A measured or calibrated height above the ground or above sea level.
The standard aircraft altimeter is an aneroid (without liquid) barometer that measures the ambient or static air pressure outside the airplane. It is calibrated through the use of a Standard Atmosphere model so that it presents this pressure to the pilot as an altitude. Because the air pressure on the ground varies a great deal with the movement of air masses across the country, an offset can be introduced by the pilot to make the indicated altitude equal to the actual altitude of an airport before takeoff and while approaching to land. The offset, if any, is indicated by the reading in a window, known as the Kollsman window, on the face of the altimeter.
The Standard Atmosphere model is based on an arbitrarily chosen, midlatitude, average value for the pressure, temperature, humidity, and density of the air at sea level. It assumes a sea-level pressure of 29.92 inches (76.00 centimeters) of mercury, a sea-level temperature of 59 degrees Fahrenheit (15 degrees Celsius), 0 percent humidity, and an air density calculated from the ideal gas law. It further assumes that the temperature decreases linearly with an increase in altitude at the rate of 3.566 degrees Fahrenheit for every 1,000 feet for the first 36,000 feet, the troposphere, and then is constant, the stratosphere. These assumptions, along with the gravitational and thermodynamic laws, yield the Standard Atmosphere, uniquely defining the “standard” air pressure, density, and temperature at every altitude.
It should be noted that pressures expressed as a height of mercury are not using true pressure units but are reflecting a common way to measure pressures. An accurate mercury thermometer can be made by bending a 6-foot-long glass tube into the shape of an upright “U,” filling it half-full of mercury, and attaching a vacuum pump to one end. Atmospheric pressure at the other end then pushes the mercury down, and the difference in mercury heights is a direct measure of the atmospheric pressure. Pressure expressed in inches of mercury can be converted to pressure expressed in units of pounds per square inch (psi) by multiplying by 70.73.
Because the altimeter is calibrated in feet of altitude, or in kilometers in Europe and elsewhere, but is really measuring only atmospheric pressure, the actual air pressure can be obtained by consulting tables of the Standard Atmosphere. This is important because the performance of engines and airplanes depends directly on the air density, and the only way to determine air density is to calculate it from the measured air pressure and temperature, using the ideal gas law. Pilots adjust their altimeters so that 29.92 appears in the Kollsman window—that is, with no offset from the Standard Atmosphere—and the indicated altitude is then called the pressure altitude.
Whenever a pilot is flying above an indicated altitude of 18,000 feet, the altimeter must be set to 29.92 (no offset) to simplify vertical separation of aircraft. All aircraft flying in busy airspace are also required to use transponders that report both position and pressure altitude to air traffic control.
When the pressure altitude is combined with the outside air temperature through the ideal gas law, the density of the air can be calculated. It is most convenient to express this air density in terms of the altitude in the Standard Atmosphere, which is defined to have this density. This calculation is called the density altitude. The performance of airplanes, in terms of the available engine thrust or power, takeoff distance, climb rate, cruise speeds, and landing distances, depends directly on density altitude, or air density, and is specified as such in aircraft flight manuals. It is very helpful for a pilot to realize intuitively that low pressures (especially due to high elevations) and high temperatures result in very low air density (high-density altitudes) and that in high-density altitudes, aircraft performance will be greatly reduced from sea-level values.
Density altitude is easily calculated from the pressure altitude and the temperature using either an E6-B circular slide rule or an electronic calculator. The current density altitude is also broadcast at many high-altitude airports.
When approaching to land, it is important to enter the appropriate offset, or altimeter setting, into the altimeter, so that it will both give guidance regarding obstacle clearance on the approach and read field elevation after landing. However, usually the variation of pressure with altitude above the airport will not follow the Standard Atmosphere model, and the indicated altitudes of obstructions will still not exactly equal their true altitudes.
Air pressure varies more rapidly with altitude if the air is colder than that assumed by the Standard Atmosphere. Therefore, if a pilot flies into air that is colder than standard, or if the altimeter has not been adjusted en route while flying toward a region of lower pressures, the airplane’s true altitude is lower than the indicated altitude, and safety may be compromised, especially in mountainous terrain. Unstable weather conditions and high winds around mountain ranges can also produce locally lower air pressures that result in erroneously high indicated altitudes.
The radar altimeter measures an aircraft’s height above the ground by measuring the time it takes a radio wave to return, using the known speed of light. Transport and other complex aircraft find the radar altimeter to be a valuable aid in avoiding obstructions during the landing approach.
Altitude can also be derived by geometry from three or more satellites in the Global Positioning System (GPS) used for navigation. This may become the preferred altimeter for high-altitude, oceanic flight.
Barnard, R. H., and D. R. Philpott. Aircraft Flight. 2d ed. Essex, England: Addison-Wesley Longman, 1995. An excellent, nonmathematical text on aeronautics. Well-done illustrations and physical descriptions, rather than equations, are used to explain all aspects of flight. Gleim, Irvin N. Federal Air Regulations and Aeronautical Information Manual. Gainesville, Fla.: Gleim Publications, 1999. A republication of official Federal Aviation Administration information for pilots, with discussion of altimeter errors and setting procedures. U.S. National Oceanic and Atmospheric Administration. U.S. Standard Atmosphere, 1976. Washington, D.C.: U.S. National Oceanic and Atmospheric Administration, 1976. Covers the basics for computation of atmospheric properties, the elemental constituents of the atmosphere, and tables of atmospheric properties to 1,000 kilometers.
An altimeter measures an aircraft’s altitude in feet, thousands of feet, and tens of thousands of feet.