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In a jet engine, each major rotating section usually has
a separate gauge devoted to monitoring its speed of
rotation. Depending on the make and model, a jet
engine may have an N1 gauge that monitors the low
pressure compressor section and/or fan speed in
turbofan engines. The gas generator section may be
monitored by an N2 gauge, while triple spool engines
may have an N3 gauge as well. Each engine section
rotates at many thousands of r.p.m. Their gauges
therefore are calibrated in percent of r.p.m. rather than
actual r.p.m., for ease of display and interpretation.
[Figure 15-4]
Direction of Flight
Inlet
Air
Exhaust
Combustion
Combustion
Fan Air
Fan Air
Figure 15-2.Turbofan engine.
Ch 15.qxd 5/7/04 10:22 AM Page 15-2
15-3
The temperature of turbine gases must be closely
monitored by the pilot. As in any gas turbine engine,
exceeding temperature limits, even for a very few
seconds, may result in serious heat damage to turbine
blades and other components. Depending on the make
and model, gas temperatures can be measured at a
number of different locations within the engine. The
associated engine gauges therefore have different
names according to their location. For instance:
• Exhaust Gas Temperature (EGT)—the temperature
of the exhaust gases as they enter the tail
pipe, after passing through the turbine.
• Turbine Inlet Temperature (TIT)—the temperature
of the gases from the combustion section of
the engine as they enter the first stage of the turbine.
TIT is the highest temperature inside a gas
turbine engine and is one of the limiting factors
of the amount of power the engine can produce.
TIT, however, is difficult to measure. EGT
therefore, which relates to TIT, is normally the
parameter measured.
• Interstage Turbine Temperature (ITT)—the
temperature of the gases between the high
pressure and low pressure turbine wheels.
• Turbine Outlet Temperature (TOT)—like EGT,
turbine outlet temperature is taken aft of the
turbine wheel(s).
JET ENGINE IGNITION
Most jet engine ignition systems consist of two igniter
plugs, which are used during the ground or air starting
of the engine. Once the start is completed, this ignition
either automatically goes off or is turned off, and from
this point on, the combustion in the engine is a
continuous process.
CONTINUOUS IGNITION
An engine is sensitive to the flow characteristics of the
air that enters the intake of the engine nacelle. So long
as the flow of air is substantially normal, the engine
will continue to run smoothly. However, particularly
with rear mounted engines that are sometimes in a
position to be affected by disturbed airflow from the
wings, there are some abnormal flight situations that
could cause a compressor stall or flameout of the
engine. These abnormal flight conditions would usually
be associated with abrupt pitch changes such as
might be encountered in severe turbulence or a stall.
In order to avoid the possibility of engine flameout
from the above conditions, or from other conditions
that might cause ingestion problems such as heavy
rain, ice, or possible bird strike, most jet engines are
equipped with a continuous ignition system. This system
can be turned on and used continuously whenever
the need arises. In many jets, as an added precaution,
this system is normally used during takeoffs and landings.
Many jets are also equipped with an automatic
ignition system that operates both igniters whenever
the airplane stall warning or stick shaker is activated.
FUEL HEATERS
Because of the high altitudes and extremely cold outside
air temperatures in which the jet flies, it is possible
to supercool the jet fuel to the point that the small
Figure 15-3. Jet engine power controls.
Figure 15-4. Jet engine r.p.m. gauges.
Ch 15.qxd 5/7/04 10:22 AM Page 15-3
15-4
particles of water suspended in the fuel can turn to ice
crystals and clog the fuel filters leading to the engine.
For this reason, jet engines are normally equipped with
fuel heaters. The fuel heater may be of the automatic
type which constantly maintains the fuel temperature
above freezing, or they may be manually controlled by
the pilot from the cockpit.
SETTING POWER
On some jet airplanes, thrust is indicated by an engine
pressure ratio (EPR) gauge. Engine pressure ratio can
be thought of as being equivalent to the manifold
pressure on the piston engine. Engine pressure ratio is
the difference between turbine discharge pressure and
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