So a bright spark suggested chopping the UK up into lots of pressure setting regions and in each region, use the lowest forecast QNH. That way everyone in the region can fly on the same setting to make it easier to avoid one another. The lowest setting is used as the datum because your altimiter will in effect under read in most of the area. As a result you fly higher giving you a bit more clearance from the terrain.

Ah, but what if I wanted to fly over the ocean where there are no nearby airfields who can give me a local QNH or even a regional pressure setting (RPS)?

Why don't we have a standard pressure setting so that everyone can avoid each other. But then it won't tell you exactly how far above sea level you are. No but, then at that stage you are more worried about hitting other planes than the ground, so we'll just make sure we are well clear of the ground before we use it. If you fly with the standard pressure setting on set on your altimiter (1013mb) then you fly at set levels. (ie. If I am pottering around in the vicinity of my local airfield in uncontrolled airspace I can fly at 3781 feet if I wish. Not particularly easy to read on the dial so I might round it off and fly at 3800 feet. If using the standard pressure setting I would aim to fly at 4000 feet.) All very well and good, but to complicate the matter, we don't say we are flying at x thousand feet, we say we are at a flight level. That tells everyone listening that I am flying at a set level with 1013mb set on my altimeter. The level is referred to as a flight level and is simply the altitude on the dial with the last 2 zero's knocked off, so 4000 feet becomes flight level 40. 35000 feet becomes flight level 350 etc.

The standard pressure setting is used primarily to avoid aircraft hitting each other and so tend's to be used when aircraft can't easily see and avoid each other. As a result it is most commonly used in instrument flying and in controlled airspace. Due to the fact that it is only a rough guide to your height above the sea level and the terrain beneath you, it doesn't get used at low levels. Thus there is a transition altitude/level (3000 feet in the UK). Below that imaginary line you should not use the standard pressure setting or flight levels, instead referring to our old friends QFE,QNH or RPS. Above the line you can use QNH or RPS in good weather outside of controlled airspace; or flight levels in poor weather or in controlled airspace.

:ugh: :ugh: :ugh: :{ Clear as mud.

On a seperate note, Grummaniser I fear you are incorrect. The pressure is low look out below verse applies to flying from an area of high pressure into an area of low pressure. The result is a descending aircraft even though the altimeter stays the same. Near to airways, the rule to remember is....

As you wind the pressure setting up the altitude goes up.

eg.

The base of the airway is FL40. You are at 3800 feet on a pressure setting of 1006. Are you in the airway?

On the basis that 1mb = 30' the pressure difference of 7mb (1013-1006) equates to 210 feet.

To get from your current pressure setting of 1006 to 1013, you would need wind the pressure setting up to 1013. As you do so, you will see you altitude go up 210' putting you inside the airway!

Try it on the ground next time. Wind the pressure setting up and see what happens to your altitude.

Hope this is of some use to someone somewhere, but I'm going to bed now Taraaaaa.

Obs cop

Obs - I am sending you a virtual beer after that one - cheers :ok:

I reckon this should be a sticky for a while as I am willing to bet there are hundreds of people (mostly newbies like me) who ponder over this :confused:

Regards

Maz

Obs, Grummaniser,

I think Grummaniser's principle is correct, but he applied that principle to the wrong thing. As the pressure drops, the base of the airway drops relative to the QNH not the QNH relative to the airway.

I suppose, for airways you could modify the saying to "If the pressure is lower, get a call from ATC on the blower."

Um, sorry about that :\