673. Airplanes can land at many different altitudes above sea level. How does the altimeter work at these different landing altitudes to show zero when the plane has finally landed? — BC, Canada
Their altimeters don't read zero once they have landed—they read the altitude of the airport! Each airport's altitude is reported on the navigational maps that pilots use. As the pilot approaches the runway, the pilot watches the altimeter and expects it to reach the airport's altitude about the time that the plane touches the runway. Before the next take off, the pilot adjusts the altimeter using the airport's official altitude as a calibration point for the altimeter. Some modern planes also used radar equipment to determine the distance to the ground beneath the plane. These devices do read zero at landing. The satellite-based Global Positioning System (GPS) also provides altitude information to pilots. Since this system reports altitude above sea level, it gives the altitude of the airport at landing, not zero.