The Jet Engine
Flying Handbook Menu > Transition to Jet Powered Airplanes > Operating The Jet Engine
In a jet engine, thrust is determined by the
amount of fuel injected into the combustion chamber. The power
controls on most turbojet and turbofan powered airplanes consist
of just one thrust lever for each engine, because most engine
control functions are automatic. The thrust lever is linked
to a fuel control and/or electronic engine computer that meters
fuel flow based upon r.p.m., internal temperatures, ambient
conditions, and other factors. [figure15-3]
figure15-3. Jet engine power controls.
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. [figure15-4]
figure15-4. Jet engine r.p.m. gauges.
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
• Turbine Outlet Temperature (TOT)—like EGT, turbine
outlet temperature is taken aft of the turbine wheel(s).