Altimetry and altitudes/levels

[BadgerBadger]

Couple of questions that hopefully aren't too ridiculous. Please correct me if I've got the wrong end of the stick here....

i) If 1 mb = 30 ft in terms of air pressure change related to altitude, whereby a drop of 1 mb eqautes to a gain in altitude of 30ft, how is it that there are not enough millibars in a QNH of say 1000 to accomodate an altitude past 30,000ft? For example, QNH 1000 = 0ft amsl, pressure 500 = 15,000ft amsl, pressure 0 = 30,000ft amsl. Obviously the air pressure at 30,000ft is not zero millibars, so how does this 1 mb = 30 ft yardstick work?

ii) If aircraft change to the standard setting 1013.2 mb through a transition altitude, is it possible that 1,000ft vertical separation would be compromised if there was an aircraft at say FL40 on 1013.2 and another at 3000ft on, for example, QNH 990?

I've spent a good deal of time trying to get my (relatively ATC-novice) head round these - if anyone can assist me in my quest to work them out, that would be much appreciated.

[HEATHROW DIRECTOR]

i) Some genius will answer that..

ii) An a/c flying at 3000ft on 990 will not be separated from someone flying at FL40 on 1013 as they will be only 300-odd feet apart.

Here's a tip from an instructor of mine when I was a kid - helps with all altimeter questions. If you wind the subscale up, the altimeter reading goes up. So... if the subscale is at 990 and the altimeter is reading 3000 ft.... if you turn the subscale up to 1013 the altimeter reading increases to around 3690 ft. It may sound simply but it answers question ii.

Good luck...

[Farmer 1]

i) I believe the official altitude change for 1mb is 27ft, but that is for the International Standard Atmosphere, which does not exist anywhere on Earth. However, 30ft is used by one and all as an easier number with which to work. If you ascend several thousand feet, the pressure is much less than at sea level, so 1mb will be worth more than 30ft, if you can understand that. In other words, the rate of decrease in pressure decreases as altitude increases. If the pressure is half that at sea level, 1mb would be equivalent to 60ft (or 54ft if you are being pedantic).

ii) Yes, you are correct. If the QNH is 33mb either side of 1013.2, which you set climbing through the transition altitude of 3,000ft, your flight level would be either 020 or 040 approximately. The transition level would be adjusted if necessary to the lowest level above 3000ft.

[Eva San]

I guess that the answer lies in the definition of the Standard Atmosphere... It's a theoritical model, some kind of an approximation, this part being only valid for lower altitudes. Sorry not to be very precise, others will probably be.

Second question: this is exactly why the first usable level in IFR is transition level plus ten at least my little country called France...
As the Transition Level is the first level above Transition Altitude there will always be at least 1000 ft between TA an TL+10.

Edit: damned... beaten on the line !

[Mark 1]

In lieu of a genius, I'll attempt to answer 1)

The pressure lapse rate is proportional to air density (which in turn is proportional to pressure and inversely to temperature).

At sea level, the density is 1.225 kg/m^3, so the vertical column of air responsible for a pressure of 1mb (=100N/m^2 or 10.2 kgf/m^2) is <10.2/1.225>m = 8.3m or about 27'.

As you go up in altitude the density decreases, so the lapse rate decreases proportionately, or the change in altitude for a 1mBar drop in pressure will increase in inverse proportion.

At FL200 1mBar will be about 50' in altitude.

[smithgd]

BB

Q1) Think of it as 30ft =1mb rather than 1mb = 30ft. This approximation is used when working out the altitudes & heights of aircraft and /or airfields when low level since any aircraft flying up high is likely to be flying a FL using 1013mb. For example if you know the QFE on the airfield and the elevation of the airfield you can work out the QNH (roughly).

Eg. QFE = 998mb Elev = 300ft using 1mb = 30ft

then QNH = (300/30) + 998 = 1018mb

This can be used in reverse.


Q2) As said before.

[Lee-a-Roady Moor]

QNH = (300/30) + 998 = 1018mb

Sticky fingers perhaps but, 300/30 + 998 = 1008

Incidentally, shouldn't we be referring to hPa's...?

[nibog]

Yes, 1mb/hPa is equal to approx 27ft at sea level ISA (1013.25 @ +15oC). The higher up you go, the colder it gets, the less dense the atmosphere, the more altitude each hPa is equal to.This continues till approx 56,000ft if I'm not mistaken. Above 56,000 the temp increases with altitude as you enter the ionosphere?(it's been a long time, correct me if required). Adove this altitude, conventional altimiters are not used.

The senario you give would indeed break the required separation standard of 1000ft vertical, but should never happen because of the Transition Layer. Each country or region will have a fixed transition altitude (TA) which will be calculated individually depending on the highest obstacle in that country or region. As an aircraft passes through the TA the pilot will change the Alt sub scale to 1013 and oberate on FL above. Depending on the aerodrome pressure, QNH(alt) or QFE(height), the Transition Level will be calculated and will be the Lowest useable FL at that time.

The Transition Layer will always be there and it will always be 1000ft - 1500ft thick.

For example: If the TA is 5000ft and aerodrome QNH is 1014 then the TL is FL60, but, if pressure drops by 1hPa to 1013, then the TL becomes FL65. The TL will be adjusted as necessary to maintain at least a 1000ft buffer.

I know a guy who makes TransLevel computers for ATC units if anyone wants one. Simple little things that fit in a strip holder. Very cheap, and effective. PM me for details.

[BadgerBadger]

You guys are the best! Thanks for your comprehensive and understandable replies..... they make sense and I can sleep at night again!

Thanks again.