novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

### Altimeter Setting Definition

Altimeter setting definition:

"Altimeter setting is defined as station pressure reduced to sea level."

This probably is a stupid question to ask, though i don't understand this. Using this definition for example take Denver International airport which sits at 5,000ft so does it take the pressure at 5000ft and then use the international lapse rate to reduced the setting to sea level? which even at that would probably not give you an accurate pressure altitude as it is unlikely for the lapse rate to be standard. However maybe i'm looking are interpreting this this definition the wrong way.

Pihero
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Joined: Mon Jan 31, 2005 5:11 am

### RE: Altimeter Setting Definition

 Quoting Novice (Thread starter):that would probably not give you an accurate pressure altitude as it is unlikely for the lapse rate to be standard.

Don't complicate your subject.
Basically, we do not care about rates that are outside of the "International standard atmosphere ".
The simpler way of getting the QNH for a given place would just be to grab an altimeter andf turn the setting button until the - known - altitude of the placve is shown by the instrument : the altimeter setting, io.e the QNH will be set in the pressure window.
Aviation altimetry is just that : an arbitrary way of measuring two different pressures and converting the differencve into a virtual altitude reading.
Of course, if you fly away from a point of QNH reference, you should redo your calculation, especially if your reading has been done on a high-elevation airfield.

If you're interested, this is the computation of QNH : NOAA QNH calculator
and the formula used : QNH formula
you can recognise that on the two elements between brackets, the first involves standard temperature rate, the second the height rate per Hpa .
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novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

### RE: Altimeter Setting Definition

Thanks for that Pihero

Mir
Posts: 19358
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### RE: Altimeter Setting Definition

 Quoting Novice (Thread starter):Using this definition for example take Denver International airport which sits at 5,000ft so does it take the pressure at 5000ft and then use the international lapse rate to reduced the setting to sea level?

Yes.

 Quoting Novice (Thread starter):which even at that would probably not give you an accurate pressure altitude as it is unlikely for the lapse rate to be standard.

But the altimeter calibration scale is standard, which is all that matters. So if you can figure out using the standard lapse rate what an altimeter would need to be set to in order to read correctly at the airport elevation, that's all you need to do.

-Mir
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Starlionblue
Posts: 17836
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Altimeter Setting Definition

 Quoting Mir (Reply 3):Quoting Novice (Thread starter): which even at that would probably not give you an accurate pressure altitude as it is unlikely for the lapse rate to be standard. But the altimeter calibration scale is standard, which is all that matters. So if you can figure out using the standard lapse rate what an altimeter would need to be set to in order to read correctly at the airport elevation, that's all you need to do.

Quite. The important thing is that everyone in the area has the same altimeter setting, not that the altimeter reads correctly to the last foot. The altimeter can read incorrectly by up to 75 feet and still be fine for flying anyway.

Interestingly, transponders transmit altitude information using standard pressure, not local.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

### RE: Altimeter Setting Definition

 Quoting Pihero (Reply 1):Basically, we do not care about rates that are outside of the "International standard atmosphere ". The simpler way of getting the QNH for a given place would just be to grab an altimeter andf turn the setting button until the - known - altitude of the placve is shown by the instrument : the altimeter setting, io.e the QNH will be set in the pressure window.

Which, when I worked at the FBO at an uncontrolled field, is how we gave the current altimeter setting to aircraft over the UNICOM. The altimeter had to be calibrated every 6 months by the FAA   Interesting, considering that altimeter/wind/temperature/active runway infromation given over the UNICOM is considered "advisory" only...e.g. you can't use such information for shooting an instrument approach in actual IMC. Most uncontrolled fields in the US that have an instrument approach with decent minimums now have an AWOS on the field...

You couldn't use the absolute pressure setting, as at a high altitude field, that would be off the scale for most altimeters (and is one of the reasons why we set altimeters to field elevation, not "zero" here in the US!). Didn't it used to be common practice amongst GA operators in Europe to set the altimeter to an elevation of "zero" for a local flight? That would have been quite a few years ago...
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Goldenshield
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Joined: Sun Jan 14, 2001 3:45 pm

### RE: Altimeter Setting Definition

 Quoting KELPkid (Reply 5):Didn't it used to be common practice amongst GA operators in Europe to set the altimeter to an elevation of "zero" for a local flight? That would have been quite a few years ago...

Many glider clubs both in the US and abroad like to set their altimeters to zero. This isn't so bad for a local flight, but is no good for a cross-country.

My glider club is situated next to/under class B airspace. Using zero altitude there could very easily set you up for a violation.
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KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

### RE: Altimeter Setting Definition

 Quoting Goldenshield (Reply 6):My glider club is situated next to/under class B airspace. Using zero altitude there could very easily set you up for a violation.

As I recall, mode S returns strictly pressure altitude, thus denying pilots the opportunity to cheat the system on an altitude bust    You can't hide a busted altitude by changing the Kollsman setting   I know, slightly off topic from your post...
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Goldenshield
Posts: 5017
Joined: Sun Jan 14, 2001 3:45 pm

### RE: Altimeter Setting Definition

 Quoting KELPkid (Reply 7):As I recall, mode S returns strictly pressure altitude, thus denying pilots the opportunity to cheat the system on an altitude bust You can't hide a busted altitude by changing the Kollsman setting I know, slightly off topic from your post...

Actually, it's slighly ON-topic now. The SSA and AOPA are pushing for operators, clubs, and individuals install mode-S transponders into their aircraft. Gliders and and balloons tend to have the battery life issue, but there are now transponders now that are amazingly power conservative.
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