and a hydraulic pressure selector switch and dual
pointer indicator, both located on the pilots
The transmitters operate on the Bourdon tube
principle. Expansion and contraction of the Bourdon
tube is transmitted by mechanical linkage to the rotor
of a transmitter synchro. The synchro transmits an
electrical signal through wiring to the pressure
indicator. The indicator contains two synchros
mechanically attached to the two separate pointers.
When the HYD PRESS SELECTOR switch (fig.
7-37) is in the No. 1 and No. 2 FLT CONT position,
the pointers (marked "1" and "2") indicate the
pressure in their respective systems, independent of
each other. When the HYD PRESS SELECTOR
switch is in the UTILITY position, the synchros are
connected in electrical parallel, and the pointers align
with each other and act as one.
Although the Aviation Electricians Mate is
responsible for inspecting and maintaining all the
aircraft gauges and other instruments, you must know
how to read the hydraulic pressure gauge to inspect
and maintain the hydraulic system.
Pressure gauges on some naval aircraft are
calibrated to register from 0 to 2,000 psi; on others
they register from 0 to 4,000 psi. The gauge in
figure 7-37 is an example of the latter type.
As shown in figure 7-37, on gauges designed for
a range of 0 to 4,000 psi, the dial is calibrated with
four major markings with the numerals 1,2,3, and 4.
One major intermediate graduation between each
numeral and four minor intermediate markings
between the major markings are for reading to the
Figure 7-38.Hydraulic pressure gauge.
nearest 100 psi.
On these gauges, the numeral
reading must be multiplied by 1,000 to obtain the
actual pressure in psi.
On gauges designed for a range of 0 to 2,000 psi,
the dial is calibrated with two major markings, the
numerals 1,000 and 2,000, and four intermediate
graduations for reading to the nearest 200 psi. A
gauge of this type is shown in figure 7-38.
GAUGE AND PRESSURE TRANSMITTER
A gauge and pressure transmitter snubber is a
hydraulic component located upstream of pressure
gauges and pressure transmitters. Its purpose is to
damper out system pressure surges that could cause
possible damage to gauges and pressure transmitters.
Snubbers also prevent cockpit hydraulic indicators
from oscillating and fluctuating, which makes
accurate reading of the gauge not only difficult but
Without the use of a snubber,
pressure oscillations and other sudden pressure
changes existing in hydraulic systems could affect the
delicate internal mechanism of both gauges and
This may cause either complete
destruction of the gauge or transmitter or, often
worse, partial damage, resulting in false readings.
The basic components of a snubber are the
housing, fitting assembly with a fixed orifice
diameter, and the pin and plunger assembly, as shown
in figure 7-39. The snubbing action is obtained by
metering fluid through the snubber.
assembly orifice restricts the amount of fluid that
flows to the gauge or pressure transmitter, thereby
snubbing the force of a pressure surge. The pin is
pushed and pulled through the orifice of the fitting
assembly by the plunger, keeping it clear and at a
Figure 7-39.Gauge and pressure transmitter snubber.