Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat My Starred Topics | Profile | New Topic | Forum Index | Help | Search
 Altimeter Setting Definition
 Novice From United Kingdom, joined Aug 2012, 90 posts, RR: 0Posted Thu Nov 29 2012 14:36:12 UTC (3 years 6 months 2 days 8 hours ago) and read 4806 times:

 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 From France, joined Jan 2005, 5174 posts, RR: 78 Reply 1, posted Thu Nov 29 2012 16:30:00 UTC (3 years 6 months 2 days 7 hours ago) and read 4782 times:

 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.

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 .

 Contrail designer
 Novice From United Kingdom, joined Aug 2012, 90 posts, RR: 0 Reply 2, posted Thu Nov 29 2012 19:46:15 UTC (3 years 6 months 2 days 3 hours ago) and read 4730 times:

 Thanks for that Pihero
 Mir From United States of America, joined Jan 2004, 22866 posts, RR: 53 Reply 3, posted Thu Nov 29 2012 22:59:27 UTC (3 years 6 months 2 days ago) and read 4687 times:

 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

 7 billion, one nation, imagination...it's a beautiful day
 starlionblue From Greenland, joined Feb 2004, 17667 posts, RR: 65 Reply 4, posted Fri Nov 30 2012 05:20:21 UTC (3 years 6 months 1 day 18 hours ago) and read 4622 times:

 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."
 KELPkid From United States of America, joined Nov 2005, 6833 posts, RR: 3 Reply 5, posted Fri Nov 30 2012 11:52:12 UTC (3 years 6 months 1 day 11 hours ago) and read 4544 times:

 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...

 Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 Goldenshield From United States of America, joined Jan 2001, 6170 posts, RR: 13 Reply 6, posted Fri Nov 30 2012 12:24:16 UTC (3 years 6 months 1 day 11 hours ago) and read 4536 times:

 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.

 Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
 KELPkid From United States of America, joined Nov 2005, 6833 posts, RR: 3 Reply 7, posted Fri Nov 30 2012 13:41:40 UTC (3 years 6 months 1 day 9 hours ago) and read 4514 times:

 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...

 Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 Goldenshield From United States of America, joined Jan 2001, 6170 posts, RR: 13 Reply 8, posted Fri Nov 30 2012 13:47:14 UTC (3 years 6 months 1 day 9 hours ago) and read 4512 times:

 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.

 Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
 Top Of Page Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat Forum Index

Reply To This Topic Altimeter Setting Definition
• Tech/Ops related posts only!
• Not Tech/Ops related? Use the other forums
• No adverts of any kind. This includes web pages.
• No hostile language or criticizing of others.
• Do not post copyright protected material.
• Use relevant and describing topics.
• DETAILED RULES

 Similar topics: More similar topics...
Why Is 29.92 The Standard Altimeter Setting? posted Sat Jan 8 2011 12:29:59 by c5load
Altimeter Setting posted Sat Dec 11 2004 23:10:21 by IndianaPilot
Altimeter Setting Below Sea Level posted Thu Jul 25 2002 22:46:57 by N79969
Aircraft Altimeter Pressure Setting posted Tue Sep 2 2008 05:51:23 by Oly720man
737 Stab Trim Setting Indicator posted Tue Oct 9 2012 12:23:47 by smartt1982
V-speed Definition Question posted Thu May 31 2012 01:08:03 by Starlionblue
MD-80 Flap Setting posted Sun Feb 12 2012 09:14:50 by e38
Effects Of Takeoff Stabilizer Trim Setting posted Sat Nov 12 2011 17:53:45 by aerotech777
Speed Bug Setting On 727 posted Mon Nov 1 2010 02:49:55 by Oli
North Central Super Storm And Altimeter Settings posted Mon Oct 25 2010 17:04:30 by N6238P