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Chapter 5 - Flight Instruments: Clocks, Tops & Toys 5-25

rigid position. That’s why the child’s
top remains upright until is stops
spinning. It’s this property that
allows the attitude indicator’s hori-
zon card to portray airplane attitude.
The internal workings of the atti-
tude indicator are displayed in Figure
42. While this is a highly simplified
drawing, it does allow you to under- Fig. 46
stand the principles upon which the
attitude indicator works. Notice the
circular disk in the center of the
instrument. This is the gyroscope. It
is mechanically connected to the
Fig. 43 sky/ground horizon card on the face 45). This system is known as the vac-
of the indicator. When spun, this disk uum system (and this isn’t something
takes on gyroscopic properties and you use to keep your airplane clean!).
maintains its position, fixed in space, Notice that the vacuum pump is
relative to the earth. Thus, the hori- connected to two instruments—the
zon’s face card, which is mechanical- attitude indicator and the heading
ly connected to the gyro, also remains indicator. Both these instruments
fixed in space. From inside the air- use gyroscopes that are typically
plane, the horizon face card accurate- spun by vacuum air pressure. You
ly represents the real horizon in may also come across an airplane
either a right or left hand turn as with a heading indicator whose gyros
shown in Figure 43. As you can clear- are electrically spun. Either way, the
known as rigidity in space. Figure 41 ly see, the airplane rotates about the spinning of a gyro allows these
shows a child’s top in full spin. A attitude indicator’s gyro-stabilized instruments to work their magic.
spinning top, just like a spinning (fixed) horizon card. Malfunctions of the airplane’s air-
wheel inside the attitude indicator Most light airplane attitude gyros spun gyro instruments are usually
(Figure 41), acquires the unusual are spun by air pressure. A vacuum caused by the vacuum pump provid-
property known as rigidity in space. pump (Figure 44) sucks air through ing insufficient vacuum pressure. An
A small wheel (a gyro) spun at high the instrument and over the gyro, airplane’s vacuum gauge (Figure 46)
speed tends to remain in a fixed or spinning it at high velocity (Figure keeps you informed about the

THE AIRPLANE’S VACUUM PUMP

Fig. 44
Fig. 45