Classic British Flight Sim

Thanks for the laugh, Pete. Judging by your post I'd say this model flies pretty accurately.

Brian

'Scuse me.....I only have a cursory knowledge of FL rules.
Am I right in thinking that although the FL isn't going to necessarily match actual aircraft altitude, the difference is not enough to be of concern, as FLs go in blocks of 500feet, and it's a case of "pretty close"?

Ahhh, altimetry - my favourite subject, Mike.

You're right when you say that the aircraft's altitude may be different from its flight level. In simple terms, altitude is defined as the vertical distance above sea level and a flight level is the vertical distance above the 1013.2 hPa pressure line. So, if the sea level pressure is 1013.2, then the aircraft's altitude and flight level - in this case, 6000ft and FL60 - will be the same. If the pressure at sea level is higher than the 1013.2, then FL60 will be higher than than 6000ft and conversely, if the seal level pressure is lower than 1013, FL 60 will be lower than 6000ft.

To give an example using the (very rough) rule of thumb that 1 hPa (used to be millibars) = 30 ft -

Sea level pressure 1003 - FL60 is at 5700ft amsl (the 1013 hPa pressure datum is 300ft below sea level)
Sea level pressure 1013 - FL60 is at 6000ft amsl (the 1013 hPa pressure datum is at seal level)
Sea level pressure 1023 - FL60 is at 6300ft amsl (the 1013 hPa pressure datum is 300ft above sea level)

This can make a big difference when operating in areas such as the London TMA where controllers are using both altitudes and flight levels - if the pressure is 1013 or above then 6000ft is separated by the minimum required 1000ft from FL70 but if the pressure is below 1013, the minimum usable flight level (to give at least 1000ft) becomes FL80.

Pete

Thank you Pete,
You've explained it brilliantly!

He's great isn't he I swear to god that had he been one of my teachers at school.. I'd have left with a lot better qualifications rather than the ones you get for just turning up

ATB
DaveB

Oh, you've made me blush. I did, though, spend 10 of my 40 years in ATC in full-time training and examining, 7 of them as Head of ATC Training so it's not the first time I've done that explanation.
Pete

Credit where it's due Sir Whenever you've explained anything here be it regarding ATC or on any other subject.. I've always been thoroughly impressed by the tone, manner and clarity of the explanation. It's always 'a good read' and never leaves one in a position of doubt or 'WTF was that all about'! It's a gift that all too few people in instructional positions possess. Maybe it's coincidence that Mike and I are receptive to your particular style but I doubt that very much


ATB
DaveB

Pete,If you don't mind me asking, I always thought that the transition level in Europe was 6000 feet but I seem to recall reading somewhere that this isn't always the case. If it does vary could explain why? Thanks EricT

PeteP wrote: ↑

03 Jul 2017, 08:02

Ahhh, altimetry - my favourite subject, Mike.

You're right when you say that the aircraft's altitude may be different from its flight level. In simple terms, altitude is defined as the vertical distance above sea level and a flight level is the vertical distance above the 1013.2 hPa pressure line. So, if the sea level pressure is 1013.2, then the aircraft's altitude and flight level - in this case, 6000ft and FL60 - will be the same. If the pressure at sea level is higher than the 1013.2, then FL60 will be higher than than 6000ft and conversely, if the seal level pressure is lower than 1013, FL 60 will be lower than 6000ft.

To give an example using the (very rough) rule of thumb that 1 hPa (used to be millibars) = 30 ft -

Sea level pressure 1003 - FL60 is at 5700ft amsl (the 1013 hPa pressure datum is 300ft below sea level)
Sea level pressure 1013 - FL60 is at 6000ft amsl (the 1013 hPa pressure datum is at seal level)
Sea level pressure 1023 - FL60 is at 6300ft amsl (the 1013 hPa pressure datum is 300ft above sea level)

This can make a big difference when operating in areas such as the London TMA where controllers are using both altitudes and flight levels - if the pressure is 1013 or above then 6000ft is separated by the minimum required 1000ft from FL70 but if the pressure is below 1013, the minimum usable flight level (to give at least 1000ft) becomes FL80.

Pete

I'm sorry he asked


Graham

Tomliner wrote: ↑

03 Jul 2017, 14:57

I always thought that the transition level in Europe was 6000 feet but I seem to recall reading somewhere that this isn't always the case. If it does vary could explain why? Thanks EricT

Hi Eric,

What you're referring to is the Transition Altitude not the Transition Level. The Transition Altitude is a fixed and published altitude but it can vary from place to place and country to country. For example, the published Transition Altitude in the UK is 3000ft but in certain areas - the controlled airspace around the London airports, for example - it's 6000ft. I seem to remember from my Radar Contact beta team days that Germany is (or was) 5000ft, Belgium 3500ft and so on with each country having their own fixed value.

Your question was why? I can only surmise that each country has gone its own way with procedures as ATC developed after the war and has chosen figures to suit their own needs. The good news - at least I think it's good - is that when I retired 11 years ago, serious consultations were just beginning on harmonising the the various TAs in Europe to a single figure under the auspices of Eurocontrol. 18000ft started as the favourite with 10000ft coming up on the rail but they're still working on it and, although I don't pay too much attention to this sort of thing these days, I think 18000ft is now the front runner again for the UK but 10000ft for some European countries.

Although the Transition Altitude is fixed for a particular location, what does change as the pressure changes for the reasons I've explained in my reply to Mike, is the Transition Level.

Pete

PS this is all Brian's fault! He must have known that posting some screen shots of a Deux Ponts would inevitably lead to this. Some people have no consideration for others.
P.