Flight Training - July 2019 - 17

HOW IT WORKS

BY JULIE SUMMERS WALKER

THE ALTIMETER
HOW HIGH IN THE SKY ARE YOU?

IT LOOKS LIKE A CLOCK, YOU READ IT LIKE A CLOCK, BUT IT'S

not a clock. The big hand is at hundreds of feet, the little hand
is at thousands of feet. If there's a small white line, it reads
in tens of thousands of feet. What is this? It's the altimeter,
which proved its worth in aviation on the first "blind" instrument flight by James "Jimmy" Doolittle in 1929. (He would
later lead the daring attack on Japan from the aircraft carrier
USS Hornet after the United States entered World War II.)
The sensitive altimeter still used in aviation was patented
in 1936 by Paul Kollsman, its inventor. The measurement of
altitude is altimetry (bonus question: What is the measure
of water depth? Bathymetry). The altimeter measures the

height of an aircraft above a fixed level. The instrument
senses this by taking the ambient air pressure from the static
port. That air is plumbed through the back of the panel and
into the back case of the altimeter. Inside the altimeter is a
sealed disc called an aneroid, or bellows. As the aircraft goes
up, the pressure inside the case increases and the bellows
expand. The opposite happens as the aircraft descends. The
bellows are mechanically connected to the face of the instrument through gears. It is Kollsman's invention that assists
the pilot: The window on the front of the instrument allows
the pilot to set the altimeter to the current local pressure, so
it will display an accurate height above sea level.

10,000-foot pointer

1,000-foot pointer

Aneroid
wafers
Gears
Kollsman window
(altimeter setting)

Static port

100-foot pointer
Adjustment knob

ALTITUDE ATTITUDE
True: Actual height above sea level.
Indicated: What's read on the instrument.

STEVE KARP

Absolute: Height above ground level (agl).
Pressure: What's read on the instrument with the pressure
set to standard (29.92 inches).
Density: Pressure altitude corrected for nonstandard
temperature (what the aircraft thinks the altitude is).
AOPA.ORG/FT  17



http://www.AOPA.ORG/FT

Flight Training - July 2019

Table of Contents for the Digital Edition of Flight Training - July 2019

Contents
Flight Training - July 2019 - Intro
Flight Training - July 2019 - Cover1
Flight Training - July 2019 - Cover2
Flight Training - July 2019 - Contents
Flight Training - July 2019 - 2
Flight Training - July 2019 - 3
Flight Training - July 2019 - 4
Flight Training - July 2019 - 5
Flight Training - July 2019 - 6
Flight Training - July 2019 - 7
Flight Training - July 2019 - 8
Flight Training - July 2019 - 9
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Flight Training - July 2019 - 11
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Flight Training - July 2019 - 16
Flight Training - July 2019 - 17
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Flight Training - July 2019 - Cover3
Flight Training - July 2019 - Cover4
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