Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 54

Indicated altitude is the altitude that is read directly from the altimeter and is uncorrected for any errors. In the United States, below FL180 the altimeter is set to the current altimeter setting of the field you are departing from or arriving to, or is given by air traffic control for the current area you are flying in. In the United States, when flying at or above 18 000 ft, altitude is measured in Flight Levels (e.g. FL180 for 18 000 ft). At FL180, the indicated altitude will be equal to pressure altitude as the altimeter setting is set to 29.92″, standard pressure, or QNE. The altitude at which the crew changes to 29.92 is called the transition altitude (TA). When the crew descends for landing, the altitude at which they return the altimeter setting to local barometric pressure corrected to sea level (QNH) is called the transition level (TL). (Remember it this way: 29.92 is selected at the TA, and the “A” stands for aloft, as in climbing or cruise. When returning to land, the TL is set on descent, and “L” stands for low, or landing.)

When QNE is lower than 29.92, the lowest usable flight level is no longer FL180. The lowest usable FL is obtained from the aeronautical publications. For instance, in the United States, if the pressure in the area of operations is between 29.91 and 29.42″, the lowest usable enroute altitude is FL185. It should also be noted that the TA and TL outside the United States will not always be 18 000. ICAO members set their own values.

Incidentally, QFE is the reference pressure set in the altimeter if the pilot wishes to know the elevation above the airfield. When the aircraft is on the airfield, the altimeter reads zero. QFE is seldom used as it would be of limited value when away from the immediate vicinity of the airfield.