FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 12. Aircraft Avionics Systems | 3. Ground Operational Checks for Avionics Equipment (Non-Electrical) | 12-38. Pneumatic Gyros
AC 43.13-1B CHG 1
9/27/01
b. Vacuum Pump Systems. In order to
overcome the major drawback of the venturi
tube, that is, its susceptibility to ice, aircraft
were equipped with engine driven vacuum
pumps and the gyro instruments were driven
by air pulled through the instrument by the
suction produced by these pumps. A suction
relief valve maintained the desired pressure
(usually about four inches of mercury) on the
attitude gyro instruments, and a needle valve
between one of the attitude indicators and the
turn and slip indicator restricted the airflow to
maintain the desired 2 inches of suction in its
case. Most of the early instruments used only
paper filters in each of the instrument cases,
but in some installations a central air filter was
used to remove contaminants from the cabin
air before it entered the instrument case.
(1) The early vacuum pumps were
vane-type pumps of what is called the wet
type-one with a cast iron housing and steel
vanes. Engine oil was metered into the pump
to provide sealing, lubrication, and cooling,
and then this oil, along with the air, was blown
through an oil separator where the oil collected
on baffles and was returned to the engine
crankcase. The air was then exhausted over
board. Aircraft equipped with rubber deicer
boots used this discharge air to inflate the
boots. But before it could be used, this air was
passed through a second stage of oil separation
and then to the distributor valve and finally to
the boots. (See figure 12-2.)
(2) The airflow through the instruments
is controlled by maintaining the suction in the
instrument case at the desired level with a suc
tion relief valve mounted between the pump
and the instruments. This valve has a
spring-loaded poppet that offsets to allow
cabin air to enter the pump and maintain the
correct negative pressure inside the instrument
case.
(3) The more modern vacuum pumps
are of the dry type. These pumps use carbon
vanes and do not require any lubrication, as the
vanes provide their own lubrication as they
wear away at a carefully predetermined rate.
Other than the fact that they do not require an
oil separator, systems using dry air pumps are
quite similar to those using a wet pump. One
slight difference, however, is in the need for
keeping the inside of the pump perfectly clean.
Any solid particles drawn into the system
through the suction relief valve can damage
one of the carbon vanes, and this can lead to
destruction of the pump, as the particles bro
ken off of one vane will damage all of the
other vanes. To prevent particles entering the
relief valve, its air inlet is covered with a filter,
and this must be cleaned or replaced at the in
terval recommended by the aircraft manufac
turer.
c. Positive Pressure Systems. Above
about 18,000 feet there is not enough mass to
the air drawn through the instruments to pro
vide sufficient rotor speed, and, to remedy this
problem, many aircraft that fly at high altitude
use positive pressure systems to drive the gy
ros. These systems use the same type of air
pump as is used for vacuum systems, but the
discharged air from the pump is filtered and
directed into the instrument case through the
same fitting that receives the filtered air when
the vacuum system is used. A filter is installed
on the inlet of the pump, and then, before the
air is directed into the instrument case, it is
again filtered. A pressure regulator is located
between the pump and the in-line filter to con
trol the air pressure so only the correct amount
is directed into the instrument case.
System Filters. The life of an air-driven gyro
instrument is determined to a great extent by
the cleanliness of the air that flows over the
rotor. In vacuum systems, this air is taken
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