Two minor points: At 18,000 feet (varies in some areas) aircraft change their altimeters from whatever was at the nearest ground station to the standard setting of 1013.2. This way everyone at higher altitude is using the same standard and, assuming they are properly calibrated, all altimeters read the same.
Unless the ground setting is also 1013.2, the airplane will not be at the actual height indicated; therefore, altitudes above 18,000 feet are called Flight Levels. FL 180, FL 190, etc. Pressure altimeters lose accuracy when the air gets thin. For this reason, things change at FL 290. Above that, it's FL 310, 330, 350, 370, 390, etc. 290/330/370/410 etc. are eastbound, 310/350/390 etc. westbound. Note that there is no FL 300; therefore, cub reporters writing about an airplane being at "30,000 feet" are wrong, unless the plane just happens to be passing through that altitude in a climb or descent. But it sounds nice in a story. Why feet and not meters? After World War II the aviation industry in mainland Europe was devastated, and the USA (primarily) and the UK could have their way. Aviation in the USA was in fairly good shape because it hadn't had the bejeesus bombed out of it like what happened in Europe. There is some talk about using the intermediate flight levels above 290 now, with more accurate instrumentation (search around the web to find out more). But every time talk happens about US aviation going to meters, someone brings out the scary weather forecast of possible widely scattered aluminum. Carleton -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Terry Simpson Sent: Saturday, December 13, 2003 11:18 To: U.S. Metric Association Subject: [USMA:27823] RE: Airplane altitudes > Of Nick Kocharhook >I've often thought that 33,000 feet is a pretty >strange number for athe cruising altitude It is in 'thousands of feet'. Odd thousands for aircraft going east. Even thousands for aircraft going west. The numeric value does not actually describe altitude, it describes the pressure of the air through which the aircraft is flying. As the surrounding air pressure rises and falls, the aircraft will climb and descend to maintain the same pressure. There are regions of the world where altitude is expressed in metres. Aircraft change from metres to feet and back as they fly in and out of these regions, of course. There are also regions where both are in use at the same time e.g. military using metres, civilian using feet. >proximity to 10 km more than just coincidence? It is a coincidence. >And, why don't they tell us that we'll be cruising at 6 and a quarter miles? An excellent question. * If you use a wide range of values, it is best to stay with the same units. You avoid the changes and conversions as you move up and down the range. * All communication will be in the same expected units so errors are less likely. * Equipment can be a simple. * You do not distribute learning a 'feel' over several different units, your learning of 'feel' is focussed on one. This is one of the key features of the metric system. Each quantity has only one unit.
