User Guide

Here, too, we find advantages and shortcomings. The DG’s
advantage, compared to a traditional magnetic compass, is that it’s
much steadier and easier to read. In rough air, a regular compass
swings back and forth all the time. Even in smooth air, it’s only
accurate in straight flight. The earth’s magnetic field has a vertical
component as well as the obvious horizontal one, and since air-
planes bank when they turn, the old-fashioned compass will lag
way behind for part of the turn, then rush ahead, then lag again -
it’s only accurate (and not very, then) when you’re passing right
through due east or west. The DG, on the other hand, neither
knows nor cares about magnetic north; it simply tries to hold a
rigid position in space, so its indication is smooth and constant.
And therein lies its disadvantage, too: since it doesn’t know
where north is, it also doesn’t know if it’s accurate or not. Even the
best gyros drift a bit with time (and even a theoretical “driftless”
gyro, rigid in space, would appear to do a slow flip every 24 hours
as the world turned beneath it). That’s why the 172’s directional
gyro has to be cross-checked every ten minutes or so against the
old-fashioned magnetic compass in the middle of the windshield,
and reset as necessary using the knob at the 7 o’clock position.
And, as with the gyro horizon, if both vacuum pumps fail, all bets
are off...
If you’re new to this, you’ll notice that neither the DG nor the
“whiskey” compass in the windshield (so called because its damp-
ing fluid is mostly alcohol) are marked off in the traditional N, S, E,
and W. Instead, we use degrees, with 0 for north, 90 for east, 180
for south, and 270 for west. In aircraft instruments, they’re called out
every 30 degrees, with the last zero left off - thus, “9” is east, “24”
would be southwesterly at 240 degrees, and so on.
THE VERTICAL SPEED INDICATOR
Finally, at the bottom right of the “sacred
six,” we find the vertical speed indicator (VSI).
This is a very simple unpowered instrument
which simply reads whether the airplane is
climbing or descending, and how fast (up to
2000 feet per minute either way). Unless in a very
strong thermal or mountain wave, no 172 has ever
climbed at 2000 fpm except in the dreams of the Cessna market-
ing department, but a 2000-fpm descent, while ear-popping, is not
unheard-of. Something to remember about the VSI is that its indi-
cations lag behind what’s really happening by up to 10 seconds.
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POWERPLANT INSTRUMENTS
The most important engine instrument -
and one you’ll refer to quite frequently
when setting power - is the tachometer,
located just below the VSI. Airplane
engines turn a lot slower than their auto-
motive counterparts; you’ll notice that
this one is redlined at only 2400 rpm.
Engine RPM is controlled directly by
the throttle - but in an airplane like the 172,
with its simple fixed-pitch propeller, it’s
also controlled indirectly by airplane
speed. It’s rather like driving a car with only one gear. Shove the
throttle in all the way with the airplane at a standstill on the
ground, and it’ll only turn up around 2100 rpm - but as its speed
increases in a climb, it’ll pick up a bit more. Level off in cruise, let
the airplane accelerate, and it’ll nudge redline. That’s a perfectly
acceptable way to operate, as long as you don’t exceed 2400 rpm
- but if you start a descent without reducing power, the engine will
overspeed very readily. Just pay attention to the tach and you
won’t have a problem.
The other engine instruments, which monitor its “health,” are
the smaller ones to the left of the main flight instrument panel. At
the bottom left is a dual indicator showing both oil pressure and oil
temperature - the latter important since in an air-cooled engine like
this one, the oil plays an important role in cooling as well as lubri-
cation. To its right, another dual indicator monitors how much vac-
uum the dual air pumps are producing to run the gyro instruments
(warning lights elsewhere will show if either pump fails), and
whether the electrical system is charging or discharging the battery.
Flight Instruction
Flight Instruction
Tachometer
Oil Temperature &
Pressure Gauge
Vacuum Gauge &
Amperage