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Altimeters inform the pilot the altitude of the aircraft. The information provided by an altimeter ranges from important for Visual Flight Rules (VFR) flying to critical for an instrument approach. The altimeter is a device that senses atmospheric pressure. As an aircraft climbs though the earths atmosphere, the pressure decreases. The altimeter detects this reduction in pressure and displays the reading by mechanical or electronic means to the pilot.
Prior to takeoff, pilots will set the local altimeter reading (current atmospheric pressure provided by a weather station on the airfield and is measured by inches of mercury (Hg)). This will set the altimeter to the Mean Sea Level (MSL) of the airfield (also referred to as true altitude). After takeoff, if the aircraft is under Instrument Flight Rules (IFR) or radar control, the pilot will use the altimeter to climb to the assigned altitude.
Anchorage Departure Control: "Dice 01, radar contact climb and maintain 12,000 feet."
In this example, the pilot will climb and level the aircraft at 12,000 feet on the altimeter. The key to this procedure is having the proper altimeter setting. Since the altimeter senses pressure, it can only provide the correct MSL altitude with the correct local altimeter setting.
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Updated: March 12, 2004