However, the effect of the increase in dielectric
needed, and pressure registers on the gauge only
constant is greater than the effect of the increase
during these periods.
in density. The results are an incorrect indication.
The pressures of hydraulic systems vary for
The system reduces this error by varying the
different models of aircraft. In older pressure
capacitance of the reference capacitor in the
systems, the gauges registered from 0 to 2,000
bridge leg opposite the immersed tank unit.
PSI. With later model aircraft, the pressure ranges
have increased. Some aircraft have systems with
COMPENSATION. --The reference capacitor
pressure ranges as high as 4,000 PSI. The trend
is varied by connecting a compensator unit in
is away from the direct reading pressure gauge and
parallel with it. The compensator, like the tank
towards the synchro (electric) type of gauge.
probe, is a variable capacitor. However, the
Figure 6-69, view A, shows the hydraulic
compensator mounts at the lowest level of fuel
pressure indicator of a late model naval aircraft.
This aircraft has three hydraulic systems. The
so it is completely immersed until the tank is
almost dry. Its capacitance depends on the
indicating system shows the No. 1 and No. 2 flight
dielectric content of the fuel rather than the
control systems and the utility hydraulic system
quantity. The compensator connects into the
pressures. The indicating system consists of three
common reference leg for both phases of the
remote pressure transmitters, a hydraulic pressure
bridge circuit. This allows it to become a part of
selector switch, and a dual pointer indicator. The
the reference capacitance. A change in the
system uses 26-volt, 400-hertz, single-phase
dielectric constant of the fuel affects both the
alternating current from the 26-volt, single-phase
tank probe and the compensator capacitance.
bus.
Therefore, the current change in the tank probe
Each hydraulic pressure system line contains
leg of the bridge is counteracted by a similar
a Bourdon tube type of pressure transmitter.
change in the reference leg of the circuit.
Expansion and contraction of the Bourdon tube
There are various capacitor-type fuel quantity
travels by mechanical linkage to the rotor of the
systems that operate on the principle just
described. Indicators, tank probes, and power
units may differ as to shape, size, and specifi-
cations from system to system. For this reason,
you should always consult the manufacturer's
manuals for specific information on a particular
system.
HYDRAULIC PRESSURE INDICATORS
In most naval aircraft, the hydraulic system
operates the landing gear, flaps, speed brakes,
bomb bay doors, and certain other units. Aircraft
hydraulic pressure gauges show either the pressure
of the complete system or the pressure of an
individual unit in the system. A typical direct
reading gauge contains a Bourdon tube and a
gear-and-pinion mechanism. The mechanism
amplifies and transfers the tube's motion to the
pointer. The position of the pointer on the
calibrated dial shows the pressure in pounds per
square inch.
The pumps supplying pressure for operating
the aircraft's hydraulic units are driven by an
aircraft engine, an electric motor, or by both.
Some installations employ a pressure tank or
accumulator to maintain a reserve of fluid under
hydraulic pressure. In such cases, the pressure
gauge registers continuously. With other installa-
Figure 6-69.-(A) Hydraulic pressure indicator; (B) hydraulic
tions, operating pressure builds up only when
pressure indicating schematic.
