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Re: Friday, August 4, 2006
Hey Doc, From the Trivia Department-- "Flight Level 370" is not technically 37,000 feet above sea level, as one might think. Instead, it is the altitude at which a plane's altimeter will read 37,000 feet when the sea level pressure is set to 29.92 inches of mercury, which is considered the "standard pressure". The actual sea level pressure at a given location varies day by day, hour by hour, which is why when planes are flying at lower altitudes (and most importantly, when they're about to land) they need to be told the actual pressure at the nearest available location so they can set their altimeters to local conditions and get an accurate reading of their altitude. At high altitudes, however, it is less important that they be at an accurate altitude than that they all be at the same altitude relative to each other. Since they don't all want to be resetting their altimeters every ten minutes to reported local pressures, instead they all agree that when they fly above 18,000 feet they all set their altimeters to the standard pressure of 29.92, and then their altimeter reading becomes their "flight level", and it is described with the last two zeroes truncated. So, to use your example, Flight Level 370 is the altitude at which a plane's altimeter reads 37,000 feet above sea level once its been set to a pressure of 29.92. This altitude could be substantially above or below 37,000 feet depending on whether the actual pressure is below or above 29.92, and as the plane flies into areas of higher or lower pressure, its actual altitude goes up or down even as its altimeter continues to read 37,000 and it considers itself to be flying consistently at Flight Level 370. More than you wanted to know, huh! Cheers, Steve Tulsky
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