When you're low to the ground - small airplanes in particular - height above the ground is important. You want your altimeter set close to actual pressure so you can compare your displayed height to the airport and ground elevations shown on your aeronautical chart. (Most of us found radar altimeters well out of reach, and their primary use is on instrument approaches anyway.)
On the other hand, if you're an A330 at cruising altitude, it's unlikely you're going to hit the ground anywhere, except maybe around Nepal. You are more concerned with separation from everyone else. So if everyone has the altimeter set to the same thing, they can more accurately maintain vertical separation. If the displayed altitude is a bit off from actual height above the ground - well, at those altitudes, it doesn't really matter. Note that if meters were universally used, vertical separation would be lower if 500 m were the levels - even thousands westbound and thousands + 500 eastbound, perhaps - current separation 2000 ft, new separation 500 m, or about 1640 ft. Probably still safe enough. Carleton -----Original Message----- From: John S. Ward [mailto:[EMAIL PROTECTED] Sent: Monday, December 15, 2003 22:05 To: [EMAIL PROTECTED]; U.S. Metric Association Subject: Re: [USMA:27853] RE: Airplane altitudes Carleton, It seems like the killer solution to metricating flight altitudes would be to directly use pressure in hPa. It's simply wrong to call it "feet" or "meters" if airplanes really are flying on surfaces of constant pressure. Any idea why this isn't standard practice? Have you seen this discussed before in the pilot circles? John On Monday 15 December 2003 04:00, Carleton MacDonald wrote: > Feet and kPa are not directly related. > > What it does mean is that an altimeter, set to standard pressure (1013.2), > will read the altitude indicated, if the outside pressure at that altitude > is the amount given. > > And the main reason for the common setting at and above FL 180 is to make > sure everyone up there is using the same standard, so they are separated > with relationship to each other. > > Carleton > Former flight instructor > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf > Of Terry Simpson > Sent: Sunday, December 14, 2003 20:52 > To: U.S. Metric Association > Subject: [USMA:27849] RE: Airplane altitudes > > >can we know what range of altitude the plane is actually at? > > Near Norway right now, the pressure is 968 hPa. A pilot with an altimeter > set to the standard 1013 hPa therefore has a pressure error of 45 hPa. > There are about 10 m (30 ft) per 1 hPa. It will read '33 000 ft' when it is > around 31 500 ft. > > Over Portugal right now, the pressure 1033 hPa. A pilot with an altimeter > set to the standard 1013 hPa therefore has a pressure error of 20 hPa. It > will read '33 000 ft' when it is around 33 700 ft. > > >How did you come up with 33 000 ft = 26.2 kPa? > > http://mtp.jpl.nasa.gov/notes/altitude/altitude.html
