Answer Geek: How Planes Measure Altitude

-- 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.

Because planes flying below 18,000 feet can be assigned to flying levels that vary by 500 feet — for example, if there are three planes flying in the same area, one might be told to fly at 15,000 feet above sea level, another at 15,500 feet, and the third at 16,000 — pilots flying below the 18,000-foot level have also have to adjust their altimeters regularly, usually on the order of once every 100 miles or so. There is another reason to keep the altimeter in adjustment at lower altitudes, of course — the possibility of running broadside into a very tall building or a mountain.

Above 18,000 feet, normal procedure is to adjust the barometric altimeter to a standard pressure of 29.92, regardless of the local pressure. Planes cruising at that altitude are assigned flying levels in 1,000-foot increments. Because all planes use this uniform “pressure altitude,” there is no danger that two planes in the same area will be flying at the same altitude.

These days, the barometric altimeter is not the only device pilots use for ascertaining their altitude. There’s also something called a radio altimeter, which bounces a radio signal off the ground and measures the return time. It then calculates a plane’s altitude in relationship to the ground below (which can be very different from height above sea level). Radio altimeters are sometimes called “absolute altimeters” or “terrain clearance indicators.”

Well-Grounded in Fact

A number of readers also wrote to ask about groundspeed. In addition, several pilots pointed out that what I described in my earlier column (“Taking Flight”) was what they call “indicated airspeed” and that there are a number of other types of airspeed that pilots keep track of, including “calibrated airspeed,” “equivalent airspeed,” and “true airspeed.” These are all quite fascinating if you are a pilot, but a little outside the scope of this particular column.

Groundspeed, on the other hand, is worth a look, because if you don’t know what your groundspeed is, you can’t calculate how long it will take to get from one airport to the next, which is an important factor in determining whether you have enough fuel to reach your intended destination.

Simply put, groundspeed is true airspeed corrected for the effects of the wind. True airspeed is the indicated airspeed corrected for the effects that temperature and pressure have on the airspeed indicator. (See the “Taking Flight” column for more on that.) So, if your true airspeed is 400 mph and there is a 100-mph tail wind, your ground speed is 500 mph. Easy enough.

Increasingly, groundspeed, altitude, and other important bits of data that are essential to the safe flying of an airplane are provided by Global Positioning Systems and other electronic navigation systems, which feed all that information directly into an onboard computer. High-tech military aircraft have even more elaborate instrumentation, tracking groundspeed with Doppler radar, for example, and using a navigational device that fixes on the sun or a star to provide directional information to the plane’s computers.

All that high-tech stuff is great, but I say you still want that good old-fashioned barometric altimeter and a pilot who remembered to adjust for the local air pressure. Because if something goes awry with all those cool gadgets, a well-tuned barometric altimeter is your best friend when it comes time to land.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.