As one who used to fly -
Local altimeter settings (pressure) are intended to have the altimeter in the airplane roughly approximate the elevation of the runway. So if an airport is (sorry, it's what they unfortunately do) 3522 feet above sea level, an airplane with its altimeter set properly would have an altimeter that reads about that when the airplane lands. Above FL 180 (flight level 180, which is only 18,000 feet if the atmosphere in that area is standard), everyone sets the altimeters to the standard atmosphere setting. It may not accurately reflect height above ground, but more importantly everyone is at the same height with respect to EACH OTHER which is more important really. If actual atmospheric pressure is below standard then FL 180 may not be usable, as it may be less than 18,000 ft above sea level, and airplanes would then lose minimum (1000 ft) separation with those below them at 17,000 ft; those airplanes are using local altimeter settings instead. Sorry for the feet but until large parts of the world get their acts together that's still how many countries measure height in aviation. Carleton From: [email protected] [mailto:[email protected]] On Behalf Of Martin Vlietstra Sent: Sunday, September 04, 2011 17:40 To: U.S. Metric Association Subject: [USMA:51071] Re: planes collide I believe that the aviation industry uses air pressure to calibrate altimeters. Changing the standard atmosphere from 101.325 kPa to 100 kPa will result in an apparent 130 m difference in reference altitude. I don't think that they are willing to take the risk with this change. _____ From: [email protected] [mailto:[email protected]] On Behalf Of Michael Payne Sent: 04 September 2011 21:27 To: U.S. Metric Association Cc: U.S. Metric Association Subject: [USMA:51069] Re: planes collide In 1999, the International Union of Pure and Applied Chemistry <http://en.wikipedia.org/wiki/International_Union_of_Pure_and_Applied_Chemis try> (IUPAC) said that for the purposes of specifying the properties of substances, "the standard pressure" should be defined as precisely 100 kPa (?750.01 torr <http://en.wikipedia.org/wiki/Torr> ) or 29.53 inHg <http://en.wikipedia.org/wiki/InHg> rather than the 101.325 kPa value of "one standard atmosphere" (From Wikipedia). I think we're talking about 2 different things here, the Standard Atmosphere and the International Standard Atmosphere. I know the latter is purely a set of units set up to calibrate and test things worldwide to the same standard. 15C, 1 Atmosphere of 1013,25 hPa. It's heavily used in the aerospace industry for calibration and performance. Everyone has to be on the same page for these two items. A bit more research is needed. Mike Payne On 04/09/2011, at 07:39 , John M. Steele wrote: One (standard) atmosphere is 101.325 kPa. However, properties for many chemicals are specified at 1 bar (100 kPa) rather than 1 atm. There is also a 'technical atmosphere" 98.0665 kPa (based on kilogram-force), which is now deprecated and rarely used.. --- On Sun, 9/4/11, Michael Payne <[email protected]> wrote: From: Michael Payne <[email protected]> Subject: [USMA:51064] Re: planes collide To: "U.S. Metric Association" <[email protected]> Date: Sunday, September 4, 2011, 6:58 AM I believe one atmosphere is still 100 kPa, the 1013,25 comes from the International Standard Atmosphere. Commonly referred to as the ISA it's what manufacturers use to calibrate instruments and what aircraft makers use to calibrate the performance of aircraft. In the aviation world we constantly refer to the temperature being below or above ISA, the lapse rate is fixed under ISA as 1.98 C up to 36090 (11000 m) where it's supposed to be constant. Obviously in the real world it changes which has an affect on performance. Mike Payne On 03/09/2011, at 13:57 , G. Stanley Doore wrote: The 1013.25 mbar (101.325 kPa) pressure for altimeter settings is NOT "arbitrary" as Kilopascal & Wiki write. The standard altimeter setting for worldwide altimeter settings was determined from the mean surface pressure level. Stan Doore On Sep 3, 2011 11:36 AM, "Michael Payne" <[email protected] <http://us.mc824.mail.yahoo.com/mc/[email protected]> > wrote:
