The main part of the direct reading gauge is the
Bourdon tube. The Bourdon tube is a curved metal
tube that is oval in cross-sectional shape (fig. 12-36).
One end of the Bourdon tube is closed, while the other
end has a fitting for connecting it to a pressure source.
The fitting end is fastened to the gauge frame, while the
other end is free to move so it can operate the
Assume that fluid pressure enters the Bourdon
tube. Since fluid pressure will be transmitted equally
in all directions and the area on the outside radius of the
tube is greater than that of its inside, the force will also
be greater on the outside radius, which tends to
straighten the tube. As the movable end of the tube
tries to turn outward, it turns the pivot segment gear.
Figure 12-37.--Typical hydraulic pressure indicator.
This gear meshes with a smaller rotary gear to which a
pointer is attached, and its movement causes a reading
The indicator system consists of three pressure
on the pressure gauge. The gauge dial is calibrated so
that the needle points to a number that corresponds to
and a hydraulic pressure selector switch and dual
the exact pressure that is applied. When the pressure is
pointer indicator, both located on the pilot's instrument
removed, the Bourdon tube acts as a spring, and returns
to its normal position.
principle. Expansion and contraction of the Bourdon
tube is transmitted by mechanical linkage to the rotor
of a transmitter synchro. The synchro transmits an
On most newer aircraft, an electrically operated
electrical signal through wiring to the pressure
(synchro) pressure indicator is used. Figure 12-37
indicator. The indicator contains two synchros
shows the pressure indicator of a typical naval aircraft.
mechanically attached to the two separate pointers.
This aircraft is equipped with three hydraulic
systems--No. 1 flight control system, No. 2 flight
When the HYD PRESS SELECTOR switch (fig.
control system, and utility system. One indicator
12-37) is in the No. 1 and No. 2 FLT CONT position,
provides pressure indication for all three systems. This
the pointers (marked "1" and "2") indicate the pressure
type of arrangement is desirable because it saves
in their respective systems, independent of each other.
instrument panel space.
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 Electrician's 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
12-37 is an example of the latter type.
As shown in figure 12-37, 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
Figure 12-36.--Bourdon tube.
and four minor intermediate markings between the