At Airport A, the altimeter setting is 30.74 inches of mercury. When the airplane’s altimeter registers 25.74 inches of mercury over Airport A, the altimeter will read 5,000 feet.
But, the pilot does not obtain the altimeter setting for Airport B, where the setting is 29.74 inches of mercury. The airplane is only 4,000 feet above sea level over Airport B.
Re-created from the AMS Weather Book: The Ultimate Guide to America’s Weather.
Measuring height above the ground
Sophisticated sensors give absolute altitude
To measure an aircraft’s height above the ground, you need a radio altimeter (also called a radar altimeter). It sends radio waves to reflect off the ground and calculates height above the ground using the time radio waves take to reflect back to the airplane. In addition to being expensive, radio altimeters work well only at relatively low altitudes.
Air carriers use radio altimeters as a part of autoland systems that allow safe landings in 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, and a primary component of terrain awareness warning systems, which alert pilots they are approaching the ground or a mountain peak. —JW
Altimetry definitions
Absolute altitude. The vertical distance of an aircraft above the terrain. Pilots usually use the term 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 hilltops or the tops of towers.
Indicated altitude. The altitude read directly from the 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.
Most altitudes in aviation are given in height above sea level. Determining height above sea level requires a simple barometer, which measures atmospheric pressure. An altimeter is a barometer labeled in feet above sea level.
Altimeters rely on the principle that atmospheric pressure decreases on average at a regular, known rate with altitude. For relatively low altitudes where most general aviation airplanes fly, a common rule of thumb says that atmospheric pressure decreases by one inch of mercury for each 1,000 feet in altitude gained. Standard atmospheric pressure at sea level is 29.92 inches of mercury. The actual atmosphere changes with the weather, and pilots must adjust their altimeters to account for the atmosphere in which the aircraft is flying. Pilots do this by using the latest local altimeter setting.
When you set your altimeter to the correct setting, it should read within 75 feet of the elevation of the ramp where you’re sitting. If it doesn’t, your altimeter might need calibration. If you are taking off from an airport without regular weather reports, you can turn the altimeter setting knob until the altimeter reads the airport’s height above mean sea level.
On a flight to another airport, you should regularly adjust your altimeter using information from airports along the way. Listen to the automated weather broadcasts of airports en route and your destination airport, or get the altimeter setting from air traffic control.
The wrong altimeter setting can produce dangerous errors in altitude readouts. Remember, “High to low (pressure), look out below.” When traveling into an area of lower pressure without changing your altimeter setting, you are closer to the tops of nearby hills or broadcast towers than you think. The “high to low, look out below” rule applies to temperature as well as to air pressure, but unless you are flying into extremely cold weather, you aren’t as likely to run into trouble from not setting your altimeter correctly.
PILOT SPEAK
In the soup
Campbell’s has nothing to do with this. The soup in this instance is the clouds that envelop you and your aircraft and make you realize getting an instrument rating truly makes you a better pilot.
Check your six
This means to look behind you (the six o'clock position) for possible problems. When someone says he or she has “got your six” it means he or she has your back; that person will take care of you. See, people can be nice.
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