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The Weather Never Sleeps
Truth or consequences
Understanding altitude and altimeters

On the face of it, understanding altitude sounds simple-it's how high you are in the
air. As many flight students have discovered, however, altitude is far from simple,
especially when they have to answer knowledge test questions about the topic. To
basic concepts and some simple ways to keep confusion at bay.

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?

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.
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
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?

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 "(a) lower than indicated
altitude."

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

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.

By Jack Williams
No Dumb Questions
Q.My airplane has an encoding altimeter. Can I "adjust" the
altitude it reports (through the transponder) to ATC by
changing the setting in the altimeter's Kollsman window?
A. No. Whether an aircraft has an encoding altimeter or a blind
encoder, the altitude that either device feeds the transponder
for transmission to ATC is always based on an altimeter setting
of 29.92. If pilots could adjust their encoded altitude, it would
effectively disable the purpose of Mode C altitude reporting
and TCAS (traffic collision avoidance system) - vertical
separation of aircraft.
As pilots well know, altimeter settings change with location, and if a pilot forgets to
reset the altimeter to the local setting, the altimeter will read either above or below
the actual altitude. Using 29.92 as the basis for encoded altitude precludes this
problem because all aircraft in the same area will be reporting the same altitude
regardless of the actual altimeter setting. ATC computers adjust encoded altitude
from 29.92 to the local altimeter setting before it reaches the controller's radar
screen. ATC must verify the encoded altitude's accuracy, so if a pilot doesn't report
his altitude in 100-foot increments when checking in with ATC, the controller will
request it
Encoded altitude is the basis for TCAS, installed on air carrier and many turbine-
powered business aircraft. A TCAS unit examines all the transponder signals from
aircraft around it, looking for those that are at the same encoded altitude, or
converging. When it finds one that poses a potential collision threat, it warns the
pilot and suggests specific evasive action.
An encoding altimeter has the encoder contained within the altimeter, while a blind
encoder is separate from the altimeter. In the end they both do the same thing.
Using digital information supplied by the encoder, the transponder transmits the
aircraft's altitude to ATC in 100-foot increments. If ATC says your encoded altitude
doesn't match your reported altitude, your first action should be to check your
altimeter setting (the barometric pressure set in the altimeter's Kollsman window) to
make sure it's correct. If your altimeter setting is correct, your altitude encoder is
not transmitting the correct altitude. If the difference is 300 feet or more, ATC likely
will ask you to "Stop altitude squawk; altitude differs (number of feet) feet."
Recycling the transponder by turning it off then on may fix this problem. If it doesn't,
the airplane must visit the avionics doctor for pitot-static, encoder, or transponder
repair.

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