Altimeter

The altimeter is a sensitive barometer that measures air pressure. It's calibrated to display that air pressure as height, usually in feet above mean sea level (MSL).

How It Works

The altimeter is connected to the static ports. The air pressure inside the instrument case decreases as the airplane climbs and increases as it descends. As the pressure in the case drops, sealed wafers in the instrument case expand. Increasing pressure squeezes the wafers. As the wafers expand and contract, needles connected to them rotate around the altimeter dial like hands on a watch.

Reading the Altimeter

Most small aircraft are equipped with two-needle altimeters. The long needle shows hundreds of feet. The short needle points to thousands of feet. A wedge-shaped striped indicator appears whenever the current altitude is less than 10,000 ft (3,048 m).

For example, if the long needle is on 5 and the short needle is between the 2 and 3, you're at 2,500 ft (762 m) MSL. If the striped indicator isn't visible, the same needle orientation shows that you're at 12,500 ft (3,810 m) MSL.Jets and other high-performance aircraft typically have "needle and drum" altimeters. A long needle shows hundreds of feet and an odometer-like display shows altitude in numerical form.

Setting the Altimeter

To display altitude accurately, the altimeter must be set to the current barometric pressure adjusted to sea-level pressure. This setting appears in the Kohlsman window—the scale between the "2" and "3" on the dial. Before takeoff, the pilot turns a setting knob to set the correct pressure. When properly set, the altimeter indicates the airport elevation—not zero—before the airplane takes off.

Pilots can get the current altimeter setting from ATIS broadcasts, air traffic controllers, and Flight Service Stations (FSS). If one of these sources isn't available, the pilot should set the altimeter so that it displays the elevation of the departure airport.

TIP: To set the altimeter to the current atmospheric pressure, press B. For a realistic flying experience, you can make the atmospheric pressure change gradually in Flight Simulator as you fly. For more information about this feature, see the topic "Changing Barometric Pressure" in the Using Flight Simulator section of Help.