The Mil and their use of QFE
The 30ft per millibar difference is only valid up to (very approx) 20 - 23,000ish ft. At 35,000ft the difference can easily be 70ft per millibar!
Try it out at FL350 some time.
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bookworm, let us take an example:
The calculations below show actual feet/mb for various altitudes assuming ISA:
Zero ft 30 ft per mb
5000 ft 30 ft per mb
10000 ft 31.45 ft per mb
20000 ft 36.25 ft per mb
30000 ft 40.84 ft per mb
40000 ft 47.84 ft per mb
50000 ft 52.75 ft per mb
60000 ft 62.96 ft per mb
If an aircraft is flying at 20000 ft on an RPS of 1033 (20
mbs x 36.25) the aircraft is 725ft lower than an aircraft at FL200. This differs from our traditional assumtion that, using 30ft/mb, the first aircraft would only be 600ft lower.
If we then use this example in an ATC coordination scenario, we may find that ATC would agree for an aircraft not to be below 21600ft RPS to ensure standard separation against traffic at FL200. Under the 30ft/mb assumption this would be OK as 21600ft would equate to 21000ft 1013.2mbs. However, taking into account the 36.25ft/mb accurate equation, in this particular circumstance the aircraft on RPS would actually be at the equivalent of 20 875 ft 1013.2 mbs thus creating actual separation between the 2 aircraft of 875ft. This, in ATC separation parlance, is not enough.
If we take the argument up to the 35000ft territory and are in airspace where the temperature bit of ISA is significantly different from the assumed, then the problem gets significantly more acute.
The calculations below show actual feet/mb for various altitudes assuming ISA:
Zero ft 30 ft per mb
5000 ft 30 ft per mb
10000 ft 31.45 ft per mb
20000 ft 36.25 ft per mb
30000 ft 40.84 ft per mb
40000 ft 47.84 ft per mb
50000 ft 52.75 ft per mb
60000 ft 62.96 ft per mb
If an aircraft is flying at 20000 ft on an RPS of 1033 (20
mbs x 36.25) the aircraft is 725ft lower than an aircraft at FL200. This differs from our traditional assumtion that, using 30ft/mb, the first aircraft would only be 600ft lower.
If we then use this example in an ATC coordination scenario, we may find that ATC would agree for an aircraft not to be below 21600ft RPS to ensure standard separation against traffic at FL200. Under the 30ft/mb assumption this would be OK as 21600ft would equate to 21000ft 1013.2mbs. However, taking into account the 36.25ft/mb accurate equation, in this particular circumstance the aircraft on RPS would actually be at the equivalent of 20 875 ft 1013.2 mbs thus creating actual separation between the 2 aircraft of 875ft. This, in ATC separation parlance, is not enough.
If we take the argument up to the 35000ft territory and are in airspace where the temperature bit of ISA is significantly different from the assumed, then the problem gets significantly more acute.
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....the CAA limit on the distance at which controllers can provide a radar service without holding an Area Radar rating.
For at least two years now, the only restriction on the maximum range that a civil radar service can be provided using an Approach Radar ticket is radar coverage, adjacent airspace arrangements and the procedures written into the unit's MATS Part 2 ('Controllers' Order Book').
On the subject of QNH vs QFE, why would Brit mil aircrew 'want it' when US mil & Canadian mil appear to be able to function without???
Surely, if you fly an SRA you're given the threshold elevation and advisory altitudes i.e. QNH-based check altitudes, if you fly an ILS you simply follow the glidepath and the approach plate will give you check heights (altitudes) at the FAF and in tabulated form against ranges from touchdown, and if you fly a PAR you just do what the nice talkdown controller tells you to do over the earphones.
As for publishing a CTR with an upper limit defined as an altitude of 3500' when the TA is 3000', or an AIAA with an upper limit of altitude 5000', (note: altitude and so based on QNH) this really is a load of complete and utter bo
s; next time I'm near the Brize CTR I'll ask Zone for the QNE...
The calculations below show actual feet/mb for various altitudes assuming ISA:
Zero ft 30 ft per mb
5000 ft 30 ft per mb
10000 ft 31.45 ft per mb
20000 ft 36.25 ft per mb
30000 ft 40.84 ft per mb
40000 ft 47.84 ft per mb
50000 ft 52.75 ft per mb
60000 ft 62.96 ft per mb
Zero ft 30 ft per mb
5000 ft 30 ft per mb
10000 ft 31.45 ft per mb
20000 ft 36.25 ft per mb
30000 ft 40.84 ft per mb
40000 ft 47.84 ft per mb
50000 ft 52.75 ft per mb
60000 ft 62.96 ft per mb
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bookworm, I cannot put my hands on the source of the table, i will keep looking. In the meantime, the attached make interesting reading.
http://www.auf.asn.au/meteorology/section1a.html
http://www.auf.asn.au/groundschool/umodule3.html
http://www.auf.asn.au/meteorology/section1a.html
http://www.auf.asn.au/groundschool/umodule3.html
WBS
I'm confused by your table of numbers. If you're attempting to work out the change in level corresponding to a change in pressure as a function of level in the ISA, then they're incorrect.
The variation is much higher -- some approximate numbers are shown in the pressure gradient section of the reference you quoted. Even those are broad averages.
I previously used 100 ft/mbar in the mid-300s, which comes from the knowledge that the 250 mbar level is conventionally FL340, and the 200 mbar level is conventionally FL390, so that's an average of 100 ft/mbar.
Maybe you're using the average gradient between sea level and the level indicated?
If that's what the numbers are meant to be, then I can only reiterate that it's not relevant, for the reasons that I've tried to set out.
If I try to put it another way, you need to realise that changing the altimeter setting by 1 mbar is not the same as moving the aircraft up or down by one mbar of pressure in the atmosphere. When you change the altimeter setting, you shift the reference level, at which the altimeter would read zero.
So in your example, your aircraft at 20,000 ft on the RPS of 1033 is 20,000 ft above the 1033 mbar level. Your aircraft at FL200 is 20,000 ft above the 1013 mbar level. The vertical distance between them is therefore the level difference between the 1033 mbar level and the 1013 mbar level, and that's about 20 mbar x 30 ft/mbar = 600 ft because the 1033 mbar level and the 1013 mbar level are both close to sea level.
If you were to measure the actual pressure outside the FL200 aircraft, it would be about 435 mbar. If you were to measure the actual pressure outside the 20,000 ft on 1033 aircraft, it would be about 446 mbar. There would only be 11 mbar difference in pressure between them, even though their altimeter settings are 20 mbar apart.
I'm confused by your table of numbers. If you're attempting to work out the change in level corresponding to a change in pressure as a function of level in the ISA, then they're incorrect.
The variation is much higher -- some approximate numbers are shown in the pressure gradient section of the reference you quoted. Even those are broad averages.
I previously used 100 ft/mbar in the mid-300s, which comes from the knowledge that the 250 mbar level is conventionally FL340, and the 200 mbar level is conventionally FL390, so that's an average of 100 ft/mbar.
Maybe you're using the average gradient between sea level and the level indicated?
If that's what the numbers are meant to be, then I can only reiterate that it's not relevant, for the reasons that I've tried to set out.
If I try to put it another way, you need to realise that changing the altimeter setting by 1 mbar is not the same as moving the aircraft up or down by one mbar of pressure in the atmosphere. When you change the altimeter setting, you shift the reference level, at which the altimeter would read zero.
So in your example, your aircraft at 20,000 ft on the RPS of 1033 is 20,000 ft above the 1033 mbar level. Your aircraft at FL200 is 20,000 ft above the 1013 mbar level. The vertical distance between them is therefore the level difference between the 1033 mbar level and the 1013 mbar level, and that's about 20 mbar x 30 ft/mbar = 600 ft because the 1033 mbar level and the 1013 mbar level are both close to sea level.
If you were to measure the actual pressure outside the FL200 aircraft, it would be about 435 mbar. If you were to measure the actual pressure outside the 20,000 ft on 1033 aircraft, it would be about 446 mbar. There would only be 11 mbar difference in pressure between them, even though their altimeter settings are 20 mbar apart.
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HD says <Now you can bet some loony will ask how military controllers can use 500 ft vertical inside a civil control zone!!!! Tee hee!!>
Looks like HD has shown a typical civil controllers lack of understanding of the military task.
Shame, so I'll throw my tuppence worth in and say we use RVS in 'CTZs' because our limited airspace requires such controlling skills. Indeed, we are good enough to wheel and deal OAT/GA traffic in very limited space, we have too. We do seek the civil pilots permission to apply RVS and if they are happy they get a good view of a fast jet and the pleasure of flying on QFE too!!
I will also go further and say that unlike HD's 'routine' job as a LHR Director, where evreything either comes off BOV,BPK,BIG or LAM and does the same thing day in day out. The Mil Controller has to cope with a far more dynamic, fast moving,unpredictable traffic flow from all points of the compass, with conflictors and the odd lack of SSR day. I would like HD to try recovering 13 Tornados (pattern speed of Concorde, remember?) with a number of emergencies, fuel priorities and the weather going downhill quickly in limited airspace. Throw a few transits in the way and 500' RVS is a luxury that Mil Controllers need to achieve their task.
Hope this loon has answered the question.
Looks like HD has shown a typical civil controllers lack of understanding of the military task.
Shame, so I'll throw my tuppence worth in and say we use RVS in 'CTZs' because our limited airspace requires such controlling skills. Indeed, we are good enough to wheel and deal OAT/GA traffic in very limited space, we have too. We do seek the civil pilots permission to apply RVS and if they are happy they get a good view of a fast jet and the pleasure of flying on QFE too!!
I will also go further and say that unlike HD's 'routine' job as a LHR Director, where evreything either comes off BOV,BPK,BIG or LAM and does the same thing day in day out. The Mil Controller has to cope with a far more dynamic, fast moving,unpredictable traffic flow from all points of the compass, with conflictors and the odd lack of SSR day. I would like HD to try recovering 13 Tornados (pattern speed of Concorde, remember?) with a number of emergencies, fuel priorities and the weather going downhill quickly in limited airspace. Throw a few transits in the way and 500' RVS is a luxury that Mil Controllers need to achieve their task.
Hope this loon has answered the question.
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Ah yes Northolt is absolutely awash with fast jets arriving unanounced from all directions with fuel emergencies! 
As for using 500ft separation in places other than Heathrow Zone - simple - because JSP whatever says it can be done.
However just because it says it can be done does not make it safe. If 500ft separation was safe in civil operations we would have double the capacity in the airways!
Similarly one has to question the safety of the altimeter setting procedures in current use.
Regards,
DFC
As for using 500ft separation in places other than Heathrow Zone - simple - because JSP whatever says it can be done.
However just because it says it can be done does not make it safe. If 500ft separation was safe in civil operations we would have double the capacity in the airways!
Similarly one has to question the safety of the altimeter setting procedures in current use.
Regards,
DFC
Let's not allow a good bit of banter to cloud the issue. Mil pilots want to fly on QFE in the vicinity of the aerodrome. As mentioned earlier QNH was tried (posed no problems to ATC) but the aircrew didn't like it. If the primary customer is using QFE, it makes sense to me that all traffic likely to mix it should also use QFE. Procedures are very clear cut, well tried and tested and all Mil controllers are fantastic at sums! (I should know 
)Whilst A Good Headin might have gone a tad OTT, his/her sentiment is not a million miles wide of the mark - in very busy, constantly changing scenarios, the procedures really do work!
Last edited by Canary Boy; 22nd Feb 2005 at 22:43.
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AGH says <where evreything either comes off BOV,BPK,BIG or LAM>
Looks like AGH has shown a typical military controllers lack of understanding of the civil task.
Surely they all come off either BOV, OCK, BIG or LAM?!? Much harder i'm sure!! 
FB
Looks like AGH has shown a typical military controllers lack of understanding of the civil task.
Surely they all come off either BOV, OCK, BIG or LAM?!? Much harder i'm sure!!
FB
Having spent circa 2,000 hrs in the back of the simulator, training and checking, and having seen both QFE and QNH ops I know which I prefer - definitely QNH - as has been said by Chilli Monster the number of missed approaches I have seen where QFE is used and we get an alt bust due to not/late setting of QNH.
Stateside they dont even call it QNH - its THE altimeter setting.
Lastly, reminds of going into Crete on a nice weather day and we were about number three in the visual approach "queue" to join right base RW 27. The Greek ATCO was working like a one armed paper hangar with arrival and departure tracks the same and much traffic - no radar, all separation based on DME. In the middle of all the RT chat Danair who had been cleared for a visual asks for the QFE - the controller replies in rather exasperated voice "Danair, the QFE is 3 millibars less than the QNH!" - I could hardly control my laughter - you had to be there to appreciate it!
Stateside they dont even call it QNH - its THE altimeter setting.
Lastly, reminds of going into Crete on a nice weather day and we were about number three in the visual approach "queue" to join right base RW 27. The Greek ATCO was working like a one armed paper hangar with arrival and departure tracks the same and much traffic - no radar, all separation based on DME. In the middle of all the RT chat Danair who had been cleared for a visual asks for the QFE - the controller replies in rather exasperated voice "Danair, the QFE is 3 millibars less than the QNH!" - I could hardly control my laughter - you had to be there to appreciate it!
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I do enjoy a bit of banter with civvy controllers and they usually get me back when they flash their pay chits at the end of the month. CB sums up the original question/answer nicely,thnks. Feel free to abuse me with your paychits as the end of the month is upon us!
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Lord Sven, Thank you for some sanity amongst all this blah. This country is not Canada or the Ukraine, or even Holland where you could land with a negative value on your QNH set altimeter. The Military choose to use QFE for recovery and the Civvies QNH. There is no need for a review, because we are all able to do maths or we wouldn't be in the jobs we are. Bookworm, you are quite correct. Move on!!!!!!
Oh yeah, and lets not get onto the subject of Millibars, Inches, Metres and Feet!\ 
Oh yeah, and lets not get onto the subject of Millibars, Inches, Metres and Feet!
\
Just a thought - of the replies posted from aircrew - how many are military?
