Answer Geek: How Planes Measure Altitude
<br> -- Q U E S T I O N: Well you have answered one of my two longtime questions about airplanes, which is how do you measure airspeed. Now, how do you measure altitude?
— Gordon O.
Q U E S T I O N: How does a plane determine its ground speed?
— Bob G.
A N S W E R: If you’ll remember back a couple of weeks, we spent some time looking at how automobiles and airplanes measure speed. (See related articles in right column.) That prompted a number of interesting questions — many from people who, like Gordon, were curious about how airplanes keep track of altitude, a very important thing to know if you are a pilot and wish to avoid an abrupt and unplanned landing against, say, the side of a mountain.
Technology provides a number of ways to measure altitude, but there is one instrument that almost all planes use, and it is called a barometric altimeter. This is a pretty simple device, and not all that different in concept from the device used by meteorologists to calculate barometric pressure down here on the ground.
But instead of measuring air pressure in inches of mercury, the altimeter gives a reading in hundreds and thousands of feet. To do that, the barometric altimeter contains a sealed bellows that expands or contracts as the plane climbs or descends and is subject to the change in air pressure that comes with a change in altitude. Gears translate the movement of the bellows into the movement of pointers on a dial, which shows the pilot the altitude of the plane in relation to sea level.
Weathering the Storm
All this raises yet another interesting question: How do pilots compensate for changes in air pressure that are related not to altitude, but to local weather conditions?
The answer depends on altitude. For planes flying at an altitude greater than 18,000 feet above sea level, there is one set of rules to follow. For those below, there is a different procedure.
Let’s start with those flying below 18,000 feet. Day-to-day differences in barometric pressure as weather systems come and go can mean a difference of a couple hundred feet on an uncorrected barometric altimeter. For planes landing or taking off, this can be a critical variation. To compensate, barometric altimeters have a knob that pilots use to adjust for the local barometric pressure. When a pilot is preparing to take off, standard procedure is to contact the control tower or the nearest weather service to get the current barometric pressure and then turn that knob to the proper setting. This yields an altitude reading that is quite accurate. For obvious reasons, it’s also pretty important to do that shortly before landing.
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