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pressure indicators are similar to those for
temperature and pressure indication.
42. On some engines, a fuel differential pressure
switch, fitted to the low pressure fuel filter, senses the
pressure difference across the filter element. The
switch is connected to a warning lamp that provides
indication of partial filter blockage, with the possibility
of fuel starvation.
Fuel flow
43. Although the amount of fuel consumed during a
given flight may vary slightly between engines of the
same type, fuel flow does provide a useful indication
of the satisfactory operation of the engine and of the
amount of fuel being consumed during the flight. A
typical system consists of a fuel flow transmitter,
which is fitted into the low pressure fuel system, and
an indicator, which shows the rate of fuel flow and the
Controls and instrumentation
141
Fig. 12-8 A typical double element thermocouple system.
total fuel used in gallons, pounds or kilogrammes per
hour (fig. 12-10). The transmitter measures the fuel
flow electrically and an associated electronic unit
gives a signal to the indicator proportional to the fuel
flow.
Vibration
44. A turbo-jet engine has an extremely low
vibration level and a change of vibration, due to an
impending or partial failure, may pass without being
noticed. Many engines are therefore fitted with
vibration indicators that continually monitor the
vibration level of the engine. The indicator is usually
a milliammeter that receives signals through an
amplifier from engine mounted transmitters (fig. 12-
11).
45. A vibration transmitter is mounted on the engine
casing and electrically connected to the amplifier and
indicator. The vibration sensing element is usually an
electro-magnetic transducer that converts the rate of
vibration into electrical signals and these cause the
indicator pointer to move proportional to the vibration
level. A warning lamp on the instrument panel is
incorporated in the system to warn the pilot if an
unacceptable level of vibration is approached,
enabling the engine to be shut down and so reduce
the risk of damage.
Controls and instrumentation
142
Fig. 12-9 Oil temperature and pressure transmitters and indicators.
Fig. 12-10 Fuel flow transmitter and
indicator.
46. The vibration level recorded on the gauge is the
sum total of vibration felt at the pick-up. A more
accurate method differentiates between the
frequency ranges of each rotating assembly and so
enables the source of vibration to be isolated. This is
particularly important on multi-spool engines.
47. A crystal-type vibration transmitter, giving a
more reliable indication of vibration, has been
developed for use on multi-spool engines. A system
of filters in the electrical circuit to the gauge makes it
possible to compare the vibration obtained against a
known frequency range and so locate the vibration
source. A multiple-selector switch enables the pilot to
select a specific area to obtain a reading of the level
of vibration.
Warning systems
48. Warning systems are provided to give an
indication of a possible failure or the existence of a
dangerous condition, so that action can be taken to
safeguard the engine or aircraft. Although the various
systems of an aircraft engine are designed wherever
possible to ’fail safe1, additional safety devices are
sometimes fitted. Automatic propeller feathering
should a power loss occur, and automatic closing of
the high pressure fuel shut-off cock should a turbine
shaft failure occur, are but two examples. On some
engine types, the fuel system is fitted with a control
to enable the engine to be operated by manual
throttling should a main fuel system failure occur.
49. In addition to a fire warning system (Part 14), a
number of other audible or visual warning systems
can be fitted to a gas turbine engine. These may be
for low oil or fuel pressure, excessive vibration or
overheating. Indication of these may be by warning
light, bell or horn. A flashing light is used to attract the
pilot’s attention to a central warning panel (C.W.P.)
where the actual fault is indicated.
50. Other instruments and lights warn the pilot of
the selected position of the thrust reverser, the fan
reverser or the afterburner variable nozzle, when
applicable. Gauges also inform the pilot of such
things as hydraulic pressure and flow and generator
output, which are vital to the correct operation of the
aircraft systems.
Aircraft integrated data system
51. The aircraft integrated data system (A.I.D.S.) is
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