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Why Is 29.92 The Standard Altimeter Setting?  
User currently offlinec5load From United States of America, joined Sep 2008, 917 posts, RR: 0
Posted (3 years 11 months 2 weeks 1 day 5 hours ago) and read 21283 times:

In the US, once you get to 18,000 ft, you set your altimeter to 29.92. My question is why? Unless the pressure outside is 29.92 your true altitude and your altimeter will not match up. The pressure doesn't change just because you reach 18,000 ft. So why can't you leave your altimeter at the real pressure setting the entire flight?


"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
12 replies: All unread, jump to last
 
User currently offlineFly2hmo From , joined Dec 1969, posts, RR:
Reply 1, posted (3 years 11 months 2 weeks 1 day 5 hours ago) and read 21279 times:

Quoting c5load (Thread starter):
My question is why?

Real simply, It gives a common reference setting for everybody cruising at higher altitude levels thus preventing everyone else from bumping into each other.

In some areas, notably Europe, you set the standard sea level pressure from much lower, sometimes 5000ft AGL or so.

[Edited 2011-01-08 12:39:42]

User currently offlinemoose135 From United States of America, joined Oct 2004, 2399 posts, RR: 10
Reply 2, posted (3 years 11 months 2 weeks 1 day 5 hours ago) and read 21279 times:

What pressure reading would you use? The one from the airport you took off from? How about the guy coming the other way, with a different pressure reading from the airport 1,500 miles away which departed from? To provide positive altitude control, everyone uses the same setting, so you don't have aircraft that should be at different altitudes running into one another.


KC-135 - Passing gas and taking names!
User currently offlineb78710 From United Kingdom, joined Mar 2006, 344 posts, RR: 0
Reply 3, posted (3 years 11 months 2 weeks 1 day 5 hours ago) and read 21261 times:

your exact altitude doesn't matter that high up. as long as you and the others are reading off the same scale

User currently offlineNorthwest727 From United States of America, joined Jul 2005, 491 posts, RR: 0
Reply 4, posted (3 years 11 months 2 weeks 1 day 4 hours ago) and read 21185 times:

What moose said, and in addition, at FL180 and above (essentially class A airspace to FL600), the airspace is occupied mostly by high speed enroute jet aircraft. Imagine how many times a pilot would theoretically have to reset his/her altimeter on a coast-coast flight, especially crossing multiple pressure systems (mid latitude cyclones).

Like others said, at such altitudes and speeds, actual altitude doesn't matter; its that everyone is using the same scale and not bumping into each other is what matters.


User currently offlineHaveBlue From United States of America, joined Jan 2004, 2124 posts, RR: 1
Reply 5, posted (3 years 11 months 2 weeks 1 day ago) and read 21059 times:

It's so everyone has a common barometer to base their seperation by. Does it matter what the real altimeter reading is at your original airport or destination airport? ATC can't provide positive seperation, much less of 1,000' or less, if everyones using a different 'standard'. You set it to 29.92 and everyone is 'on the same page'. Its like Zulu time, your local figures dont matter you have to have a common denominator to provide safety. I can't imagine the chaos if everyone was using their own localy correct barometric pressure. Its standardized for safety.


Here Here for Severe Clear!
User currently offlineShyFlyer From , joined Dec 1969, posts, RR:
Reply 6, posted (3 years 11 months 2 weeks 1 day ago) and read 21023 times:

To further expand, 29.92 is used as it is the pressure used in the International Standard Atmosphere model.

http://www.adac.aero/Documents/US_Standard_Atmosphere.pdf
http://en.wikipedia.org/wiki/International_Standard_Atmosphere


User currently offlineflymia From United States of America, joined Jun 2001, 7270 posts, RR: 6
Reply 7, posted (3 years 11 months 2 weeks 21 hours ago) and read 20944 times:

As explained above it is too keep everyone in the world on the same page. it is critical for safe operations.

Another reason is 29.92 inches is the avearage standard air pressure at sea level.



"It was just four of us on the flight deck, trying to do our job" (Captain Al Haynes)
User currently offlineN243NW From United States of America, joined Jul 2003, 1640 posts, RR: 20
Reply 8, posted (3 years 11 months 2 weeks 18 hours ago) and read 20894 times:

Quoting Northwest727 (Reply 4):
Imagine how many times a pilot would theoretically have to reset his/her altimeter on a coast-coast flight, especially crossing multiple pressure systems (mid latitude cyclones).

One can't forget the associated chaos with ATC constantly reading altimeter settings to aircraft in the vicinity. I hear it enough on an IFR flight plan under FL180, traveling a measly 130kts...can't imagine the headache if I were traveling at jet speeds.



B-52s don't take off. They scare the ground away.
User currently offlinerwessel From United States of America, joined Jan 2007, 2410 posts, RR: 2
Reply 9, posted (3 years 11 months 2 weeks 16 hours ago) and read 20860 times:
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Another thing to consider is that small missettings create larger and larger errors in altitude as altitude decreases. At sea level a 1 inch underset (admittedly rather a large error) on your altimeter results in about a 940 ft (assuming something close to a standard day). At 20,000ft it's 1800ft, and at 40,000ft, the error is some 4100ft.

So even small differences in altimeter settings at high altitude can cause aircraft to miss their assigned altitude significantly, making normal separation impossible. At low altitudes, there's enough slack.


User currently offlinezeke From Hong Kong, joined Dec 2006, 9229 posts, RR: 76
Reply 10, posted (3 years 11 months 2 weeks 16 hours ago) and read 20840 times:

Quoting ShyFlyer (Reply 6):
To further expand, 29.92 is used as it is the pressure used in the International Standard Atmosphere model.

No it is not, it is 1013.25 HPa. 1013 HPa is the standard sea level pressure used internationally in aviation, only a few countries used the non-standard 29.92, they are also normally the ones that use FAA TERPS instead of ICAO PAN-OPS.

Like the old joke

ATC: Pan Am 1, descend to 3,000 ft on QNH 1019.

Pan AM 1: Could you give that to me in inches?

ATC: Pan Am 1, descend to 36,000 inches on QNH 1019



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlinemandala499 From Indonesia, joined Aug 2001, 6965 posts, RR: 76
Reply 11, posted (3 years 11 months 2 weeks 13 hours ago) and read 20796 times:

Quoting moose135 (Reply 2):
What pressure reading would you use? The one from the airport you took off from? How about the guy coming the other way, with a different pressure reading from the airport 1,500 miles away which departed from? To provide positive altitude control, everyone uses the same setting, so you don't have aircraft that should be at different altitudes running into one another.

You take off using the pressure at your departure airport.
After airborne... if you enter an area where the "area QNH" is different or your current area has a change.... ATC will tell you to change to new setting.
Once you go above the transition altitude (11000ft where I am... 18000 in US... and whatever it is in the regulation valid for whichever area you're in) you go to 1013mb or 29.92 (US)... the QNE...
You stay at the QNE until you descend below the transition level (FL130 where I am... FL180 in the US)... the ATC will tell you the area QNH that's valid at the time... or you might get given the QNH for your destination airport immediately... and use it once you go below the transition level.

If your flight never go above the transition altitude... then you get given whatever QNH is valid for your area... new QNH when there's a change or when you enter a new area... until your destination.

That way... for low levels... everyone at the same area will use the same QNH.

Mandala499



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineShyFlyer From , joined Dec 1969, posts, RR:
Reply 12, posted (3 years 11 months 2 weeks 5 hours ago) and read 20613 times:

Quoting zeke (Reply 10):
No it is not, it is 1013.25 HPa.

Which translates to 29.92. Not exactly, but since we aren't calculating performance values (which would use 1013.25 hpa and +15*C) it will suffice for this discussion, especially since the OP is in the US and asking about a US procedure. You are the most technically correct, though, and I appreciate the correction/additional information.

From my first link:

Quote:
The most commonly used atmosphere model is the International Civil Aviation Organization (ICAO) Standard Atmosphere, of which the main part is the International Standard Atmosphere (ISA). At all altitudes of interest to an aircraft performance engineer, the ISA model is the same as that of the 1976 U.S. Standard Atmosphere, and the MIL-STD Standard Atmosphere.


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