1013 or 2992?
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In my jet it doesn't matter that much - press "STD" on the way up, if you don't, then an amber thing happens.
ICAO DOC 8168 OPS/611, (Aircraft Operations) Vol 1, Flight Procedures, (5th edition 2006) states in Ch1 (Introduction to Altimeter Setting Procedures):
"1.1(b) In flight above the transition altitude, the vertical position of an aircraft is expressed in terms of flight level, which are surfaces of constant pressure based on an altimeter setting of 1013.2 hPa."
I confess I am surprised by the absence of any mention here of 29.92in.
It further notes, however:
\
"1.2 This method provides flexibility to accommodate variations in local procedures without compromising the fundamental principles"
ICAO DOC 8168 OPS/611, (Aircraft Operations) Vol 1, Flight Procedures, (5th edition 2006) states in Ch1 (Introduction to Altimeter Setting Procedures):
"1.1(b) In flight above the transition altitude, the vertical position of an aircraft is expressed in terms of flight level, which are surfaces of constant pressure based on an altimeter setting of 1013.2 hPa."
I confess I am surprised by the absence of any mention here of 29.92in.
It further notes, however:
\
"1.2 This method provides flexibility to accommodate variations in local procedures without compromising the fundamental principles"
Per Ardua ad Astraeus
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Originally Posted by Avionero
Doesn´t that depend on which airspace you are flying in?
Dayglo pointer at standard
I asked once before on this forum why the instrument makers do not simply paint a dayglo triangle pointer on the scale (both inches & Pa) - would that not prevent some of the setting errors discussed above?
I just done my hundredth post - someone give me a Noddy badge! Can I put it in my log book...??
I just done my hundredth post - someone give me a Noddy badge! Can I put it in my log book...??
Std pointer
Thanks for the badge, boac (may I call you boke?), much appreciated!
My pointer idea - it would tell you at a glance if you were on standard or off standard - if you are off standard, then you are presumably at your last setting, which is fine until time to reset (and maybe you should check now). It is simply to avoid having to read off a number to register that you are set standard; obviously, you need to read the number once entering/checking QNH
The beauty of it is that the same pointer shows both inches and Pa (the air molecules don't know the difference...)
My pointer idea - it would tell you at a glance if you were on standard or off standard - if you are off standard, then you are presumably at your last setting, which is fine until time to reset (and maybe you should check now). It is simply to avoid having to read off a number to register that you are set standard; obviously, you need to read the number once entering/checking QNH
The beauty of it is that the same pointer shows both inches and Pa (the air molecules don't know the difference...)
Only half a speed-brake
No, we are not wondering whether to set 1013 or 1013,2 because PANS-OPS clearly says (as stated above) it is 1013,2; full stop.
1013,2 is 29,91978 inhg so using 29,92 is PRECISE. 411A is not happy because of a cowboy nature, but because he is right.
Before we frown upon this pee-sized
0,2 hPa let's not forget that:
- at FL380 0,2 hPa equals to 17,4 ft ref: Manual of the ICAO Standard Atmosphere Doc 7488
What would you, captain, do to me when ATC say QNH 29,58 and I just dial in something that, upon closer inspection, reads 29,60 ? [17,4 ft approx ,019 inhg]
. . . .
. . . . Yes sir. No sir! Thank you sir.
JAA TGL #6 Guidance material on the approval of aircraft and operators for flight in airspace above FL290 where a 300 m (1000') VSM is applied :
- At the point in the envelope where the mean ASE [altimetry system error] reaches its largest absolute value that value should not exceed 25 m (80 ft);
- An automatic altitude control system is required capable of controlling altitude within ±20 m (±65 ft) about the selected altitude
Supposing that I keep 1013 at FL 380, my AP features something called "SOFT ALT mode" and will ignore variations +/- 50 ft. Added together to 67,4 I may have busted the certified aircraft RVSM envelope. Certainly by a very small value, but intentionally
.
This post is a mire of bull-poo because, as thankfully pointed out by HF3000 at #37 below, eventough 0,2 hPa is 17,4 ft at FL380, changing the sea-level reference of the altimeter by 0,2 hPa would only make a 5,5 ft difference on the readout. FD.
---------------------------------------
And now for THE REAL nit-picks:
1013,2 / 29,92 or 1013,25 / 29,9213?
Manual of the ICAO Standard Atmosphere Doc 7488:
Sea level atmospheric pressure P0 = 101,325 x 10^-3 Pa.
PANS-OPS 1.1.1.1 :
Flight level zero shall be located at atmospheric pressure level of 1013,2 hPa. Consecutive flight levels shall be separated by a pressure interval corresponding to at least 500 ft (152,4 m) in the standard atmosphere.
Solved.
FD (the un-real)
1013,2 is 29,91978 inhg so using 29,92 is PRECISE. 411A is not happy because of a cowboy nature, but because he is right.
Before we frown upon this pee-sized
0,2 hPa let's not forget that:- at FL380 0,2 hPa equals to 17,4 ft ref: Manual of the ICAO Standard Atmosphere Doc 7488
What would you, captain, do to me when ATC say QNH 29,58 and I just dial in something that, upon closer inspection, reads 29,60 ? [17,4 ft approx ,019 inhg]
. . . .
. . . . Yes sir. No sir! Thank you sir.JAA TGL #6 Guidance material on the approval of aircraft and operators for flight in airspace above FL290 where a 300 m (1000') VSM is applied :
- At the point in the envelope where the mean ASE [altimetry system error] reaches its largest absolute value that value should not exceed 25 m (80 ft);
- An automatic altitude control system is required capable of controlling altitude within ±20 m (±65 ft) about the selected altitude
Supposing that I keep 1013 at FL 380, my AP features something called "SOFT ALT mode" and will ignore variations +/- 50 ft. Added together to 67,4 I may have busted the certified aircraft RVSM envelope. Certainly by a very small value, but intentionally
.This post is a mire of bull-poo because, as thankfully pointed out by HF3000 at #37 below, eventough 0,2 hPa is 17,4 ft at FL380, changing the sea-level reference of the altimeter by 0,2 hPa would only make a 5,5 ft difference on the readout. FD.
---------------------------------------
And now for THE REAL nit-picks:
1013,2 / 29,92 or 1013,25 / 29,9213?
Manual of the ICAO Standard Atmosphere Doc 7488:
Sea level atmospheric pressure P0 = 101,325 x 10^-3 Pa.
PANS-OPS 1.1.1.1 :
Flight level zero shall be located at atmospheric pressure level of 1013,2 hPa. Consecutive flight levels shall be separated by a pressure interval corresponding to at least 500 ft (152,4 m) in the standard atmosphere.
Solved.
FD (the un-real)
Last edited by FlightDetent; 16th Apr 2008 at 14:52.
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Are you guys for real?
You people filled two pages of completely useless *********
Even if the error amasses to twenty feet at altitude - How tall is your plane - and where is the static port mounted - does it really matter?
You all have way too much time on your hands...
You people filled two pages of completely useless *********
Even if the error amasses to twenty feet at altitude - How tall is your plane - and where is the static port mounted - does it really matter?
You all have way too much time on your hands...
The last time I read the Jepps it said something like when setting standard QNH, it was to be 1013.2 hpa or 29.92".
That being said, almost nobody uses inches anymore, so as mentioned above the international standard of 1013 would be a good thing.
That being said, almost nobody uses inches anymore, so as mentioned above the international standard of 1013 would be a good thing.
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I find that discussion really interesting here, lots of good arguments, but very academic.
We are talking about an error that amounts to the thickness of an airliner´s fuselage after all.
Flying to Russia or China, the conversion to Meters produces way bigger discrepancies, yet the Chinese still incorporated RVSM.
I don´t understand why I should confuse myself with a unit of measurement that is used in only one of 190 countries on earth, when the benefit has no practical meaning to me.
I´m always puzzled by that strange kind of patriotism when it comes to aeronautical standards.
We have a little bit of everything anyway, (ft,m,kts, etc..), why do Russians have to stick to Meters, Americans to Inches, French to flying odd levels westbound?
We are talking about an error that amounts to the thickness of an airliner´s fuselage after all.
Flying to Russia or China, the conversion to Meters produces way bigger discrepancies, yet the Chinese still incorporated RVSM.
I don´t understand why I should confuse myself with a unit of measurement that is used in only one of 190 countries on earth, when the benefit has no practical meaning to me.
I´m always puzzled by that strange kind of patriotism when it comes to aeronautical standards.
We have a little bit of everything anyway, (ft,m,kts, etc..), why do Russians have to stick to Meters, Americans to Inches, French to flying odd levels westbound?
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Actually, as correctly stated at the beginning, an error of 0.2 hPa on the altimeter subscale isn't 17 feet, it is 6 feet.
This is because it is a DATUM (ie it represents standard atmosphere at sea-level). It doesn't matter if you are at FL350, you are only changing the sea-level datum by 0.2 hPa which is 6 feet.
This is because it is a DATUM (ie it represents standard atmosphere at sea-level). It doesn't matter if you are at FL350, you are only changing the sea-level datum by 0.2 hPa which is 6 feet.
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Barometric Conversion
I have had this for ages: I seem to recall that it came from an RAF Support Command (ah, as it then was), Safety Review and Digest of the 1960s, so I hope that I am not going to incur any MoDWrath by reproducing it... but it seems too masterly not to pass on... 
Aesop for Aviators (with apologies to Stan Unwin)
Once in the era long before the Greddle Squeeze an unmentionable aircrafty was flying from Luddle Heaprow to a Foreign Land. The crew were all from the Country Clump — a body vastly experienced in VIP Frights.
As they neared the moment when they were to reunite the Dundollop roundy-wheels with The Good Old Mother Earth, they entered a Lotty Mount of strato claggimus. This necessitated an instrumental-electromagical letting down.
‘Hello Good Old Mother Earth down there,’ said the Communicator, ‘how is your meaty-roger-tickle Roger Over?
‘Allo Roger Over,’ spoke up the bold Air Traffic Confuser from the Good Old Mother Earth. ‘Ve iss havink ze vind up ze vest, ze driddle am driddling und zer bermetricks of Doo-Niner-Doo-Tree.’
‘Ha-har!’ shouted the Squabling Leader Capital. ‘Why do not these garlic-eating. . . speak in milliminibars instead of hinges of mercy.’
‘Clapting I have converted it to 986.5 miniwhatnots’ said the Communicator.
‘It is set upon the snail on my heighti-meter’ answered the Clapting.
Said the Navvigman ‘You are over the bacon now, turn onto two two two degrades.’ And so they settled onto the Slide path. ‘Mine breaking height is 250 feet above the Good Old Mother Earth’ quoth the Capstan. At 250 feet on his heightimeter they broke through the cloud and there was a tall conifurry tree up to 150 feet above the roots and they missed it by ten foots. And all their hair turned white.
Moral: Hinges of mercy and millibuses mixed in the wrong way can cause many a hair to turn grey.
Aesop for Aviators (with apologies to Stan Unwin)
Once in the era long before the Greddle Squeeze an unmentionable aircrafty was flying from Luddle Heaprow to a Foreign Land. The crew were all from the Country Clump — a body vastly experienced in VIP Frights.
As they neared the moment when they were to reunite the Dundollop roundy-wheels with The Good Old Mother Earth, they entered a Lotty Mount of strato claggimus. This necessitated an instrumental-electromagical letting down.
‘Hello Good Old Mother Earth down there,’ said the Communicator, ‘how is your meaty-roger-tickle Roger Over?
‘Allo Roger Over,’ spoke up the bold Air Traffic Confuser from the Good Old Mother Earth. ‘Ve iss havink ze vind up ze vest, ze driddle am driddling und zer bermetricks of Doo-Niner-Doo-Tree.’
‘Ha-har!’ shouted the Squabling Leader Capital. ‘Why do not these garlic-eating. . . speak in milliminibars instead of hinges of mercy.’
‘Clapting I have converted it to 986.5 miniwhatnots’ said the Communicator.
‘It is set upon the snail on my heighti-meter’ answered the Clapting.
Said the Navvigman ‘You are over the bacon now, turn onto two two two degrades.’ And so they settled onto the Slide path. ‘Mine breaking height is 250 feet above the Good Old Mother Earth’ quoth the Capstan. At 250 feet on his heightimeter they broke through the cloud and there was a tall conifurry tree up to 150 feet above the roots and they missed it by ten foots. And all their hair turned white.
Moral: Hinges of mercy and millibuses mixed in the wrong way can cause many a hair to turn grey.
It is for this very reason that I always compare the reported QNH with the forecast (TAF) QNH as an independent gross error check. This is especially necessary when flying an RNP RNAV approach where a minima around 250 ft is the norm (just like in the story above).
All TAF's in Australia include forecast QNH and temperatures at 3 hour intervals. It's a shame that this is not universal around the world.
All TAF's in Australia include forecast QNH and temperatures at 3 hour intervals. It's a shame that this is not universal around the world.
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I very much like it wot everyone who mentioned aircraft thickness in hPa said and I heartily congratulate the one wot said it first wiv the capital/little letters in the right places