Cessna 402C - Page 237

'M~L402C
SECTION 7
AIRPLANE & SYSTEMS DESCRIPTIONS
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The exhaust air side of each attitude gyro is connected to the vacuum
manifold thus providing a smooth steady vacuum for the gyros. The vacuum
pressu re be i n9 appl i ed to the gyros is cons tantl y presented on the su cti on
gage. This gage also provides failure indicators for the left and right
vacuum pumps. These indicators are small red buttons located in the lower
portion of the suction
gage Which are spring-loaded to the extended
(failed) position. When nonnal vacuum is applied in the manifold, the
failure oottons are pulled flush with the gage face. Should insufficient
vacuum occur on either side, the respective red button will extend. No I
corrective action is required by the pilot, as the system will automatic-
ally isolate the failed vacuum source, allowing normal operation on the
remaining operative
vacuum pump.
The inlet air side of the attitude gyros are connected to a common
vacuum air filter which cleans the ambient cabin air before allowing it to
enter the gyros.
FLIGHT INSTRUMENTS
The basic flight instruments, see Figure 7-1, consist of airspeed,
altimeter and rate-of-climb indicators, electric turn-and-bank and vacuum
horizon and directional gyros.
Operation of the
airspeed, altimeter and rate-of-climb indicators can be
determined by cross-checking the copilot's instruments, if installed.
Also, when a climb or descent is initiated, these instruments should indi-
cate the appropriate change. If no change is indicated, it is reasonable
to assume static source blockage has occurred and the alternate static
source should be selected. If the possibility of static source icing is
present, actuation of the stall and vent heat switch might deice the static
sources, allowing a return to the normal static source, if the optional
heated
static sources are installed. If only the airspeed indicator ap-
pears to
be affected when the climb or descent is initiated, it is rea-
sonable
to assume a pitot system blockage has occurred. If the possibility
of pitot source icing is present, actuation of the pitot heat switch will
clear the ice blockage. Reference the optional copilot's instruments and
optional angle-of-attack indicator for airspeed information until a reli-
able airspeed indication can be obtained. If neither optional system is
installed, fly attitude and power references.
Operation of the turn-and-bank needle
can be checked by initiating a
standard
rate turn and cross-checking the turn rate with the directional
gyro. An indicated standard rate turn should show a turning rate of 3
degrees per second
on the directional gyro. Pushing the PRESS-TO-TEST
button adjacent to the annunciator panel will illuminate the T &B TEST
annunciator light if power is being applied to the turn-and-bank indicator.
After shutdown of the airplane on the ground, abnonnal noise cCflling from
the turn-and-bank can indicate a near failure condition. The ball part of
the turn-and-bank
is virtually failure proof. Inaccuracy can result only
if the indicator is not level in the instrument panel. With the airplane
on level ground, the ball should be centered in the race.
1
November 1979
Revision 1 - 2 Jan 1980
7-33