problems. The two most common casualties that you
can identify just by noting variations in the system
pressure are loss of pitch control and loss of hydraulic
Loss of Pitch Control
A loss of pitch control can be caused by either a
mechanical or an electrical failure. Mechanical failures
tend to occur more frequently. You should be aware,
however, that electrical problems can occur that will
occasionally produce a loss of control. Normally, an
electrical problem, such as a broken or loose cannon
plug or loss of feedback position, will require the system
to be shifted to manual control. In DDG-51 class ships,
however, the CPP system has both a normal and an
alternate system by which the pitch position can be
monitored. If the normal system fails to provide
command or feedback data, the CPP system can be
shifted to the alternate system. When investigating a
loss of pitch control on any gas turbine-powered ship,
you must be aware of the components in the CRP/CPP
systems that are most likely to fail. The following are
some of the components you will have to monitor most
ELECTROHYDRAULIC SERVO VALVE.
The most common component failure is the
electrohydraulic servo valve. This valve is the primary
component for remote operation and control. If this
valve were not installed, all operations would require
personnel to be stationed at the OD box at all times.
You can easily identify a faulty electrohydraulic
servo valve. Any of the following symptoms should
alert you as to the possible failure of this valve:
Pitch fails to respond to a desired integrated
throttle control (ITC) change.
Pitch changes (fluctuations) occur without a
pitch change command.
Hub servo pressure increases steadily without a
change in system demands.
AUXILIARY RELIEF VALVE. A faulty
auxiliary relief valve also will cause a loss of pitch
control. If the valve fails in the open position, all of the
control oil will be ported back to the sump. Pitch cannot
be changed without control oil to position the auxiliary
servo piston in the OD box. In addition to the loss of
pitch control, you should investigate any loss of
pressure. You should be able to spot a pressure loss by
checking the HOPM pressure gauges. This should be
one of your first steps in checking the system.
REDUCING VALVE. A faulty reducing valve is
another cause for a loss of pitch control. If the reducing
valve fails in the closed position, the flow of control oil
will be cut off to the auxiliary servo piston, and pitch
will fail to respond. Like the auxiliary relief valve, this
loss of pressure will have to be viewed at the HOPM
during the initial system investigation.
Loss of Hydraulic Oil Pressure
Usually, a loss of hydraulic oil pressure will cause
an alarm to be generated at the PACC/PCC. The
generation of this alarm, of course, will immediately
alert the operator to a problem. The alarm will sound
when casualties occur either to the main relief valve or
to a sequencing valve. The alarm may or may not sound,
however, when a major leak occurs. Look at the three
main causes of hydraulic oil pressure loss and the
MAIN RELIEF VALVE. A faulty main relief
valve can be identified by a low-pressure alarm at the
console, but the actual answers can be found at the
HOPM. This component failure can be easily identified
by the operator. The operator simply looks at the HOPM
pressure gauges and notes that all pressures are
extremely low or nonexistent.
SEQUENCING VALVE. You may suspect that a
sequencing valve is faulty after a loss of pitch control as
well as a loss of hydraulic oil pressure, depending on
how the valve fails. For instance, if the valve fails in the
open position, then all the oil would become
high-pressure oil and the low-pressure alarm would not
sound. In this case, the auxiliary servo supply (control
oil) pressure would be drastically low and the system
control would fade. On the other hand, if the sequencing
valve fails in the closed position, a low-pressure alarm
would sound and alert the operator. In this instance, the
operator would also be able to see an extremely sluggish
pitch response time.
MAJOR LEAK. A major leak can provide the
same symptoms as a loss of hydraulic oil pressure,
depending on the location of the leak.
We have just told you about some of the components
you should check when you detect a loss of pitch control
or a loss of hydraulic oil pressure. Now, we will discuss
some of the most common maintenance procedures you
as a supervisor will expect your personnel to perform.