Weather /

When a new student pilot first sees the instruments spread across the panel, the altimeter might seem to be the easiest one to understand. It tells you how high you are. Right? Unfortunately, it’s not that simple. The first thing an instructor probably explains is that an aircraft’s altimeter doesn’t tell you how high you are above the ground. It tells you how high you are above mean sea level (msl).

Why worry about mean sea level? The federal aviation regulations and air traffic control assign altitudes for pilots to fly, which help keep aircraft from colliding. Measuring altitudes at heights above mean sea level instead of above the ground makes this much simpler.

If you’re flying between airports under visual flight rules in an area where you’re not required to contact air traffic control, the rules say that if you’re lower than 18,000 feet above mean sea level, and you’re on a magnetic course of zero through 179 degrees, you should fly “any odd thousand foot msl altitude plus 500 feet (such as 3,500, 5,500, or 7,500).” If you’re on a magnetic course of 180 degrees through 359 degrees, you should fly “any even thousand foot msl altitude plus 500 feet (such as 4,500, 6,500, or 8,500).”

If altitudes were heights above the ground, trying to have airplanes follow ATC clearances or conform to the rule about even and odd altitudes would be nearly impossible. In addition, measuring an aircraft’s height above mean sea level is easier and less expensive than measuring height above the ground.

MEASURING HEIGHT ABOVE THE GROUND. To measure an aircraft’s height above the ground, you need a radio altimeter (also called a radar altimeter). Its antenna sends radio waves to reflect off the ground. The radio altimeter calculates height above the ground based on the time radio waves take to reflect back to the airplane. Radio altimeters work well only at relatively low altitudes, and they are expensive.

Airliners use radio altimeters as a part of autoland systems that allow safe landings in very poor visibility. The radio altimeter tells the autopilot when to begin flaring for a landing and controls the autothrottles. Radio altimeters also are the core of ground proximity warning systems that alert pilots that they are too close to the ground or a mountain peak.

MEASURING THE HEIGHT ABOVE SEA LEVEL.Ordinary airplane altimeters are called pressure altimeters because they use atmospheric pressure to measure height above sea level. In simple terms, an altimeter is a barometer—an instrument that measures atmospheric pressure—that has been calibrated to read in feet above sea level instead of inches or mercury or metric system millibars, as ordinary barometers do.

A barometer makes a good altimeter because atmospheric pressure decreases with altitude, as shown on this page, which shows the standard atmosphere for temperature and atmospheric pressure up to 10,000 feet above mean sea level. The numbers show that atmospheric pressure doesn’t decrease at a regular rate with altitude. Nevertheless, for relatively low altitudes such as the ones at which most general aviation airplanes fly, a common rule of thumb says atmospheric pressure decreases by one inch of mercury for each 1,000 feet in altitude gained.

You should use this figure for altimeter questions on FAA knowledge tests that require calculations. Knowledge tests probably are the only time you’ll need to use the one inch of mercury per 1,000 feet figure.

ALTIMETER DEFINITIONS. A good way to begin learning about altimeters is by learning the terms used in relation to measuring aircraft height above sea level.

Absolute altitude: The vertical distance of an aircraft above the terrain. Pilots usually use agl for “above ground level.”

Elevation: The height above mean sea level of an object or location that’s fixed to the ground, such as mountains or the tops of towers.

Indicated altitude: The altitude read directly from an altimeter after it is set to the current altimeter setting.

Pressure altitude: The altimeter reading when the pressure-setting scale is set to 29.92 inches of mercury, which is sea level pressure in the standard atmosphere.

True altitude: The true vertical distance of the aircraft above sea level—the actual altitude. Elevations of airports, terrain, and obstacles found on aeronautical charts correspond to true altitudes.

HOW ALTIMETERS WORK. Altimeters in most training aircraft are sensitive aneroid barometers. An aneroid is a flexible metal bellows from which some air was removed before it was sealed. Pressures lower than the inside pressure allow the bellows to expand; higher pressures squeeze it, making it thinner. A lever attached to one end of the bellows moves as the pressure changes to point at a number on a dial, indicating the surrounding atmospheric pressure.

Newer altimeters replace mechanical aneroids with pressure transducers, which convert pressures into electrical signals that can be used to show altitude in digital form instead of with the pointer of an aneroid altimeter.

SETTING AND READING AN ALTIMETER. You don’t have to learn much about weather to realize that the weather affects altimeters, because atmospheric pressure changes are associated with weather changes. Atmospheric pressures lower than the surrounding pressure are associated with foul weather because air is rising from surface areas of low pressure. As the rising air cools, its water vapor condenses to form clouds and maybe precipitation.

Air is sinking from higher altitudes to form surface areas of high pressure, which generally have clear skies because sinking air warms—evaporating clouds and keeping new clouds from forming. Pilots must adjust their altimeters to account for surface air pressures at airports they are using, and for the surface below when they are in the air.

You adjust your altimeter using a figure that all weather stations report called the altimeter setting, which you can obtain via radio if the airport has automated weather reporting or a tower with an automated terminal information service (ATIS). If you’re taking off from an airport without regular weather reports, you should use the nearest available report.

When you have the altimeter setting you turn the dial-setting knob seen on the altimeter on page 51 until the altimeter setting number is next to the pointer in the altimeter setting window in the figure.

When you set your altimeter to the correct setting, it should read the elevation of the ramp where you’re sitting. If it’s more than 75 feet off, your altimeter might need calibration. If a nearby altimeter setting isn’t available, set your altimeter to show the airport elevation when you are still on the ground.

You can think of the altimeter setting as the atmospheric pressure measured at the bottom of a hole dug down to sea level at the airport. Before computers, weather observers calculated the altimeter setting using the weather station’s elevation and the station’s actual barometric pressure, called the station pressure. Today’s automated weather observing systems do these calculations.