Because the altimeter does not measure pressure at altitude, only distance (feet or meters).
There is a setting where you put in either (1) ground level pressure at the appropriate ground station, if you are not above the transition altitude; this corrects for non-standard pressure, or (2) standard pressure (1013.2 mb or, in the old USA system, 29.92 inches of mercury) if you are above the transition altitude; this puts everyone on an equal standard. Then you read the height shown by the altimeter. It will never show you what the pressure at altitude is. Carleton -----Original Message----- From: John S. Ward [mailto:[EMAIL PROTECTED] Sent: Tuesday, December 16, 2003 21:30 To: [EMAIL PROTECTED]; U.S. Metric Association Subject: Re: [USMA:27862] RE: Airplane altitudes Why not give the maximum safe pressure that would easily clear Mt. Everest in any weather, since you are in fact measuring pressure? As many people have pointed out, some aspects of aviation are not metric and should be fixed. Why not fix it right by sticking with SI? On Tuesday 16 December 2003 09:14, [EMAIL PROTECTED] wrote: > The aeronautical chart for that area would show a minimum safe altitude > that would easily clear Mt. Everest no matter what the actual atmospheric > pressure was. > > cm > > > >Of Michael Payne > > >Mountains are not shown with heights in hPa. > > > > Pay attention at the back :-) > > > > The unit 'feet' used to describe cruising aircraft altitude is not the > > same as height above mountains. An aircraft described as flying at 30000 > > 'feet' could crash into Everest at 29000 feet.
