User Guide

THE ALTIMETER
The third instrument in the top row on the right
is the altimeter. Basically a glorified barometer,
this utilizes air pressure to read the airplane’s
altitude above sea level - not above the
ground. In other words, you could be flying
along near Denver with the altimeter read-
ing a comfortable 6000 feet...but you’d be
only about 700 feet above the ground (or
well below it once you got a few miles west).
There are three clock-like hands. The big one reads hun-
dreds of feet; the small one, thousands, so if the altimeter were
reading “half past three” you’d be at 3500 feet above sea level.
The smallest hand - the one that looks like a little triangle at the
outer edge of the scale - reads tens of thousands; with the 172’s
modest ceiling, you’re unlikely to see it much beyond “half past
one.”
Finally, there’s a little setting window at the 3 o’clock posi-
tion, controlled by a small knob at the 7 o’clock point. This is
called the Kollsman window, because the first altimeters to have it
were made by that firm; it’s become a generic term, like Kleenex
or Ductape. Since the altimeter measures barometric pressure,
which changes with the weather, the Kollsman window is used to
compensate for those changes by setting in the local barometric
pressure; otherwise, the altimeter could be in error by several hun-
dred feet. This can be embarrassing when you’re depending on it
to keep you clear of the ground during an instrument approach.
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THE TURN COORDINATOR
At the left of the lower row is another gyro
instrument, called a “turn coordinator.” Where
the attitude gyro directly indicates angle of
bank, the turn coordinator does so indirectly,
indicating instead whether the airplane is actu-
ally turning - changing its direction - to the left
or right. It doesn’t show any pitch information,
and is labeled to warn you of that shortcoming.
In return for that seeming failure, though, it has a lot going
for it. First of all, it’s a lot simpler and more rugged than the gyro
horizon, and thus less prone to failure. Second, the gyro horizon
and the directional gyro (which is explained next) are powered
pneumatically, using vacuum pumps on the engine; the turn coor-
dinator is electric. Vacuum pumps are notoriously unreliable,
which is why the 172R has two of them - but even then an air leak
could leave the gyro horizon unusable. This way, you have two
different types of gyro instruments, powered by completely differ-
ent systems, in the hope that no combination of malfunctions
would deprive you of everything at once.
At the bottom of the turn coordinator is a curved glass tube
with a metal ball, damped by liquid, sliding back and forth inside.
This so-called “skid and slip” ball indicates whether you have the
right bank angle for the rate at which you’re turning (or, converse-
ly, that you’re turning at the right rate for the bank angle you’re
using). You’ll control it with the rudder pedals, if you have them;
if not, FLY! can be configured to take care of that for you auto-
matically. The skid ball neither has, nor needs, any type of airplane
power at all.
THE DIRECTIONAL GYRO
Next in line, in the center and directly
below the attitude gyro, we find the other
air-driven gyro instrument, the directional
gyro or gyrocompass.
Flight Instruction
Flight Instruction