Types of Altitude

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The altimeter in an airplane is designed to show height above sea level, MSL. The instrument is calibrated to show that height under standard atmospheric conditions. The current temperature and pressure rarely match standard conditions, however, so pilots must understand several types of altitude and know how to correct for altimeter errors caused by nonstandard conditions.

	Indicated altitude is the altitude shown on the altimeter. If the altimeter is set to the current atmospheric pressure corrected to sea level, indicated altitude is approximately equal to the aircraft's height above sea level (MSL).
	Pressure altitude is the altitude shown on the altimeter when the pressure is set to 29.92 in. of mercury (or 1012.2 mb). Pressure altitude is important in calculating density altitude, a critical factor in determining aircraft performance, true airspeed, and true altitude. In the U.S., aircraft fly at pressure altitudes or "flight levels" when operating at or above 18,000 ft MSL(5,486 m). That's why you must set the altimeter to 29.92 whenever you fly at or above 18,000 ft or FL (flight level) 180 (5,486 m).
	Density altitude is pressure altitude corrected for deviations from standard temperature. You must calculate density altitude to determine how much runway your airplane will need to take off and land and its rate of climb. Calculating density altitude is especially important on a hot day when you're operating from an airport with an elevation well above sea level.
	True altitude is your actual height above sea level. If you set the altimeter to local pressure corrected to sea level, indicated altitude is approximately true altitude.
	Absolute altitude is your height at any instant above the terrain. Unless your aircraft is equipped with a radio or radar altimeter, you must estimate absolute altitude by comparing your indicated altitude with the terrain elevations shown on charts.
	Radio (radar altitude) Large aircraft are equipped with radio or radar altimeters that display absolute altitude. Pilots use radio or radar altitude during the final phases of approach and landing, particularly when the ceiling and visibility are low, to help them determine decision height.

Altimeter Errors

The altimeter is calibrated to display the correct height above mean sea level when the temperature and pressure of the atmosphere match standard conditions.Variations in temperature usually don't cause significant errors, but if atmospheric pressure doesn't change at the standard rate, the altimeter won't display the correct altitude unless the pilot periodically adjusts the altimeter setting to the local atmospheric pressure (corrected to sea level). In fact, FAA regulations require you to use the proper altimeter setting as you fly (see FAR 91.121).

From a High to a Low

For example, suppose the altimeter is set to 30.10 inches before takeoff. If the airplane travels to an airport surrounded by a low-pressure system and the pilot does not change the altimeter setting, the altimeter senses the lower pressure as higher altitude. In other words, the altimeter shows an altitude higher than the airplane's actual height above sea level.

Although the pilot thinks the airplane is at the correct altitude, it may be in conflict with other aircraft in the area whose pilots are using the correct local altimeter setting.

Cold Day - Warm Day

Because warm air has greater volume than cold air, a day that is warmer than standard (15 degrees celsius - with a 2 degree per 1000 foot lapse rate) will result in an altimeter reading that is lower than true altitude. On days that are colder than standard the reading will be higher.