1013 or 2992?

[genius747]

Hi there.

When setting STD on an analogue altimeter which has both HPa and In as the sub scale, which gives a more accurate FL readout, 1013 in the HPa window or 2992 in the In window? I have noted that 1013 and 2992 don't exactly match up in the sub scale windows. I have a feeling that this maybe because STD is exactly 1013.25HPa, and possibly that is why the two never sit perfectly in line in the sub scale window.

A bit pedantic I know, but its been bugging me for a while lately as I do those RVSM checks!!

Thanks.

[False Capture]

29.92" is what we set on the B777 standby altimeter.

[hedgehopper]

Hi genius747,

Not at all pedantic, :ok:(in my opinion) I set 29.92 for std and 29.91 for a QNH of 1013 mbs (Hpa) thereby providing a clue for r/t purposes that I'm talking FL against Altitude.

Regards

HH

[mini-jumbo]

I would think that in Europe (and anywhere that uses HPa) that setting 1013 would be appropriate.

In airspace where In is used, set 29.92

[BOAC]

Always set 29.92 as it is easier to set than the CORRECT setting of 1013.2.

[Genghis the Engineer]

A quick back of envelope sum says that 1013.25mb/hPa = 29.921in.Hg, so 29.92 would give you about 0.9ft error.

Setting 1013 when 1013.25 is technically correct is, according to my calibrated back of envelope, would give you 6.75ft error.

So, I'd say that 29.92 in.Hg is definitely more correct.

G

[411A]

Lets face it folks...inches work properly, every time.
IE: 29.92

However, having said this, 1013.2 is.....close enough.
Don't have a cow over this, relax and enjoy the view.:ok:

[BOAC]

"Setting 1013 when 1013.25 is technically correct is, according to my calibrated back of envelope, would give you 6.75ft error." - and that could ruin a good beat up:)

[Bullethead]

When the aeroplane that you are in is more than one hPa thick what does it matter? :cool:

Regards,
BH.

[NZScion]

Maybe it is just a poor reflection on the aircraft I fly, but when I set any QNH on both altimeters, there is always a slight difference (<30ft) between the pilots and the co-pilots side...

OK, I'm not flying RVSM aircraft, but I wouldn't get too pedantic about 0.25 hPa.

[foxmoth]

Airbus has a "STD" button which I presume gives it exact whichever units you have selected when you deselect STD!:p

[john_tullamarine]

... and this presumes that no-one EVER forgets to set standard going through transition ... ?

bit like wheels up landing, I suspect ....

[Old Smokey]

Interesting new units of measurement Bullethead, aircraft thickness measured in hPa! A nice change from the media's common use of "football field" units for aircraft length / wing span.

I like it:ok:

Regards,

Old Smokey

[galaxy flyer]

J_T

Was taught many moons ago-reset to STD when cleared above the transition altitude and reset, at least, the stby to local QNH when cleared below the TLv. Drives other Yanks nuts, but is very effective.

GF

[alf5071h]

Any thoughts on which is more likely to be miss-set?
28.92, 30.92 vs 1003, 1023, 1033 ?

For those who always set 1013.2, is the quest for precision more likely to act as a trigger for detecting an incorrect entry?

[Mark1234]

Well, I hope nobody is doing instrument beatups then :E

Seriously, the inherent inaccuracy in the measuring system (instrument), and static vents is going to be a lot more than 6.75 ft.... I've not seen an altimeter where you can accurately set .2 hpa either.

[john_tullamarine]

Was taught many moons ago-reset to STD

Have had colleagues who did likewise .. I get uncomfortable if more than standby is reset in anticipation .. most of us have been caught with a revised clearance to maintain on the way up ... and either forgotten, or been at risk of forgetting, to reset the clocks. Right or wrong, I put my faith in rigorous attention to transition scan so that, even if we got distracted and the scan/check was delayed, the matter would be fixed sometime in the next few thousand feet .. on the way down, our routine check at F150 was considered close enough (and unlikely to set us up for being caught out) to transition (in Oz and local, anyway) to set QNH in anticipation ...

[BOAC]

Seriously, the inherent inaccuracy in the measuring system (instrument), and static vents is going to be a lot more than 6.75 ft.... I've not seen an altimeter where you can accurately set .2 hpa either. - 'tis indeed pedantry, but the question was asked in post#1 and Flight levels ARE defined on 1013.25MB/29.921".

Many airlines, BA included, refer to "1013" as 'Standard" (presumably to reduce printing costs:)) but Boeing and Airbus at least view it as 1013.2mb/29.92 in the 'auto' setting. The point being, if you are going to set it, set it properly! For Alf, the act of confirming both '1013 and a bit'/29.92 on the subscale goes along way to eliminating the error.:ok: I have yet to see it mis-set in 45 years+.

[Avionero]

Doesn´t that depend on which airspace you are flying in?
On my a/c you can switch between hPa and In, but as we usually don´t cross the atlantic, I never touch it. Changing the unit of measurement to gain 6 feet of accuracy, just to be irritated when I have to set the QNH in hPa again sounds odd to me.

[luvly jubbly]

According to our Flight Ops Inspector, 2992 should be set.
73NGs also have the STD button, which makes things so much easier.

[cribble]

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"

[a-320crew]

i like it what you said:D

[BOAC]

Doesn´t that depend on which airspace you are flying in?
- since the two are to all intents and purposes the same, no! The original question was which was correct, 1013 or 29.92 and the answer is 1013 is technically WRONG since the correct setting is 1013.2, BUT, since it is impossible to read 0.2mb, it is easier to set it using the inches scale. Setting 'STD' on a 737NG will set 1013.2, not 1013, and presumably AB the same.

[Check Airman]

When the aeroplane that you are in is more than one hPa thick what does it matter?

:D

[james ozzie]

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...??

[BOAC]

A clever idea, james, but Alf would worry that we would just set the orange triangle rather than the correct QNH. However, have a badge.


http://img.photobucket.com/albums/v293/boacphotos/noddy-badge.jpg

[james ozzie]

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...)

[FlightDetent]

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 :ok: 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]
. . . . :ouch: . . . . 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 :yuk:.

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)

[genius747]

wow... cheers guys, 2992 it is then!!

[stue]

......Glad we got that one sorted then......:p

[Admiral346]

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...

[Fly3]

Maybe the whole problem would go away if the US would just join the real world.

[Iceman49]

Or better yet, why not a standard Altitude and Level worldwide.

[18-Wheeler]

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.

[Avionero]

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?

[HF3000]

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.

[GemDeveloper]

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.

[Blip]

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.

[slip and turn]

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 :ok:

[john_tullamarine]

GemDeveloper...

Do you assert copyright over that little gem ? .. or can anyone use it when the need arises ?

[GemDeveloper]

john_tullamarine

John, it's in my scrap book with a note on the back: "RAF Support Command Flight Safety Review and Digest 196..." and then I can't actually read the final figure, I think it's an eight... which would fit with my University Air Squadron days...

So, if it's anyone's copyright, it's some Erudite Neddie in the Minestrone of Deference... but, as that's a Government Department, and as the Neddies in all Government Departments are Crown servants, and as the Crown also is your Head of State, then I think you ought to be able to use it... with discretion and care, of course, in case some Top Neddie objects...

[john_tullamarine]

ah, you are, sir, most certainly, a gentleperson of rare sensitivity ...

[0-8]

Err, chaps, are we not missing the point of setting STD in the first place?

Who cares what you have set as long as you all have the same thing set in the window.

If you are flying around Europe with the (technically more accurate) 29.92 set but everybody else has 1013 set, it is you who has introduced the 6.75ft error!

[BOAC]

but everybody else has 1013 set - simple - they are wrong.:ok: ICAO refers as above.