The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of local barometer derived altitudes to the use of flight levels. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level.[3] Above the TA, the aircraft altimeter pressure setting is normally adjusted to the standard pressure setting of 1013.25 hectopascals (which is equivalent to millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level.