Understanding altitude and altimeters
A beginning student pilot could make the reasonable assumption that altitude refers to height above the ground because pilots need to know this to avoid running into things. This height is important, and pilots generally refer to it as the height above ground level (AGL). It's sometimes called absolute altitude. But, it isn't what air traffic controllers mean when they tell you fly at a certain altitude, and it's not what airplane altimeters measure.
Why not?
Thank about flying across the Rockies while staying 5,000 feet above the "ground," which includes peaks with steep sides. To avoid zooming and diving to follow the ground, an aircraft's true altitude is defined as its height above mean sea level (msl).
This works out well because long before airplanes were invented people measured the heights of hills and mountains-their elevations-in feet or meters above sea level. Aeronautical charts not only include such elevations, but also indicate the highest elevations in each grid on the chart.
Once you've decided to use height above mean sea level as the true altitude, you need a way to measure it. Fortunately, long before the Wright brothers came along, scientists climbed mountains carrying instruments that measure air pressure-called barometers-confirming the theory that air pressure decreases at a regular rate with elevation.
A barometer can be used to measure how high you are above sea level. Despite some complications, this works very well. This brings us to the first rule for understanding altimeters: An altimeter is a barometer that senses only one thing-the pressure of the air around the aircraft. This means, of course, that since pressure decreases as you go higher, an altimeter "sees" any decrease in air pressure as a gain in altitude-whether it's caused by the aircraft climbing or by a change in the weather.
In the United States, inches of mercury are commonly used to measure barometric pressure, including the altimeter settings that weather observers report. The term comes from the mercury barometer, a tube that's closed at one end. It's filled with mercury and turned so the open end at the bottom is in a small container of mercury that's open to the air. Near sea level, air pressure pushes the mercury about 30 inches up in the tube. The height goes up and down with air pressure.
At sea level, on the average, air pressure pushes the mercury 29.92 inches up into the tube, which is why 29.92 inches of mercury is the standard atmospheric pressure at sea level.
For the lower 10,000 or so feet of the atmosphere you can use a rule of thumb that says atmospheric pressure drops by one inch of mercury for each 1,000 feet of altitude gained. In other words, if you took off from a sea-level airport with a pressure of 29.92 inches, your altimeter would sense a pressure of 28.92 inches of mercury at an altitude of 1,000 feet; 27.92 at 2,000 feet, and so on. (The altimeter formula, not the rule of thumb, is used to relate air pressures to altitudes. Using it, you'd get a reading of 20.57 inches of mercury for 10,000 feet, which is close to the 19.92 you get using the rule of thumb.)
Altimeters, of course, are marked in feet, not inches of mercury. Since a long-tube-of-mercury barometer isn't practical in an airplane, an altimeter uses an aneroid, a mechanical device that senses air pressures and uses gears and levers to move the pointers on the instrument's face.
Now, let's see how altimeters work and how we use them.
Imagine that your airplane is based at any airport at sea level. On a nice, average day, the air pressure at the airport is 29.92 inches, and your altimeter is reading 0 feet on the ground. When you climb to 1,000 feet the altimeter senses a pressure of 28.82 feet and reads 1,000 feet. This brings us to the second rule of understanding altimeters: A correctly calibrated and set altimeter reads the airport's elevation when the airplane is on the ground.
Now, imagine going out to your airplane at the sea-level airport to check your airplane on a stormy day when the airport's barometer is reading 29.42 inches. When you check the tiedown chains, you glance into the cockpit to see the altimeter is reading 500 feet. If you did go flying that day without adjusting the altimeter, it would continuously read 500 feet high.
After the storm, when you get into the airplane, you see that the altimeter needle is below the zero mark; the air pressure is now 30.02 inches and the altimeter is trying to read minus 100 feet since an altimeter "reads" higher pressure as lower altitude. To correct this, you turn the altimeter adjusting knob until it reads 0 feet-your sea-level airport's elevation. You see that the pointer in the Kollsman window is at 30.02.
The Kollsman window is named for Paul Kollsman, who invented the first accurate altimeter. One of these was in Jimmy Doolittle's Consolidated Husky on September 24, 1929, when he made the first "blind" flight. (We'd call it an instrument flight today.) Invention of an accurate altimeter, the gyroscopic artificial horizon, and radio navigation made Doolittle's flight possible.
Setting your altimeter to read field elevation on the ground is fine, but what if you went flying for a couple of hours and the weather changed? How would you set your altimeter in the air?
A pilot at the sea-level airport could check the altimeter of an aircraft on the ground and radio the current barometer reading to you; you'd turn the knob until the pointer in the Kollsman window agrees with the reading, and when you land-assuming both altimeters are working correctly-yours will read zero.
Don't do this. You have no idea of how accurate the other pilot's altimeter is, or how good he is at reading the figure in the Kollsman window. Instead, use the altimeter setting reported by a weather observer or an automated weather station. You can safely use an altimeter setting from a few miles from where you're landing because in weather that's safe for flying, the pressure doesn't change much over a few miles. Large pressure changes over short distances create strong winds.
What about altimeter settings for airports above sea level? Remember, altitude is measured from sea level. You can't use the actual air pressure at an above-sea-level airport because an altimeters set to 29.92 inches would "think" it's a sea-level reading, since altimeters are calibrated to read altitude above sea level. Weather observers or automated stations measure the barometric pressure-called the station pressure-and use it to calculate what the sea level pressure at that time and place would be.
The third rule for altimeters is: The altimeter setting is what the barometric pressure would be at sea level if it could be measured at sea level.
Now we're ready to understand how pressure changes between airports affect what your altimeter tells you. Pressure changes can result in your indicated altitude-what the altimeter reads-being higher or lower than the true altitude, which is the actual altitude above sea level. The figure shown at left will help you to solve any problems dealing with the differences between indicated and true altitude.
In the figure, Points A and D are airports at least a couple of hundred miles apart, and to keep it simple, at sea level. Lines G, H, and I are altitudes that airplanes might fly between A and D. Imagine you tell someone at a party that you're a pilot and he says, "There's something I've always wondered about: If you fly from an area of high pressure without changing your altimeter setting, when you arrive would the altimeter indicate: (a) lower than the actual altitude above sea level; (b) higher than actual altitude above sea level; or (c) the actual altitude above sea level?
Instead of just popping off with an answer, you think a little about how the atmosphere and altimeters work, and then sketch the figure on a cocktail napkin. Make airport A the one with high pressure, say 30.00 inches of mercury, and D the airport with a lower pressure, say, 29.80. You want to fly at 3,000 feet, which means that when you set your altimeter at 30.00, it reads zero on the ground at A and reads 3,000 feet when it senses a pressure of 27.00 (30.00 minus 3 inches for 3,000 feet) at point C.
At Airport D, with a setting of 29.80, an altimeter will read 3,000 when it senses a pressure of 29.80 minus 3 inches, or 26.80.
At what altitude over D will a pressure of 27.00 be found? Subtracting 27.00 from 29.80 (D's altimeter setting) gives you 2.80 times 1,000 (feet for each inch) for an altitude of 2,800 feet. This is point E in the figure.
You confidently tell the guy at the party that the answer is "b) higher than actual altitude above sea level." In other words, since the altimeter is reading higher than the actual altitude above sea level, the airplane will be lower than the altimeter says. It's closer to the tops of hills and television towers than the pilot thinks -- obviously a dangerous situation.
This is because an airplane flying from A to D without changing the altimeter setting and holding a constant indicated altitude of 3,000 feet would follow line H from C to E. This line represents a plane of constant pressure in the atmosphere. A pilot who changes the altimeter setting along the way would follow a line close to I, from C to F.
This method of altimeter problem solving also works quite well for FAA knowledge tests. You can sketch the figure we've included on scratch paper; just don't bring it into the exam room already drawn.
The figure and the simple math we did help to illustrate what is meant by "High to low, look out below." If you fly into lower pressure without changing the altimeter, that means you'll be lower than you think you are. You'll be closer than you realize to hilltops and television antennas.
They will also help you see what happens when you fly from low to high pressure, such as from E to C back along line H, at an indicated altitude of 2,800 feet without dialing in a new setting. Line G from E to B is the path of an airplane using the correct altimeter settings along the way.
We now have the fourth rule for understanding altimeters: The 1,000 feet equals 1 inch change in barometric pressure rule of thumb, some simple math, and a sketch like the figure on page 55 will tell you the difference between true and indicated altitude. Air temperature changes, like pressure changes, complicate understanding and using altimeters. Again, the figure will help you see what's going on.
As air heats or cools it expands or contracts like other substances. Think of the line from A to C in the figure as a column of warm air. When it expands, any particular air pressure-such as 27.00, which was 3,000 feet in the question we answered above-moves up. That is, a particular pressure level is higher in warm air than in air at a "standard" temperature.
As warm air expands, the total amount of air in the column remains the same, which means the sea level pressure, and thus the altimeter setting, does not change. Line D-E in the figure represents what happens when the air is colder than normal. The true altitude of any particular pressure is lower, but the altimeter setting can remain the same. In other words, our figure can also represent a situation where the altimeter setting is the same at airports A and D, but a pilot flying from A in the warm air to D in the cold air would be lower than the indicated altitude, even if the correct altimeter setting is used.
Our fifth rule for understanding altimeters is: High to low, watch out below, and it works for high and low temperatures as well as pressures. Altimeters don't have any adjustment for temperature, although you can calculate these changes using a flight computer. Such calculations are approximations at best because you don't have enough temperature data to get a truly accurate picture of the column of air under your airplane.
As we've seen, accurate altimeters made instrument flying possible and are important today for pilots who'll never fly without a clear view of the ground. Correctly setting your altimeter and understanding what it's really telling you are important for safe flight, even in clear weather. The five rules will help you to guard against the dangerous lies that altimeters can sometimes tell you.
Jack Williams is the weather editor of USAToday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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