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engine inlet pressure. It is an indication of what the
engine has done with the raw air scooped in. For
instance, an EPR setting of 2.24 means that the
discharge pressure relative to the inlet pressure is
2.24 : 1. On these airplanes, the EPR gauge is the
primary reference used to establish power settings.
[Figure 15-5]
Fan speed (N1) is the primary indication of thrust on
most turbofan engines. Fuel flow provides a secondary
thrust indication, and cross-checking for proper fuel
flow can help in spotting a faulty N1 gauge. Turbofans
also have a gas generator turbine tachometer (N2).
They are used mainly for engine starting and some
system functions.
In setting power, it is usually the primary power
reference (EPR or N1) that is most critical, and will be
the gauge that will first limit the forward movement of
the thrust levers. However, there are occasions where
the limits of either r.p.m. or temperature can be
exceeded. The rule is: movement of the thrust levers
must be stopped and power set at whichever the limits
of EPR, r.p.m., or temperature is reached first.
THRUST TO THRUST LEVER
RELATIONSHIP
In a piston engine propeller driven airplane, thrust is
proportional to r.p.m., manifold pressure, and propeller
blade angle, with manifold pressure being the most
dominant factor. At a constant r.p.m., thrust is
proportional to throttle lever position. In a jet engine,
however, thrust is quite disproportional to thrust lever
position. This is an important difference that the pilot
transitioning into jet powered airplanes must become
accustomed to.
On a jet engine, thrust is proportional to r.p.m. (mass
flow) and temperature (fuel/air ratio). These are
matched and a further variation of thrust results from
the compressor efficiency at varying r.p.m. The jet
engine is most efficient at high r.p.m., where the
engine is designed to be operated most of the time. As
r.p.m. increases, mass flow, temperature, and efficiency
also increase. Therefore, much more thrust is
produced per increment of throttle movement near the
top of the range than near the bottom.
One thing that will seem different to the piston pilot
transitioning into jet powered airplanes is the rather
large amount of thrust lever movement between the
flight idle position and full power as compared to the
small amount of movement of the throttle in the piston
engine. For instance, an inch of throttle movement on
a piston may be worth 400 horsepower wherever the
throttle may be. On a jet, an inch of thrust lever
movement at a low r.p.m. may be worth only 200
pounds of thrust, but at a high r.p.m. that same inch of
movement might amount to closer to 2,000 pounds of
thrust. Because of this, in a situation where
significantly more thrust is needed and the jet engine
is at low r.p.m., it will not do much good to merely
“inch the thrust lever forward.” Substantial thrust lever
movement is in order. This is not to say that rough or
abrupt thrust lever action is standard operating
procedure. If the power setting is already high, it may
take only a small amount of movement. However,
there are two characteristics of the jet engine that work
against the normal habits of the piston engine pilot.
One is the variation of thrust with r.p.m., and the other
is the relatively slow acceleration of the jet engine.
VARIATION OF THRUST WITH RPM
Whereas piston engines normally operate in the range
of 40 percent to 70 percent of available r.p.m., jets
operate most efficiently in the 85 percent to 100
percent range, with a flight idle r.p.m. of 50 percent to
60 percent. The range from 90 percent to 100 percent
in jets may produce as much thrust as the total
available at 70 percent. [Figure 15-6]
SLOW ACCELERATION OF THE JET
ENGINE
In a propeller driven airplane, the constant speed
propeller keeps the engine turning at a constant r.p.m.
within the governing range, and power is changed by
varying the manifold pressure. Acceleration of the
Figure 15-5. EPR gauge.
Ch 15.qxd 5/7/04 10:22 AM Page 15-4
15-5
piston from idle to full power is relatively rapid,
somewhere on the order of 3 to 4 seconds. The
acceleration on the different jet engines can vary
considerably, but it is usually much slower.
Efficiency in a jet engine is highest at high r.p.m.
where the compressor is working closest to its
optimum conditions. At low r.p.m. the operating cycle
is generally inefficient. If the engine is operating at
normal approach r.p.m. and there is a sudden
requirement for increased thrust, the jet engine will
　
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