Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 56

True altitude is the actual altitude above mean sea level and is referenced as mean sea level (MSL). On most aeronautical charts, MSL altitudes are published for man‐made objects such as towers and buildings, as well as for terrain, since this is the altitude closest to the altitude read off the altimeter. An important note is that true altitude will only be the same as indicated altitude when flying in standard conditions, which is very rare. When flying in conditions colder than standard, the altimeter will indicate a higher altitude then you are flying, so true altitude will be lower than indicated altitude. The same dangerous situation can develop when you are flying from a high‐pressure area to a low‐pressure area and the altimeter is not corrected for the local altimeter setting. Your altimeter will interpret the lower pressure as a higher altitude and your true altitude will again be lower than your indicated altitude. From the variations in true altitude versus indicated altitude, the saying was developed “high to low, or hot to cold, look out below.” Of course, this assumes that the altimeter is never reset to local pressure for an entire flight covering a long distance with varying temperatures and pressures.